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The nine H-bonded dimers of NH3, OH2, and HF have been calculated by ab initio molecular orbital theory at the 6-31G* level with geometry optimization. Calculated dimerization energies (kilocalories/mole) are: H3N⋯HNH2, 2.9; H3N⋯HOH, 6.5; H3N⋯HF, 12.2; H2O⋯HNH2, 2.8; H2O⋯HOH, 5.6; H2O⋯HF, 9.2; HF⋯HNH2, 2.6; HF⋯HOH, 4.0; HF⋯HF, 5.9. Energies and geometries are compared with available experimental and ab initio values from the literature. The 6-31G* results are found to be internally consistent and of reasonable accuracy. The paucity of experimental measurements on gas-phase dimers makes the present set of results of special significance for understanding the mechanism of hydrogen bonding, and for stimulating further experimental work.

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... In its pair interactions, NH 3 is an excellent hydrogen-bond acceptor [40]. Thus NH 3 accepts hydrogen bonds and the structure of some complexes such as H 3 N/HCN [41], H 3 N/HF [42], H 3 N/HOH [43,44], H 3 N/HCCH [45], H 3 N … HCF 3 [46], H 3 N/ClCF 3 [47], H 3 N … HSH [48] are all characterized by linear hydrogen bonds. This was because NH 3 acts more like a Lewis-base that interacts via its lone proton-acceptor site. ...

... GC-PPC-SAFT [15] crossover SAFT [6] CP-PC-SAFT [16] SAFT-VR [5] PC-SAFT [11] SAFT-VR-Mie [7] SAFT [8] Soft-SAFT [12] PC-SAFT [9] PC-SAFT (this work) PC-SAFT [13] Soft-SAFT [10] 1300e1585 K [98] 1470e1585 K [38] 1410 K [49] 1550e1630 K [99] 1600 K [39] 1600 K [100] 1645 K [101] 1412e2212 K [102] 1863e2263 K [51] 1460 K [103] 1400e1460 K [42,104] 1400e1600 K [105] 1560 K [106] 1565 K [53] 1812 K [107] Fig. 1. Vapor pressures, liquid density (correlation-left) and heat of vaporization (prediction-right) of ammonia. ...

The phase equilibria of ammonia containing systems were investigated using the modified group-contribution PC-SAFT approach. The new proposed parameter set for ammonia with the 2B association scheme shows a better correlation/prediction of the phase behavior of ammonia and its mixtures over other published PC-SAFT parameter sets. The association energy parameter of pure ammonia and its complexes obtained in this work shows a very good agreement with the experimental/simulation data.
The assignment of the association scheme for ammonia was discussed and studied using the PC-SAFT EoS by applying the model to describe the phase equilibria of different mixtures. It was found that, in order to accurately describe the phase behavior of ammonia + aromatic, + unsaturated hydrocarbon or + refrigerant (HFCs/HCFCs) mixtures, a cross-association link was necessary. For almost considered mixtures, good vapor-liquid and liquid-liquid equilibria computation results were obtained including the double azeotropes systems.

... The presence of oxygen vacancies could be accredited to oxygen transition from its Zr 2p to Zr 4d orbital by improving the surface conductivity through solid imperfections [42]. The same trend was also observed in the case of the rGO-ZrO 2 nanocomposite, whereas the peaks corresponding to oxygen defect states of ZrO 2 and sp 2 tail band cluster of rGO were suppressed. of ammonia is very less compared with that of other test gases [43], (iv) also, the dissociation of bond at N-H occurs with the notable dissociation energy of 314 kJ/mol [44], which is relatively comparable to the other target gases, (v) clustered nanogranular morphology of ZrO 2 acts as a percolation path for ammonia molecule to diffuse into the pores within the Debye length, (vi) the oxygen vacancies and the perturbed oxygen defect centers of ZrO 2 nanostructure (as identified through PL spectra) would have increased the reactive oxygen sites and hence the better gas-solid interaction, (vii) the solid imperfections from the contribution of lattice oxygen species of ZrO 2 (transition of O from 2p to 4d) might have resulted in improving the surface catalytic behavior and thereby enhanced response, and (viii) the red shifted (Fig. 5b) from the characteristic position indicated the non-homogeneity of ZrO 2 . This has been confirmed through the formation of ZrO 2 in a mixed phase of orthorhombic and rhombohedral crystal structure as observed from XRD analysis. ...

Microwave irradiation technique provides a simple and quick synthesis route for nanomaterials at controlled experimental conditions. In this view, nanostructures of rGO, ZrO2, and rGO–ZrO2 composite were synthesized using microwave synthesis technique. Herein, we experimented the preparation of rGO using optimized conditions using convection mode at 373 K in a commercial microwave oven. Structural properties revealed the formation of mixed phases of the orthorhombic and rhombohedral crystal structure of ZrO2. The preferential plane orientation was found to be in c-axis (002) for rGO–ZrO2 nanocomposite, which revealed the predominance of rGO in the nanocomposite. Morphology of the prepared nanostructures exhibited agglomerated sheets, denser nanograins and randomly aggregated nanosheets for rGO, ZrO2, and mixed nanocomposite respectively. Room temperature gas sensing properties of the prepared nanostructures were investigated and reported. rGO showed responses of 5.3 and 5.2 towards 200 ppm of trimethylamine (TMA) and methanol. ZrO2 nanostructure showed a response of 35 towards 200 ppm of ammonia with response–recovery times of 91 and 6 s.

... However, ZPE effects lead to slightly different stability ranges for each phase, and promote distortions of the CdI2-like P3m1 phase; the onset of fully ionic phases should occur around 40 GPa at room temperature. Close to ambient conditions, the formation of molecular ammonia-water compounds is aided by energetically favorable hydrogen bonds between the two species (52). With increased compression, a different factor contributes: Proton transfer, in particular in a 2:1-ammonia:water compound, results in large stabilization due to ionic interactions and higher packing densities. ...

Significance
The mantles of icy planets comprise large amounts of water, ammonia, and methane ices. To understand their interior structure, it is crucial to study these ices at the extreme pressure conditions they likely experience. Hitherto, such studies have mostly been restricted to individual ices and not considered formation of stable mixtures. We survey here mixtures of water and ammonia and show that high pressures stabilize ammonia hemihydrate, through a transformation from a molecular crystal into a fully ionic solid that involves complete deprotonation of water. We suggest that ammonia-rich hydrates can precipitate out of any ammonia–water mixture at sufficient pressures and are an important component inside icy planets.

Molecular Simulations are increasingly entering the realm of materials syntheses. While pioneering studies were bound to simple models which could only address selected aspects of ‘real chemistry’ in the lab, recent advances in simulation methodology and computing hardware indeed paved the way to also modelling complex systems. Yet, we are hardly more than at the beginning of establishing molecular simulations as a routine tool for guiding syntheses. In the present contribution, we discuss the progress that has been made to understand ammonothermal syntheses of nitrides. This encompasses molecular dynamics simulations based on non-reactive force-fields—such as studies of liquid ammonia as a solvent, and its supercritical nature at high temperature and pressure. Moreover, we report on recent work on quantum and hybrid quantum/classical approaches for modelling the auto-protolysis of ammonia and ammonia protolyses in the course of metal ion solvation. This forms a basis for rationalizing the association of ion aggregates, size-induced proton transfer and the self-organization of amides, imides and nitrides from molecular simulations.

The interstellar medium (ISM) is home to several open shell and closed shell molecules considered to be unusual or highly unstable on earth. The chemistry of these molecules in the ISM has been widely studied over many decades. However, the concept of weak chemical interactions, which is terrestrially well studied, has not been highlighted much in interstellar chemistry. In this study, we illustrate the wide variety of possible weak interactions in the ISM occurring between a carefully chosen set of open shell (OH, SH, CN, NO, NH2, and HO2) and closed shell molecules (H2O, H2S, HF, HCl, NH3, PH3, HCN, HNC, HCP, CH3OH, and CH3SH) which are important in interstellar chemistry. We expound upon the structural, and energetic features of the weak interactions by employing electronic structure calculations (CCSD(T) and density functional theory). The nature of the weak interactions is further probed by three different techniques, viz. the Atoms-in-Molecules (AIM) method, transfer of spin densities, and the Natural Bond Orbital (NBO) method. The astrochemical implications of the weak interactions are subsequently discussed, and it is suggested that the weak interactions could impact the molecular abundances in the ISM.

