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Since 1-chloromethyl-1-fluorosilacyclohexane is a newly synthesized molecular compound its structural parameters and conformational stability is unknown. Raman and infrared vibrational spectroscopy methods were employed for analysis of this molecule. IR spectra were recorded for both gas phase and liquid sample, whereas the Raman experiments were performed in the liquid state. Additionally, low temperature matrix isolation infrared spectra were recorded after isolating the molecule in argon and nitrogen matrices. For the assignment of the experimental spectral bands, theoretical DFT/B3LYP/aug-cc-pVDZ and MP2/aug-cc-pVDZ calculations were performed. From the calculations it was found that 1-chloromethyl-1-fluorosilacyclohexane may exist in twelve different conformational forms out of which the chair axial trans conformer is the most stable form. However, there are three more chair type conformers – equatorial trans, equatorial gauche and axial gauche that are stable enough to be observed in the experimental spectra.

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... A comparison of the experimental IR and Raman results with the ones obtained from the calculations was also included in the study. Additionally, it was possible to study the influence of the size and electronegativity of the substituent atoms while com-paring current results with our previous studies, where an F atom and CH 2 Cl group are attached to the Si atom [52] . ...

... It is interesting to compare the results of this work with our recent studies of the 1-chloromethyl-1-fluorosilacyclohexane (1-ClM-1-FSiCH) molecule [52] , where the F (Fluorine) atom is bound to the Si contrary to the Cl (Chlorine) atom as it is in this work. F and Cl atoms differ in van der Waals radius (Cl-1.95 ...

1-chloro-1-chloromethylsilacyclohexane (1-Cl-1-ClMSiCH) is a newly synthesized molecular compound whose conformational analysis was performed by means of vibrational spectroscopy and theoretical calculations. Conventional ATR-FTIR and Raman spectroscopic methods were used to obtain vibrational spectra of the liquid sample. Additionally, FTIR spectra of the compound isolated in low-temperature neon and nitrogen matrices were registered to make a complete assignment of the experimental vibrational spectral bands. All theoretically possible 38 canonical ring conformations considering axial/equatorial and cis/trans/gauche-/gauche+ positions of the Cl and CH2Cl group were analyzed by utilizing MP2 and B3LYP at aug-cc-pVTZ theory level. The most stable local energy minima were investigated in detail, with the global energy minimum structure being in the chair axial trans conformation. Detailed analysis of the potential energy surface revealed the transition states (TS) and the energy barriers. The conformational path was found to be the chair→ envelope/half-chair (TS)→ skew-boat C1→ boat (TS)→ skew-boat C2. The energy barrier for ¹C4 to ¹S3 conversion was found to be 4.94 kcal/mol while for the reverse process – it was calculated to be 0.26 kcal/mol. The vibrational analysis and the experimental spectra suggest that the four lowest energy (chair ring) conformers coexist at room temperature. The energy barrier for gauche to trans conversion is 1.15 kcal/mol for axial and 1.04 kcal/mol for equatorial conformers, and it is possible to observe these processes in low-temperature matrices.

... It was observed that 1-chloromethyl-1fluorosilacyclohexane may exist in 12 different conformations, of which the chair axial trans conformer was found to be the most stable conformer. 10 It is worth noting that the aim of this study is not to investigate the conformational behavior of the chosen systems. However, the above citations have guided the understanding of the conformational behavior of the chosen systems and facilitated their geometry optimization to obtain the molecules to the correct energetic minima and conformation. ...

The application of plain cycloalkanes and heterocyclic derivatives in the synthesis of valuable natural products and pharmacologically active intermediates has increased tremendously in recent times with much attention being paid to the lower cycloalkane members. The structural and molecular properties of higher seven-membered and nonaromatic heterocyclic derivatives are less known despite their stable nature and vast application; thus, an insight into their structural and electronic properties is still needed. Appropriate quantum chemical calculations utilizing the ab initio (MP2) method, meta-hybrid (M06-2X) functional, and long-range-separated functionals (ωB97XD) have been utilized in this work to investigate the structural reactivity, stability, and behavior of substituents on cycloheptane (CHP) and its derivatives: azepane, oxepane, thiepane, fluorocycloheptane (FCHP), bromocycloheptane (BrCHP), and chlorocycloheptane (ClCHP). Molecular global reactivity descriptors such as Fukui function, frontier molecular orbitals (FMOs), and molecular electrostatic potential were computed and compared with lower members. The results of two population methods CHELPG and Atomic Dipole Corrected Hirshfeld Charges (ADCH) were equally compared to scrutinize the charge distribution in the molecules. The susceptibility of intramolecular interactions between the substituents and cycloalkane ring is revealed by natural bond orbital analysis and intramolecular weak interactions by the independent gradient model (IGM). Other properties such as atomic density of states, intrinsic bond strength index (IBSI), and dipole moments are considered. It is acclaimed that the strain effect is a major determinant effect in the energy balance of cyclic molecules; thus, the ring strain energies and validation of spectroscopic specificities with reference to the X-ray crystallographic data are also considered.