Dans ce travail, nous tentons d'expliquer l'existence et la nature des complexes intermoléculaires par liaison hydrogéne grâce au programme ab initio GAUSSIAN‐70 utilisé avec la base STO‐3G. Nous avons calculé les propriétés du dimère de l'eau. Lea résultats obtenus pour les trois structures possibles de ce système confirment que la plus stable correspond à l'enchaînement linéaire O…H‐0, où la distance intermoléculaire O…H est la plus courte (1.75 Å); l'approche des deux molécules s'effectue selon l'axe passant par l'un des centroïdes de charge de l'oxygène accepteur d'hydrogène avec un transfert de charge vers le système donneur de proton.

The electronic and vibrational theories of the hydrogen bond are reviewed. High level quantum mechanical treatments of small systems which explain and reproduce well the experimental facts have led to the development of simpler models now used in the treatment of biological and other large systems. The application of H-bond theories to spectral phenomena of H-bonding are also briefly reviewed.

In molecular physics scientists are inclined to interpret interactions between atoms, ions or molecules by a set of “forces” which usually are grouped into different classes: weak and strong, specific and unspecific, etc. Quantum mechanics, on the other hand, tells us that there is no unique wav to split a given energy of interaction into “physically” meaningful contributions as long as the charge distributions of individual atoms, ions or molecules are spread over unlimited regions due to the exponential decay of their wave functions (see eg. Ahlrichs [1976]). The source of these ambiguities is the mutual penetration of electron density distributions of the interacting subunits which can be expressed quantitatively in terms of overlap integrals.

This study focuses on the methodology and computational strategies for the determination of molecular structure of biological molecules, of central interest is how to treat intermolecular interactions in large systems. To this end a review of recent results from our laboratory and others will be discussed with the following topics analysized:

The molecular multipole moments calculated by the IEHT wavefunctions are compared with the ab initio results and the experimental values available in the literature. The results indicate that the molecular multipole moments obtained by the IEHT wavefunctions are in good agreement with the experimental values. The basis set dependence of the multipole moments calculated from ab initio wavefunctions is discussed. Hydrogen bond energies for several systems have been calculated using intermolecular interaction theory based on IEHT wavefunctions. These hydrogen bonding energies are compared with available results obtained from ab initio supermolecule calculations. The results indicate that our approach, which is computationally more practical for the applications to large biological systems, can be used to reproduce the results of the ab initio supermolecule approach.

Ab initio quantum mechanics and Monte Carlo statistical mechanics have been applied to elucidate the reaction path and solvent effects for the prototypical 1,3-dipolar cycloaddition of methyl azide and ethene. At the B3LYP/6-31G*//B3LYP/6-31G* level, the computed enthalpy and entropy of activation, 15.1 kcal mol(-1) and -38.4 cal mol(-1) K-1, are consistent with experimental results on similar systems. The unusual solvent effects, which have been observed for such reactions, were then explored by obtaining free energy profiles for the reaction in four solvents via Monte Carlo simulations with free energy perturbation theory. Rate enhancement is predicted in progressing from carbon tetrachloride to the dipolar aprotic solvent DMSO owing to an increase in dipole moment for the transition structure relative to the reactants. However, in contrast to the situation for many Diels-Alder reactions, there is not a gain in hydrogen-bond accepting ability for the transition structure of the 1,3-dipolar cycloaddition. Thus, the observed rate acceleration for the reaction of phenyl azide and norbornene in highly aqueous solvent mixtures is attributed predominantly to hydrophobic effects. Greater rate enhancements in protic solvents are expected for reactions of electron-deficient dipolarophiles and electron-rich 1,3-dipoles.

Hydrogen fluoride clusters (HF)
n
serve as prototype systems for the study of hydrogen bonding on the basis of accurate potential energy hypersurfaces. We discuss conceptual aspects of calculating, sampling, decomposing and representing such multidimensional surfaces and present the current status of molecular one-, two-and three-body potentials for HF as well as results for potential hypersurfaces of (HF)
n
, n=2–6.

The kinetics of base hydrolysis of ((αβS)(p-hydroxybenzoato)(tetraethylenepentamine)cobalt(III) has been studied in aqua-organic solvent media using MeOH, Me2CO and MeCN as cosolvents at 20.0 ≤ t0 C ≤ 40.0 ( I = 0.02 mol dm-3) with 80% (v/v) of cosolvents. Only the base catalyzed path (kobs = kOH [OH-] )has been observed. The relative second order rate constant, kOH0s / kOH0w at 1 = 0, increases nonlinearly with increase in mole fraction ( xo s ) of the cosolvents. MeCN and Me2CO exert much stronger rate acceleration than MeOH. The relative transfer free energy [ΔtG(t.s.) - ΔtG(t.s.)](s←w)298 K is positive for MeCN and negative for both Me2CO and MeCN indicating greater destabilising effect of the solvent on the transition state (t.s.) relative to the initial state(i.s.)-for MeCN-H2O while the effect is reversed for MeOH-H2O and Me2CO-H2O media. The variation of activation parameters (ΔHt and ΔSt) with xo.s.. is non-linear exhibiting extrema supporting the fact that solvent structural effects are important in controlling the energetics of the process through solvation of the initial and transition states.

It is well known that when we place objects in the way of incoming monochromatic waves, whose wavelength is comparable to the size of the objects, a diffraction pattern can be observed. This phenomenon is based on the fact that the objects behave as sources of spherical waves, whose amplitudes should be summed up to yield the picture of interference, taking into account the differences in phases. A similar phenomenon can be observed during the process of scattering of X-rays and thermal neutrons from condensed matter, as their wavelength is comparable to the distance between atoms. An observable, intensity-like quantity with a dimension length squared is needed to be able to provide a mathematical formulation of the scattering process and then connect that to the structure of the liquids under study.

Good linear correlations (r = 0.903 − 0.974) were found between the pKa, of nitrogen bases (pyridine-, aniline-derivatives, aliphatic amines) measured in aqueous solution and the minimum of the molecular electrostatic potential. The prediction of pKa for different types of amines seems possible with an accuracy of ~ 0.5 units. Using bond-increment potential calculations the estimation can be applied even for biomolecules being related to drug design. Correct consideration of both the molecular conformations and formation of the hydrogen-bonds in aqueous solutions is a prerequisite in obtaining reliable pKa values.

CHARMM force field parameters were developed for cyclopentane-modified peptide nucleic acid (cpPNA) analogs. As in the original force field parameterization, a self-consistent step-wise optimization approach was taken that involved the iterative adjustments of internal and external parameters until convergence was obtained. The geometry parameters such as standard bond lengths and bond angles were obtained by reproducing ab initio gas-phase geometries of model compounds. The internal force constants used for stretch and bend deformations were optimized to fit the calculated vibrational spectra. Torsional parameters were modified to fit the rotational barriers about single bonds in model compounds. The partial atomic charges were optimized based on interaction energies of complexes between water and the model compound. Our parameterization accurately reproduced high-level quantum mechanical calculations. The parameters were validated by series of molecular dynamics simulations of cpPNA in explicit water. Together with the existing force field for nucleic acids, these parameters will enable simulations of cpPNA complexes with RNA and DNA.

This work presents a systematic investigation into the basis set convergence of harmonic vibrational frequencies of (H2O)2 and (HF)2 computed with second-order Møller-Plesset perturbation theory (MP2) and the coupled-cluster singles and doubles method with perturbative connected triples, CCSD(T), while employing correlation-consistent basis sets as large as aug-cc-pV6Z. The harmonic vibrational frequencies presented here are expected to lie within a few cm-1 of the complete basis set (CBS) limit. For these important hydrogen-bonding prototype systems, a basis set of at least quadruple-ζ quality augmented with diffuse functions is required to obtain harmonic vibrational frequencies within 10 cm-1 of the CBS limit. In addition, second-order vibrational perturbation theory (VPT2) anharmonic corrections yield CCSD(T) vibrational frequencies in excellent agreement with experimental spectra, differing by no more than a few cm-1 for the intramonomer fundamental vibrations. D0 values predicted by CCSD(T) VPT2 computations with a quadruple-ζ basis set reproduce the experimental values of (HF)2 and (H2O)2 to within 2 and 21 cm-1, respectively.

Energies and geometries for a number of small hydrogen bonded dimers have been calculated by semiempirical method based on a perturbation approach. Results have been compared with experimental and the best theoretical data. A quite good description of equilibrium configurations has been obtained in every case when local multipoles from sufficiently accurate wave functions have been used. Hydrogen bond lengths have not been predicted with sufficient precision. Results indicate, however, that it should be possible to achieve improvement in the framework of the applied calculation scheme by modification of the parameter values.

Intermolecular potential functions have been developed for urea. Ab initio 6–31G(d) calculations were performed on urea-water complexes to obtain interaction energies and hydrogen-bond lengths that were used in developing the partial charges for the OPLS model. An important test was made by computing through Monte Carlo simulations the difference in chloroform/water partition coefficients for urea and acetamide, Δ log P. The accord between the computed result from statistical perturbation theory for TIP4P water and OPLS chloroform (2.0 ± 0.1 ) and the experimental value (1.9) is excellent. The computed absolute free energy of hydration of acetamide (-9.5 ± 0.4 kcal/mol) also matches the experimental data at 25 °C (-9.68 kcal/mol). These facts provide confidence in the computed value of −13.6 ± 0.4 kcal/mol for the absolute free energy of hydration of urea, an experimentally unavailable result. The water structure around urea and acetamide was also characterized; the average numbers of solute-water hydrogen bonds are 5 for urea and 3–4 for acetamide.

Ab initio calculations with full geometry optimization have been carried out on the planar cCc, cTc, tTc, tCt, tTt, and cCt conformers of β-hydroxyacrolein using the 4-21G basis set, and on the cCc and cCt conformers using the 4-31G basis set. The hydrogen-bonded cCc conformer is the most stable and the cCt conformer the least stable, with the other conformers following the above sequence. β-Hydroxy substitution has scarcely any influence on the geometry of the trans-acrolein structure, whereas the geometry of the cis-acrolein structure shows significant changes which depend on whether the OH group is cis or trans with respect to the CHO group about the CC bond. The ΔET values for cis → trans isomerization about the CC bond in cCt and cTc support the hypothesis that these changes in geometry are the result of a destabilizing interaction in cCt and a stabilizing interaction in cTc. The geometry of the hydrogen-bonded structure cCc sets it apart from all the other conformers: it has by far the longest CC, the longest CO, the longest OH, the shortest CC, and the shortest CO. Its formation from cCt involves a lengthening of CC, CO, and OH and a shortening of CC and CO, indicating a delocalization of charge within the ring. 4-21G calculations have also been made for a distorted cCt structure that has the same bond lengths and angles as the equilibrium cCc structure, and the distortion energy, cCt (equm. geom.) → cCt (distorted geom.), is found to be +13.1 kJ mole−1. Taking the energy of this distorted cCt structure as the baseline, the hydrogen-bonding energy in cCc is found to be —80.3 kJ mole−1.

The pseudopotential method is applied to the electronic structure determination of H 2 O, CH 3 OH and HCOOH monomers and dimers. The quantities computed are the dimerization energies, optimum geometries, charge distributions, dipole moments with their derivatives and force constants. All results compare well with those determined by all-electron calculations.

Hydrogen bonding between water and a series of small organic molecules was examined via electronic structure calculations. Several computational methods were examined, including both a hybrid density functional procedure (Becke3LYP) and second-order Møller-Plesset theory (MP2) coupled with a double-ζ basis set augmented by diffuse polarization functions on heteroatoms. The agreement between Becke3LYP and MP2 energies was generally good, as was the agreement with energies obtained using more sophisticated and costly methods. The energies and structures of 53 hydrogen-bonded complexes of water with various small organic molecules, including alcohols, thiols, ethers, thioethers, carboxylic acids, esters, amines, amides, nitriles, and nitro compounds, were then examined systematically using the Becke3LYP and MP2 procedures. The hydrogen bond geometries were generally linear, and acceptor sites corresponded closely to the positions of lone pairs as predicted by simple hybridization arguments. Structures with sulfur and chlorine atoms showed some deviation from these simple expectations and seemed to be largely determined by molecular dipole-dipole interactions. Categorization of the hydrogen bonds involved in the various complexes led to an ordering of hydrogen bond donor and acceptor abilities for some common functional groups. The strength of association was found to correlate moderately well with experimental gas-phase basicity in those cases where water acted unambiguously as the hydrogen bond donor at a single site. Interestingly, sulfur was found to be close to oxygen in hydrogen bond acceptor strength, and the surprisingly strong acceptor ability of sulfur could not be explained in terms of its enhanced polarizability relative to oxygen. Calculations were also carried out on the AT and GC base pairs and yielded results in very close agreement with the highest levels of calculation previously reported.

An approach to the general extension of the OPLS all-atom (OPLS-AA) force field to heterocycles has been explored with testing for pyridine and the diazenes, pyridazine, pyrimidine, and pyrazine. For the non-bonded interactions, the partial atomic charges are obtained from fitting to the electrostatic potential surfaces from ab initio RHF/6-31G* calculations and standard OPLS values are used for the Lennard-Jones parameters. The harmonic bond-stretching and angle-bending parameters are largely adopted from the AMBER force field. The resultant OPLS-AA force field is shown to perform well for computing the structures of the heterocycles, heterocycle-water interaction energies, and thermodynamic properties of the four pure liquids. The latter quantities were computed from Monte Carlo statistical mechanics simulations; the average errors in computed densities and heats of vaporization are 0.8% and 2.7%. Free energies of hydration were also calculated for pyridine and pyrazine, and provided errors of under 1 kcal mol-1 in comparison to experimental data.

Ab initio calculations are presented which demonstrate that nonionic
polar molecules with sufficiently large dipole moments can form stable
anions by the attachment of electrons in their dipole fields. The
resulting electron affinities are found to be considerably smaller than
for ionic molecules with comparable dipole moments. As a result of our
calculations we conclude that the dipole fields of HF and H
2O are too weak to bind an electron. On the other hand,
calculations on (HF) -2, H 3NO and CH
3CN - have been carried out which suggest that
these species are stable. HCN - appears to be a borderline
case; the present study indicates that it may be stable.

This paper presents the first ab initio attempt to construct the stretching fundamentals νFX and νF...N (X=H, D, Li) in the FX...NH3 complexes taking into account the mechanical anharmonicity. A potential‐energy surface V(rFX,RF...N) grid was generated at the self‐consistent‐field and second‐order Møller–Plesset levels. The coefficients fitting the potential‐energy surface up to the fourth order have been used to compute the νFX and νF...N stretching modes. The vibrational problem is solved by means of a variational treatment which includes the effects of mechanical anharmonicity. The results are compared with the available experimental data.

An improved method for theoretical determination of the charge redistribution, i.e., the intramolecular charge shifts, intermolecular charge transfer, and dipole moment enhancement upon complexation, is proposed. This approach consists of counterpoise correction of the basis set superposition error on calculated electron density redistribution in molecular complexes. Ab initio LCAO MO SCF calculations for HF⋅⋅⋅HNH2, H2O⋅⋅⋅HOH, H2O⋅⋅⋅HF, H3N⋅⋅⋅HF, H3N⋅⋅⋅LiF, CO⋅⋅⋅HCN, H2S⋅⋅⋅OCHNH2, H2NHCO⋅⋅⋅HSH, [Mg⋅⋅⋅OH]+ [HO⋅⋅⋅HOH]−, [Be(OH)(NH3)2]+ molecular complexes in different basis set have been performed leading to more consistent results in contrast to sometimes largely overestimated uncorrected values reported previously. In addition, the charge redistribution upon complex formation has been analyzed for some dimers in terms of atomic multipole expansion of electron density and difference molecular electrostatic potentials.

Intramolecular hydrogen bonding interactions in 1,4-dihydroxy-5,8-naphthoquinone imine (1a), its tautomers (2a–4a), and the rotamers of 1a–4a, and the utility of the SAM1 semiempirical method, in comparison to ab initio Hartree–Fock (HF) (as well as AM1 and PM3) methods, for the study of such interactions were evaluated. The N⋯H–O hydrogen bonds can be characterized as strong and were found to be stronger than, or comparable to, the strength of the O⋯H–O hydrogen bonds. This finding is in contrast with an earlier report based on NMR experimental data on 5-iminodaunomycin (II) in which the O⋯H–O hydrogen bonds were reported to be the strongest (G.L. Tong, D.W. Henry, E.M. Acton, J. Med. Chem., 22 (1979) 36; J.W. Lown, H.H. Chen, J.A. Plambeck, Biochem. Pharma., 28 (1979) 2563). The relative stabilities of the tautomers (all local minima) predicted by SAM1 and HF/6-31G∗∗ were: 4a>2a>1a>3a;4a>1a>2a>3a, respectively). 2a and 3a were not found to be saddle points whereas the structures analogous to 2a and 3a in the naphthazarin system were reported to be so. All the methods used predicted the imidine form (4a) to be the most stable tautomer. The above findings suggest the interpretation of the experimental data on the biochemical reactivity of II should be re-examined. The SAM1 barrier energies for tautomerization by intramolecular proton transfer were substantially lower than, or comparable to the HF/6-31G∗∗ ones. The SAM1 trend in stability of the tautomers and some of the SAM1 barrier energies and other hydrogen-bond parameters are found to be qualitatively more in line with preliminary DFT results, and thus, the SAM1 trend in stability is probably correct. However, the overall accuracy of the HF/6-31G∗∗ and the SAM1 methods for the study of hydrogen bonding in these systems appears to be unsatisfactory.

Intramolecular hydrogen-bonding interactions in 1,4-dihydroxy-5,8-naphthoquinone imine (1a: an oversimplified model system for 5-iminodaunomycin (5IDN)), its tautomers (2a–4a), and the rotamers of 1a–4a have been studied by the Becke–Lee–Yang–Parr (BLYP) and Becke3–Lee–Yang–Parr (B3LYP) nonlocal density functionals. The relative decreasing order of strength of the hydrogen bonds was found to be N⋯H–O (strong)≥O⋯H–O (strong)>O⋯H–N (weak/normal?), in contrast to the experimentally observed one: O⋯H–O (strong)>N⋯H–O (normal)>O⋯H–N (weak), which was based on experimental NMR data on 5IDN (G.L. Tong, D.W. Henry, E.M. Acton, J. Med. Chem. 22 (1979) 36; J.W. Lown, H.H. Chen, J.A. Plambeck, Biochem. Pharmacol. 28 (1979) 2563). The imidine, or the enaminone, form (4a) is predicted to be the most stable tautomer with the relative decreasing order in stability of the tautomers being: 4a>2a>1a>3a. The BLYP/6-31G∗ calculated barrier energies which are less than 5 kcal/mol are in the range predicted for malonaldehyde and at least in one case the proton transfer is almost barrierless suggesting that proton transfer may be feasible even at room temperature in these systems. The B3LYP/6-311G∗∗ calculated geometries and O⋯H and O⋯O hydrogen-bond distances in the enolone fragment of the tautomers are in good agreement with those determined experimentally for malonaldehyde and naphthazarin, while the BLYP/6-31G∗∗ and BLYP/6-31G∗ geometries and hydrogen-bond distances are not as good.

Dimerization energies, ED, internuclear separations, R, and directionality angles have been obtained for the electron donor substituted hydrogen bonds, HOH…OHX, XH, CH3, NH2, OH, and F, from molecular orbital calculations with the 4-31The internuclear separations and directionalities vary little over the range of substituents and ED x R is proportional to the electronegativities of the substituents.The results are explained by a simple physical model originally derived for hydride electron donors. The substituted electron donors are described in terms of an atomic pseudo-potential which replaces the OX bond. The linearity between ED x R and electron donor atom charge known to exi NH3, OH2, and FH can be extended to the substituted monomers and this provides a quantitative link with the OX pseudopotential from whi effective ΔI values are obtained.

Properties of molecular complexes formed between aliphatic amines with HF and F2, respectively, have been evaluated by the CNDO/2 method. In the amineHF hydrogen-bonded complexes the calculated properties (binding energy, amount of charge transferred, enhancement of dipole moment) are fairly constant in the series of systems studied. In the amineF2 charge transfer complexes the binding energy and the amount of charge transferred slightly increases with the lowering of the amine's ionization potential. The results obtained are discussed in relation to the electronic structure of the complexes formed.

We report results of computer simulation studies for fluid ammonia. The intermolecular potential model consists of a central Lennard-Jones part, to which are added point dipoles, quadrupoles and also polarizability. The latter introduces effective many body intermolecular interactions. The model parameters were obtained from dilute gas and crystal lattice properties. Properties calculated include dimer, liquid and solid structure and energy, transport and thermodynamic properties. The simulation results have been compared with experimental data to demonstrate the adequacy of the model for a wide range of properties over a wide range of state conditions.

The geometry of the CC(H)O group, the stretching force constant fCC2, and the coupling constant fCO,CC, calculated using the unscaled 4–31G basis set with full geometry optimization, are reported for various planar mono-substituted carbonyl compounds. The trends in rCC, rCH, ∠CCO and ∠HCO as rCO increases are investigated, and an inverse relationship established between rCO and rCC, i.e. rCO X rCC = 1.782 ± 0.013. Linear relationships are found in the plot of In fCC2 versus In rCC in accord with the general form of Clark's equation, and in the plot of fCO,CC versus the quotient rCOit/rinCC.

The CO bond length and the quadratic, cubic and quartic stretching force constants, calculated ab initio using the unscaled 4–31G basis set with full geometry optimization, are reported for three series of monosubstituted carbonyl compounds in which the atom directly bonded to the carbonyl carbon is another carbon, a nitrogen, or an oxygen atom, respectively. The data are analyzed in terms of the In ƒ versus In re relationship, and also the generalized power functions and exponential functions proposed by Herschbach and Laurie. Not only does the atom directly bonded to the carbonyl carbon affect the magnitude of re and the force constants, but the rest of the substituent group is found to be capable of exerting an even greater influence. Within each series of compounds the overall progression from the shortest to the longest CO bonds is tentatively attributed to a diminishing electron density in the bonding region.

Gas-phase reactions of vinylformate with the hydroxide ion and its hydrate have been studied using both the AM1 and ab initio MO calculations. Four competitive reaction pathways (α-H+ abstraction, vinyl-H+ abstraction, BAC2, and nucleophilic vinyl substitution (NVS)) are found to be possible. In the reactions with OH− the four processes are all exothermic, whereas in the reactions with hydrated OH− only the nucleophilic reactions proceed exothermically. The NVS reaction has the greatest exothermicity in both reactions with bare and hydrated OH−. Solvation of the hydroxide ion leads to decreases in reactivities and reactivity differences between BAC2 and NVS paths due to the smaller energy gaps between intermediates formed along the reaction coordinates, in addition to a mechanistic change in the vinyl-H+ abstraction. The reactivities of nucleophilic reactions are discussed with the HOMO (nucleophile)-LUMO (substrate) interactions in the initial states. The AM1 results tend to describe anions incorrectly and show substantial disagreement in the heats of reaction with those obtained from ab initio calculations.

Electronic population changes and the evolution of the contour maps of the charge density and its Laplacian are used to study the polarizability of hydrogen bonds as a function of the position of the proton within the hydrogen bond. Standard ab initio molecular orbital theory at the 4–31G level (44-31G* for sulphur) has been used to study the complexes between the acceptors, formaldehyde and thioformaldehyde, and the donors, hydrogen fluoride and the ammonium cation. A number of important conclusions for the interpretation of infrared spectra are discussed.

Several (NH4)+(NH3)n(H2O)w mixed clusters have been studied theoretically using the semiempirical AM1 method. In good agreement with the experimental data, our results show that while the primary solvation shell surrounding the NH+4 central core is preferentially formed by ammonia molecules, water molecules are preferred in the next solvation shell. The reasons for this preferential solvation are discussed through an analysis of the hydrogen bonds which are formed. Finally, AM1 is shown to be an adequate method for reproducing the experimental results in systems where the correct evaluation of hydrogen bonds is of major importance.

Ab initio calculations have been carried out for ammonia dimers with 4-31G and 6-31G* basis sets. The results are corrected for basis-set superposition error. For the ammonia dimer, the stability of the linear form is comparable to that of the cyclic form. The localized orbitals for both dimers are discussed.

Two stationary points have been found on the 4—31G energy hypersurface of CO2⋯HF; the linear structure of the complex corresponds to a real minimum, whereas the Cs structure represents a saddle point, Geometric and energy characteristics of the linear complex have been recalculated at the 6—31G* level with inclusion of dispersion energy (polarizability model). Finally, the formation of the complex has been treated thermodynamically.

Using the 4-31G basis set augmented with polarization functions on the S-atom all geometrical parameters have been optimized for the planar chain and ring conformers (related by 180° rotation of the OH, SH or NH group) of acrylic acid, the three monothioperformic acids,α-hydroxyacrolein, formyl and thioformylhydroxylamine, glyoxalmonoimine, formimidol and glyoxalmonooxime. The changes in bond lengths and bond angles which accompany the conversion of the chain into the ring conformer are classified under ten headings for a total of eight four-membered, nine five-membered and three six-membered ring systems. Systematic trends are found, indicative of electron transfer, which support the hypothesis that there is a hydrogen-bonding type of interaction in the four-as well as in the five- and six-membered rings. The change in total molecular energy for the conversion reaction is divided up into distortion and bonding energy components, and each partitioned in terms of the expectation energy differencnes ΔEK, ΔVee, ΔVnn and ΔVen. The nature of the distortion and bonding steps, whether attractive-dominant or repulsive-dominant, is correlated with ring size and the presence (or absence) of heavy atoms external to the ring. The internuclear disease OH, O…O and H … O in the OH … O(=C) hydrogen bridge are compared with experimental values for intermolecular hydrogen bonds, and the present findings are discussed in relation to studies in the literature on electron density distributions and chages in Mulliken gross electronic populations.

Several minimal (7, 3/3) Gaussian basis sets have been used to calculate the energies and some other properties of CH4 and H2O. Improved basis sets developed for these molecules have been extended to NH3 and HF and employed to H2CO and CH3OH. Interaction energies between XHn molecules have been calculated using the old and the new minimal basis sets. The results obtained with the new basis sets are comparable in accuracy to those calculated with significantly more extended basis sets involving polarization functions. Binding energies calculated using the counterpoise method are not much different for the new and the old minimal basis sets, and are likely to be more accurate than the results of much more extended calculations.

Electron photodetachment of solvated anions of the form ROHF- has been undertaken. Observation or non-observation of photodetachment provides information about the location of the bridging proton, i.e. whether the complex has the structure ROH·F- or RO-·HF. Where photodetachment is observed, the experimental detachment threshold energy yields the solvation energy for the corresponding free anion.

A simple model of “superexchange” interactions, taken from phenomena of magnetic ordering in non-conducting 3d-solids, is found to account quantitatively for stereospecificity and binding energy in the dimers (HF)2 and (H2O)2 and the moleculer complex H2O·HF. The exchange unit is formed by two hydrogen atoms on different molecules and the nearest heavier atom. The results are compared with those obtained on the basis of ab initio analyses with the largest available basis sets.

The influence of the β−-decay on the N—HN hydrogen bond in the tritiated-ammonia dimer and in tritiated-methyl-amine-ammonia has been studied by ab initio LCAO SCF MO calculations. The hydrogen bond, except in the ammonia dimer, is predicted to be broken following the β−decay.

The microwave spectrum of the heterodimer HCN…HF is reported, the collinearity of the nuclei established, and r0(N…F)=2.796 Å obtained. From the Stark effect, μ0(HCN…HF)=5.59 ± 0.02 D which indicates an enhancement of 0.78 D over the vector sum of the monomer values.

Ab initio LCAO‐SCF molecular orbital calculations have been carried out with an extensive basis set to determine the stabilization energies of a cyclic trimer, a cyclic tetramer, and various noncyclic oligomers of water. The cyclic trimer is shown to be less stable than the noncyclic one. It is concluded that appreciable nonadditive effects are not present in cyclic polymers of water and that these structures show no special stability (compared to noncyclic ones) other than that to be expected from their extra hydrogen bond. Finally, the results of these and similar calculations are used to calculate the lattice energy of ice I, which agrees well with the experimental value.

The Hartree‐Fock energy of the water dimer has been computed for 216 different nuclear configurations using the basis set given in the first paper of this series. Near the equilibrium configuration, a calculation was carried out using a large Gaussian basis set with optimized orbital exponents for the oxygen 3d ‐ and 4f ‐type and hydrogen 2p ‐ and 3d ‐type functions in order to get results close to the Hartree‐Fock limit. In the vicinity of the equilibrium configuration the mechanism for binding of the dimer is analyzed with the help of the bond energy analysis formalism. The importance of polarization (internal charge transfer), as pointed out previously by a number of authors, is clearly evident. The computed energies have been used to derive a simple analytical expression that reproduces the Hartree‐Fock potential energy surface to a high degree of accuracy. This analytical potential is compared with the empirical effective pair potentials proposed by Rowlinson and Ben‐Naim and Stillinger for the description of water in the condensed phase. We report preliminary results for Monte Carlo simulation of the liquid state of water using the computer program of Barker and Watts and our analytical Hartree‐Fock two‐body interaction potential. At the experimental mass densities we considered 27 water molecules in a cube with periodic boundary conditions at temperatures of 277, 298, and 348°K. The resulting pair correlation functions gO�H(2) (hydrogen bond distribution) and gO�O(2) (oxygen‐oxygen distribution) are in agreement with experimental data. It should be strongly emphasized that the properties are computed without any recourse to semiempirical data.

The theory of molecules in molecules introduced in previous articles is applied to study the hydrogen bonding interaction in the linear configuration of the dimer of FH. The transfer of localized molecular orbitals as well as the majority of the additional approximations introduced to save computational time can be justified and shown to lead to results in good agreement with those of ab initio calculations. An energy analysis of the effect of the hydrogen bond formation on the localized orbitals is given. It is seen that the effect is small, the major contribution to the binding energy is given by a first order perturbation treatment.

An extended basis set of atomic functions expressed as fixed linear combinations of Gaussian functions is presented for hydrogen and the first‐row atoms carbon to fluorine. In this set, described as 4–31 G, each inner shell is represented by a single basis function taken as a sum of four Gaussians and each valence orbital is split into inner and outer parts described by three and one Gaussian function, respectively. The expansion coefficients and Gaussian exponents are determined by minimizing the total calculated energy of the atomic ground state. This basis set is then used in single‐determinant molecular‐orbital studies of a group of small polyatomic molecules. Optimization of valence‐shell scaling factors shows that considerable rescaling of atomic functions occurs in molecules, the largest effects being observed for hydrogen and carbon. However, the range of optimum scale factors for each atom is small enough to allow the selection of a standard molecular set. The use of this standard basis gives theoretical equilibrium geometries in reasonable agreement with experiment.

Accurate SCF calculations have been carried out to investigate the potential of interaction for pairs and triplets of water molecules. The most stable pair configuration involves a linear hydrogen bond of length ROO = 3.00Å and strength 4.72 kcal/mole. Three‐molecule nonadditivities are large in magnitude and vary in sign according to the hydrogen‐bond pattern involved. In both aqueous liquids and solids, the net trimer nonadditivity effect amounts to increased binding energy, decreased neighbor distance, and slightly enhanced tendency toward perfect tetrahedral coordination symmetry. The nonadditivity furthermore is inconsistent with the phenomenology of simple mutual electrostatic polarization between neighboring molecules.

The energy surface of dimeric water is studied for the linear and bifurcated geometries within the SCF MO LCGO framework, using a gaussian basis set to approximate the avefunction. The minimum energy geometry of dimeric water is found to be linear with a hydrogen bond distance of 2.04Å and a binding energy of 4.84 kcal/mole (experimental 5.0 kcal/mole). The dipole moment was computed to be −1.69 au.

The hydrogen bond energy of the isolated linear water dimer has been investigated with the use of the bond orbital approximation. This approach provides a simple description of the relative contributions from electrostatic, overlap repulsion, and charge transfer effects. The basis set used is a minimum basis of Slater atomic orbitals. The hydrogen bond energy obtained is in reasonable agreement with the values obtained from SCF calculations.

Hartree—Fock wavefunctions are presented for the LiH(X1Σ+), BeH(X2Σ+), BH(X1Σ+), CH(X2&Pgr;r), NH(X3Σ−), OH(X2&Pgr;i), and HF(X1Σ+) molecules. These are the analytic self-consistent-field wavefunctions obtained from the solutions of the Hartree—Fock—Roothaan equations. Large sets of Slater-type functions centered on both nuclei were used as the expansion basis, and extensive optimization of the orbital exponents has been carried out. The total energies obtained for Re(exptl) are −7.98731, −15.15312, −25.13137, −38.27935, −54.97806, −75.42083, and −100.07030 hartrees, respectively, for the AH hydrides listed above. The first ionization potentials, which are obtained from the Hartree—Fock energy differences between AH and AH+ systems, are 7.02, 8.14, 8.45, 10.08, 12.82, 11.44, and 14.54 eV, respectively. In addition, potential curves, spectroscopic constants, and certain other energetic quantities are presented. Crude estimates of the correlation energy of the first-row hydrides are made and such quantities are compared within the series and with their respective united and separated atoms. These results suggest that the changes in correlation energy of AH relative to the correct united atom is independent of which hydride is involved and the change is small.

The geometry and vibrational frequencies of the water dimer are calculated using ab initio LCAO-SCF theory. Most of the results were obtained using a split valence basis set. Some calculations with a larger basis set having polarization functions are also reported. The findings are somewhat dependent on the basis set with the split valence basis set giving too strong a hydrogen bond and consequently force constants which are too large. The intramolecular frequency shifts are in reasonable agreement with experimental results. The low frequency intermolecular vibrations are also determined and are qualitatively consistent with the infrared spectrum of liquid water.

A simple level of ab initio molecular orbital theory with a split-valence shell basis with d-type polarization functions (6-31G*) is used to predict equilibrium geometries for the ground and some low-lying excited states of AHn molecules and cations where A is carbon, nitrogen, oxygen or fluorine. The results are shown to be close to the limit for single determinant wave functions in cases where corresponding computations with more extensive bases are available. Comparison with experimental results also shows good agreement although a systematic underestimation of bond lengths up to 3 per cent is evident. For systems where no experimental data are available, the results provide predictions of equilibrium geometry.

Ab initio LCAO&sngbnd;MO&sngbnd;SCF calculation for the dimeric H2O system is carried out with a minimal Slater basis set with exponents optimized for H2O. The most stable linear dimer is found to have an O···H distance of 1.80 Å, with the proton-acceptor molecule perpendicular to the donor molecule and bent by 54° trans with respect to the end OH bond of the donor. The stabilization energy calculated is about 6.55 kcal/mole. The changes in the length and the stretching force constant of the donor O&sngbnd;H bond are also discussed. A population analysis shows that the stabilization at a larger O···H distance (>2.3 Å) is essentially electrostatic, while at a smaller distance, the charge transfer becomes increasingly important.

Ab initio SCF calculations with a minimal STO−3G basis set have been performed to determine the equilibrium structures and energies of dimers having formamide as the proton donor molecule and either water or formaldehyde as proton acceptor molecules. The structures of dimers in which the N−H proton ’’s−trans’’ to the carbonyl group is hydrogen bonded (t dimers) are consistent with structures anticipated from the general hybridization model. In these dimers, there is essentially free rotation of the proton acceptor molecule about the intermolecular line. When hydrogen bond formation involves the ’’s−cis’’ proton (c dimers), a single equilibrium formamide−water and formamide−formadehyde dimer exists, the structure of which is strongly influenced by the secondary factors of dipole alignment and long−range interaction. These factors are also responsible for the increased stability of c dimers relative to t dimers. A set of 1:2 formamide:water trimers has been constructed from the equilibrium formamide−water dimers, in which the formamide molecule forms two hydrogen bonds. Only in three open−chain trimers are the hydrogen bonds stronger than those of the corresponding dimers. CI calculations have also been performed to determine the effect of hydrogen bond formation on n→π∗ transition energies in the dimers and trimers. The n→π∗ transition energy of the proton acceptor formaldehyde molecule increases in the formamide−formaldehyde dimers. In these dimers, the magnitude of the blue shift is determined by the dimer hydrogen bond energy. The formamide n→π∗ band is also blue shifted to some extent in the formamide−water and formamide−formaldehyde dimers, even though the n→π∗ transition originates in the proton donor molecule. The blue shift of the formamide n→π∗ band in the open−chain trimers having formamide as the central molecule is equal to the sum of the blue shifts in the corresponding dimers.

Ab initio minimal basis LCAOSCF molecular orbital calculations have been performed on (H2O2)2 and on the mixed H2O�H2O2 dimers. The equilibrium structures and energies of these dimers are presented and analyzed. A cyclic structure is predicted to be most stable for (H2O2)2, while open chain structures are predicted for the mixed dimers. The effect of dimerization on the trans barrier to internal rotation in H2O2 is also discussed.

The ground state binding energy (BE), rotational and vibrational energy levels, and line strengths for radiative transitions between some of these energy levels are calculated for the [H2O]2 molecule. These quantities are computed for three intermolecular potentials published for the [H2O]2 molecule, two of which were calculated by MO SCF techniques and one which was derived from an empirical point charge model for the water molecule. The value of BE calculated for two of the potentials is approximately 6 kcal∕mole, and for the third is approximately 3 kcal∕mole. The total concentration of [H2O]2 in equilibrium with H2O vapor is calculated for the sets of energy levels determined for these potentials. Results are compared with available experimental data. Using calculated values of line strengths and experimental data for the integrated absorptions for two rotational transitions, an independent value is deduced for the equilibrium concentration of [H2O]2 in fair agreement with values calculated from two of the intermolecular potentials. The theoretical calculations for BE and the equilibrium concentration of [H2O]2 are also compared with those obtained from analysis of experimental data for the second virial coefficient B2(T) of H2O vapor. The theoretical values for BE bracket the value of 3–4 kcal∕mole obtained from analysis of the data for B2(T). A discussion of our results is presented, along with suggestions for experimental work to search for the spectrum of [H2O]2 in the absorption spectrum of H2O vapor.

Quantitative gas-phase infrared intensity measurements for NH3 in the 3 μ region have been used to determine the following quantities: the energy of hydrogen bonding (4.5 ± 0.4 kcal/mole), the factor by which the integrated region intensity was increased because of hydrogen bonding (∼25), and the ratio (7.0±0.5) of the sum of the squares of the vibrational matrix elements for the transitions (010000 → 110000) and (010000 → 011100) to the sum of the squares for the transitions (000000 → 100000) and (000000 → 001100).

The hydrogen bond N·HO between the water and ammonia molecules has been investigated ab initio using the SCF LCAO MO method. The minimal and extended basis sets of Slater type orbitals were used. It was found that the energy of the hydrogen bond is equal to 6.44 kcal/mole and the equilibrium separation of the oxygen and nitrogen atoms in the dimer is 5.72 au. At this intermolecular distance there is only one minimum in the potential energy curve for the motion of proton.

The theory of molecules in molecules introduced in previous articles is applied to study the hydrogen bonding interaction between an ammonia molecule as proton acceptor and a water molecule as proton donor. The localized orbitals which are assumed to be least affected by the formation of the hydrogen bond are transferred unaltered from calculations on the fragments NH3 and H2O, the remaining orbitals are recalculated. A projection operator is used to obtain orthogonality to the transferred orbitals. Additional approximations have been introduced in order to be able to save computational time. These approximations can be justified and are seen to lead to binding energies and bond lengths which are in satisfactory agreement with the SCF values. The point charge approximation for the calculation of the interaction energy between the two sets of transferred localized orbitals is, however, not applicable in this case. An energy analysis of the effect of the hydrogen bond on the localized orbitals of the two fragments is given.

Least‐squares representations of Slater‐type atomic orbitals as a sum of Gaussian‐type orbitals are presented. These have the special feature that common Gaussian exponents are shared between Slater‐type 2s and 2p functions. Use of these atomic orbitals in self‐consistent molecular‐orbital calculations is shown to lead to values of atomization energies, atomic populations, and electric dipole moments which converge rapidly (with increasing size of Gaussian expansion) to the values appropriate for pure Slater‐type orbitals. The ζ exponents (or scale factors) for the atomic orbitals which are optimized for a number of molecules are also shown to be nearly independent of the number of Gaussian functions. A standard set of ζ values for use in molecular calculations is suggested on the basis of this study and is shown to be adequate for the calculation of total and atomization energies, but less appropriate for studies of charge distribution.

It is shown that the data of Kuipers on the absorption of the HF monomer lines can be interpreted if the line contour is expressed in a modified Lorentz form alpha=alpha0a2p2[(v-v0)1.8+a2p2]-1 The apparent absorbance at the line centers has been measured under such circumstances that the temperature variation of alpha0a2 can be obtained. It is shown that the line breadth parameter, a, has the temperature variation expected if resonant dipole intermolecular forces are for the most part responsible for the line breadth. The absorption computed from the modified Lorentz expression between the monomer lines is sometimes considerably less than the absorption that is measured. The difference is attributed to HF dimer. There result three dimer bands. The strongest is broad and featureless with a peak near 3857 cm-1, the two other bands have PQR structures with Q branch peaks at 3895 and 3965 cm-1. The temperature dependence of the dimer absorption indicates a heat of decomposition of about 6 kcal/mole.

Ab initio minimal basis LCAO SCF molecular orbital calculations have been performed to determine the energies and configurations of small groups of water molecules. It is found that polymers having OH3. OH3. OH3. chains are preferred, and that hydrogen bond energies deviate considerably from additivity. Cyclic structures are predicted to be most stable for the trimer and higher polymers.

The proton donor ability of HCl and HF are compared by carrying out ab initio molecular orbital studies on complexes of these proton donors with a number of proton acceptors. In addition, the structure and H-bond energy of the HCl dimers and HCl-HF complexes are predicted.

The energy hypersurface of the system NH3 · H2O is investigated for a number of different internuclear geometries. In the minimum energy structure involving a linear hydrogen bond, NH3 acts as proton acceptor. The binding energy of the system is calculated to be 6.28 kcal/mole and the bond distance d(NO) to be 3.07 Å. The potential energy curve of the inversion of the hydrogenbonded NH3 is computed and discussed.

The hydrogen‐bond energy and the most stable structure of the dimeric H2O system are calculated by the LCAO MO SCF method using a medium‐sized Gaussian orbital basis set. The most stable structure, found by a limited variation of the interatomic coordinates, is a linear hydrogen bond (stabilization energy 12.6 kcal mole−1) with an H⋅⋅⋅O distance of 1.72 Å, and with the hydrogen‐acceptor molecule almost freely rotating around its molecular axis. The stretching of the proton donor O–H bond is calculated to be 0.12 Å. A population anaysis near the energy minimum shows that the change in the population is distributed not only in the O⋅⋅⋅H–O fragment, but also delocalized into the neighboring O–H bonds. Hydrogen bonds of dimeric H2O other than the linear structure (cyclic and bifurcated) are also examined.

Ab initio minimal basis LCAOSCF molecular orbital calculations have been performed to determine the energies and configurations of small groups of water molecules, with particular emphasis on those aspects which are relevant to the structure of liquid water. An intermolecular potential which spans the complete range of possible relative orientations for the dimer is presented. The predicted equilibrium form of the dimer is found, together with estimates of some of the intermolecular force constants. Results of calculations on both open and cyclic polymeric water structures containing up to six molecules are included. It is found that polymers having OH⋅⋅⋅OH⋅⋅⋅OH⋅⋅⋅ chains are preferred, and that hydrogen‐bond energies deviate considerably from additivity. Cyclic structures are predicted to be most stable for the trimer and higher polymers.

High‐accuracy molecular orbital calculations have been carried out on different geometries of the hydrogen fluoride dimer and the mixed water–hydrogen fluoride dimer. A zigzag (near linear) structure is predicted for the hydrogen fluoride dimer with a dimerization energy in reasonable agreement with experiment. One geometry of the mixed water–hydrogen fluoride dimer has a very large stabilization energy (10 kcal/mole), and a microwave experiment is proposed to determine its exact structure. Changes in molecular properties and charge distribution upon dimer formation are calculated and a dimer rotational barrier determined.

The radiofrequency and microwave spectra of the K=0 states of (HF)2, (DF)2, and HFDF have been studied by the molecular beam electric resonance method. A unique hydrogen tunnelling motion involving the breaking and reforming of the hydrogen bond causes a splitting of rotational energy levels for (HF)2 and (DF)2, but not for HFDF. The electric dipole selection rules and nuclear spin statistics for the tunnelling molecules have been derived from a consideration of an extended permutation‐inversion group. Rotational constants, tunneling doublings, electric dipole moments, and deuterium quadrupole coupling constants have been determined from the observed spectra of the K=0 states.

This paper presents a survey of recent ab initio SCF calculations on water polymers and examines the relationship between calculated results and the choice of basis set used in the LCAO expansion. It is found that current studies give similar descriptions of the general features of the hydrogen bond, although variations do occur due to the nature and size of the basis set used. In many cases, differences in intermolecular properties can be correlated with differences in the calculated value of the monomer dipole moment.

High‐accuracy molecular‐orbital calculations using essentially Hartree–Fock quality atomic orbitals as a basis have been carried out on different geometries of the water dimer. Different basis sets have been considered. The molecular‐orbital approach is shown to well represent the geometry and heat of formation (− 5.3 kcal/mole) of the water dimer as well as general infrared spectral properties of the hydrogen bond. The individual molecular‐orbital energies are shown to increase for the electron acceptor and to decrease for the electron donor. This trend in energies is proposed as a quantitative organizing principle for not only H‐bond formation but all donor–acceptor interactions.

As a prelude to the study of energy transfer in the HF☒HF system, the potential energy surface for the interaction of two rigid HF molecules has been calculated within the ab initio self‐consistent‐field framework. An H(4s 1p∕2s 1p), F(9s 5p 1d∕4s 2p 1d) basis set of contracted Gaussian function was employed. The number of unique points on the surface is greatly reduced by symmetry, and only 294 points were required to give a fairly complete description of the four‐dimensional surface. Parts of the surface are illustrated by a series of contour maps. Some preliminary attempts to fit the surface to an analytic form are described. The equilibrium geometry of (HF)2 is predicted.

The analytical fit to a large number of Hartree‐Fock computations for the water‐water interaction has been reanalyzed and used to study small clusters of water molecules. With the analytically fitted Hartree‐Fock potential, thousands of possible configurations for the dimers, trimers, tetramers, pentamers, hexamers, heptamers, and octamers of water have been compared in order to determine the configuration of lowest energy (maximal stabilization energy). For the dimer two possible stable configurations are found, corresponding to an open form and a cyclic form, with the open form being more stable. For the trimers and tetramers the cyclic forms are somewhat more stable than the open structures. For the larger clusters it is concluded that it is rather meaningless to consider a single structure, but what is physically relevant is the statistical distribution of different configurations, since many configurations with significantly different geometry have nearly the same energy. The comparison of the stabilization energy per molecule of the different clusters with the corresponding value for liquid water does not support the mixture‐model theories of the structure of liquid water.

Spectra of heavy water have been obtained under high resolution between 1.25—4.1μ (2400—8000 cm—1). Approximately 4500 lines have been measured, and the majority of them analyzed into ten bands of D2O and nine bands of HDO. The analysis is described in some detail, spectra of all bands are shown and a partial table of lines and a complete table of energy levels are presented. The vibration‐rotation constants are derived and compared with those of H2O.

A simple semiempirical method is given for determining the hydrogen bond energy for water clusters in the vapor phase. This method is based on a general statistical‐mechanical theory of clustering. The partition function for a system of clusters is used to determine the equilibrium distribution of clusters. In conjunction with available thermodynamic and spectroscopic data, the cluster equilibrium constants can be used to calculate the cluster potential energy and the hydrogen bond energy. Results for the water dimer agree quite well with other reported values obtained either by quantum‐mechanical calculations or approximate thermodynamic estimates. A correct temperature dependence of the bond energies is found.

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Two experiments using an interferometer and a maser have indicated the
occurrence of submillimetre wave absorption by dimeric water. This form
of water seems to exist in the Earth's atmosphere.

A physical model of the hydrogen bond, A-H⋯B, has been deduced from ab initio molecular orbital wave functions of 36 dimers made from the monomers, NH3, OH2, FH, PH3, SH2, and ClH. Three monomer quantities are defined which characterize the model: μA-H, the A-H bond dipole; ΔI, the difference between first ionization potentials of the electron donor and the noble gas atom in its row; and l, the length of the hydrogen bonding lone pair. Dimerization energy, charge transfer, internuclear separation, directionality, stretching force constants (KAB and KAH), the dimer dipole moment, and ir intensity enhancement can be understood in terms of these quantities. The dimerization energy formula, ED = KμA-HΔI/R, where K is an energy scale factor and R, the internuclear separation between A and B, systematizes existing experimental and computational data. The tendency for strong bonding electron donors to be weak bonding proton donors and vice versa is the result of an intrinsic reciprocal relationship between μA-H and ΔI. Charge transfer is proportional to μA-H for specified B, and is ordered according to l for a given A. Internuclear separation is inversely proportional to μA-H for specified B, and has close to the same dependence on A-H for second- and third-row electron donors. The almost constant separation difference of 0.8 Å between second- and third-row electron donors results from the difference in average l between the rows. The rule of constant R for all B in a row (with given A) is found to arise from the constancy of l times I. Stretching force constants for the heavy atoms follow Badger's rule, KAB(R - dAB)3 = 1.86, with dAB dependent only on the column of the periodic table. dAB is 1.00, 0.80, and 0.55 Å for groups 5, 6, and 7, respectively. Lowering of the A-H stretching force constant, KAH, relative to the monomer, is proportional to μA-H for fixed B, variable A, and proportional to ΔI (or l) for fixed A, variable B. The model also provides qualitative explanations and some quantitative results for the properties of other hydrogen bonds: the strong hydrogen bonds found in crystal ions, the weak hydrogen bonds to π electrons in organic molecules, the multiply bonded electron donors of proteins, a variety of substituents at A and B, and the cooperativity found in trimers and higher polymers. Quantitative predictions of ED and R can be made for dimers formed with fourth-row hydride monomers.

Hydrogen-bonded dimers involving first- and second-row hydrides have been studied theoretically with ab initio molecular orbital methods, using a 431G basis set. Certain generalizations about H-bonded dimers found in a previous study2a of first-row dimers (those involving NH3, H2O, and HF) are supported by this study; others require modification. In addition to studying the dependence of H-bond energy and properties on the row of the periodic table, we examine the dependence of H-bond energies on the "hybridization" of the electron donor, including HCN, H2CO, H2CS, HNC, and HCP as electron donors. We have also studied ionic H bonds, "π" H bonds, and H-bonded trimers in an attempt to relate their properties to those of the more conventional H-bonded dimers. Can a C-H bond be an effective H-bond proton donor? We attempt to answer this question by examining the proton donor ability of CH4 and CHF3. Electrostatic potentials turn out to facilitate our understanding of H-bond energies and structures, being more useful than Mulliken populations in rationalizing H-bond energies. Finally we address ourselves to the question of predicting dimer H-bond energies from the monomers involved. Using a very simple algebraic model, we are able to predict the H-bond energy of a total 144 H-bonded complexes, using as a basis our theoretical calculations on 25 complexes.

Eine systematische: Untersuchung der Elektronenstruktur der Wasserstoffbrückenbindung in Hydriddimeren der Atome N, O, F, P, Sund Cl wird durchgeführt.

The influence of electron correlation on the linear hydrogen bond in F-H⋯F-H is studied in the frameworks of the IEPA-, CEPA-, and PNO-CI approaches. Due to compensating effects of the intrasystem and the intersystem correlation energy we find a relatively small net effect of the total correlation energy on the interaction energy, the F-F distance, and the intermolecular vibration. The following RFF and ΔE values (in au) result in the SCF-, IEPA-, CEPA-, and PNO-CI approach respectively: 5.48, -0.0055; 5.47, -0.0054; 5.46, -0.0056; 5.43, -0.0056. The intermolecular stretching vibration of the H-F bond involved in the hydrogen bond is hardly affected in comparison with the isolated HF molecule. The experimentally observed shifts of 10-15% in the absorption frequencies of infrared spectra are thus attributed to higher polymers, in agreement with previous theoretical and experimental works.

Ab initio SCF calculations with a minimal STO-3G basis set have been performed on the series of dimers ROH⋯NH3, where R may be H, or one of the isoelectronic substituents CH3, NH2, OH, or F. The equilibrium structures and energies of these dimers are presented and analyzed. The dimer structures are well described in terms of the general hybridization model for the hydrogen bond. While the electrostatic interaction is of primary importance in stabilizing hydrogen bonded dimers, dipole-dipole and long-range interactions are also shown to be important in determining hydrogen bond strengths. Comparisons are made between corresponding dimers in the two series ROH⋯OH2 and ROH⋯NH3.

An ab initio molecular orbital study of the geometries and energies of neutral systems AHn and their singly charged cations AHn+ (A = C, N, O, or F) is presented. Two previously reported basis sets are employed: the minimal, STO-3G, basis and the extended, 4-31G, basis in which valence shells are split into inner and outer parts. Comparisons are made between experimental and theoretically predicted properties.

The Shipman-Scheraga empirical intermolecular potential energy function for water (SS potential) has been applied to a study of the structure, energetics, and dynamics of the water dimer. The entire six-dimensional potential energy surface of the water dimer has been searched for minima and saddle points. Only one minimum-energy structure, the trans near-linear dimer (TNLD), has been found, while several important saddle points that serve as transition states for interconversions between TNLD configurations have been found. The TNLD's of (H2O)2 and (D2O)2 have been characterized by calculating the following properties: potential energy and O ⋯ O distance at the minimum, internal energy at absolute zero temperature, frequencies and dipole moment derivatives of the intermolecular normal vibrational modes, zero-point energy for intermolecular vibrations, principal moments of inertia, location of the center of mass, directions of the principal axes for the moments of inertia, and components of the total dipole moment along the principal axes. The important transition states for intereonversion between the TNLD's have been characterized by calculating the following properties: potential energy, intermolecular vibrational frequencies, intermolecular vibrational zero-point energy, internal energy at absolute zero temperature, and internal energy relative to the TNLD at absolute zero temperature. The familiar cyclic and bifurcated dimers have been found to be saddle points, not minima, on the six-dimensional potential energy surface. The importance of motion through the various transition states for interconversion between the TNLD's has been considered. Implications of the computed results for future spectroscopic studies are discussed.

A model to facilitate the computation of the most stable conformer of associated M H2O (M being a polar molecule) which depends upon the electrostatic interaction energy between the two associated molecules is proposed and tested. SCF electrostatic potentials for the M molecule and a suitable point charge distribution for H2O were employed in the model computations. Energies predicted by the model are found to be in good agreement with those resulting from an ab initio minimal STO basis SCF treatment of some conformations of the H2O dimer.Ein Modell zur Durchfhrung der Berechnung des stabilsten Konformeren eines Assoziationskomplexes M H2O, wobei M ein polares Molekl ist, wird vorgeschlagen und untersucht. Es basiert auf der elektrostatischen Wechselwirkung zwischen beiden Partnern, und zwar wird fr das Molekl M der elektrostatische Anteil seines SCF-Potentials und fr H2O eine angemessene Punktladungsverteilung zugrunde gelegt. Die resultierenden Energien sind in guter bereinstimmung mit denen, die sich bei einer ab initio Rechnung mit minimaler STO Basis ergeben.

The energy of the hydrogen bond N...H-O in the dimer (NH3, H2O) has been computed by the LCAO-MO method, using a minimal set of Slater-type orbitals optimized for the isolated monomers. The doubly occupied and virtual orbitals have been determined by the standard SCF technique, and electron correlation has been introduced by a complete second-order perturbation calculation, using different sets of equivalent MO's. The bonding energy is found to be equal to 7.66 kcal/mol at the SCF step and to 9.65 kcal/mol after second order corrections. The latter value is given by a set of equivalent MO's obtained by projecting the canonical MO's of the monomers into the space of the dimer MO's. The preceding values are reduced to 3.96 kcal/mol at the SCF step and to 4.63 kcal/mol at the second-order, if the basis extension arising from the vicinity of the two monomers inside the dimer is taken into account.