“Open Readings 2023” – is an international academic platform for students and young researchers from all over the world to present their ongoing research results and practices in the fields of physics and natural sciences. The 66th International Scientific Conference for Students of Physics and Natural Sciences gathers people to discuss, learn, exchange and share their scientific experience, meet key experts and enjoy an exciting programme. The conference includes lectures of well-known, leading and accomplished scientists and presentations of students’ research. “Open Readings 2023” gives an opportunity for BSc, MSc and PhD students, who are doing their scientific research, to present it to a larger audience, and get constructive criticism and helpful advice. This book gives an overview of all Open Readings 2023 conference presentations.

1-chloromethyl-1-fluorosilacyclopentane (1-ClM-1-FSiCP) was synthesized for the first time and investigated by means of vibrational spectroscopy and theoretical calculations. FT-IR and Raman spectroscopic methods were implemented to collect vibrational spectra of 1-ClM-1-FSiCP. The conformational analysis was performed utilizing FT-IR matrix isolation technique and theoretical methods such as density functional theory and ab initio calculations. The spectra of the 1-ClM-1-FSiCP isolated in the argon and nitrogen matrices, were collected before and after the annealing process. During the conformational analysis the envelope (E) and twisted (T) ring shapes with the position of the fluorine atom and chloromethyl group in terms of axial/equatorial and cis/trans/gauche-/gauche+ positions were investigated utilizing MP2/aug-cc-pVTZ and DFT/B3LYP/aug-cc-pVTZ level of theory. Results indicate three stable conformers: twisted trans (global energy minimum), twisted gauche– and twisted gauche+. The potential energy surface scans were performed to trace the energy changes and the presence of transition state structures during ring conversion and rotation of the CH2Cl group.

This review summarizes methods for the conformational control of cyclohexane derivatives and for the control of conformation-dependent
molecular properties by external stimuli affecting the intra- and intermolecular interactions of substituents. These stimuli
include complexation, change of medium acidity, change of the substituents’ size and structure, as well as formation and breaking
of labile bonds. Successful application of the compounds equipped with such conformational switches as the acid-sensitive
components of liposomes for targeted drug delivery, selective ion carriers in membrane transport, and fluorescent sensors
for cations and acids opens a way for broad development in this area toward controllable compounds and intelligent materials.
Keywordsmolecular switches–conformational triggers–conformational transmitters–allosteric systems–intelligent materials–fluorescent sensors–cyclohexano crown ethers

In the past, basis sets for use in correlated molecular calculations have largely been taken from single configuration calculations. Recently, Almlöf, Taylor, and co‐workers have found that basis sets of natural orbitals derived from correlated atomic calculations (ANOs) provide an excellent description of molecular correlation effects. We report here a careful study of correlation effects in the oxygen atom, establishing that compact sets of primitive Gaussian functions effectively and efficiently describe correlation effects if the exponents of the functions are optimized in atomic correlated calculations, although the primitive (sp) functions for describing correlation effects can be taken from atomic Hartree–Fock calculations if the appropriate primitive set is used. Test calculations on oxygen‐containing molecules indicate that these primitive basis sets describe molecular correlation effects as well as the ANO sets of Almlöf and Taylor. Guided by the calculations on oxygen, basis sets for use in correlated atomic and molecular calculations were developed for all of the first row atoms from boron through neon and for hydrogen. As in the oxygen atom calculations, it was found that the incremental energy lowerings due to the addition of correlating functions fall into distinct groups. This leads to the concept of correlation consistent basis sets, i.e., sets which include all functions in a given group as well as all functions in any higher groups. Correlation consistent sets are given for all of the atoms considered. The most accurate sets determined in this way, [5s4p3d2f1g], consistently yield 99% of the correlation energy obtained with the corresponding ANO sets, even though the latter contains 50% more primitive functions and twice as many primitive polarization functions. It is estimated that this set yields 94%–97% of the total (HF+1+2) correlation energy for the atoms neon through boron.

The method of dispersion correction as an add-on to standard Kohn-Sham density functional theory (DFT-D) has been refined regarding higher accuracy, broader range of applicability, and less empiricism. The main new ingredients are atom-pairwise specific dispersion coefficients and cutoff radii that are both computed from first principles. The coefficients for new eighth-order dispersion terms are computed using established recursion relations. System (geometry) dependent information is used for the first time in a DFT-D type approach by employing the new concept of fractional coordination numbers (CN). They are used to interpolate between dispersion coefficients of atoms in different chemical environments. The method only requires adjustment of two global parameters for each density functional, is asymptotically exact for a gas of weakly interacting neutral atoms, and easily allows the computation of atomic forces. Three-body nonadditivity terms are considered. The method has been assessed on standard benchmark sets for inter- and intramolecular noncovalent interactions with a particular emphasis on a consistent description of light and heavy element systems. The mean absolute deviations for the S22 benchmark set of noncovalent interactions for 11 standard density functionals decrease by 15%-40% compared to the previous (already accurate) DFT-D version. Spectacular improvements are found for a tripeptide-folding model and all tested metallic systems. The rectification of the long-range behavior and the use of more accurate C(6) coefficients also lead to a much better description of large (infinite) systems as shown for graphene sheets and the adsorption of benzene on an Ag(111) surface. For graphene it is found that the inclusion of three-body terms substantially (by about 10%) weakens the interlayer binding. We propose the revised DFT-D method as a general tool for the computation of the dispersion energy in molecules and solids of any kind with DFT and related (low-cost) electronic structure methods for large systems.

The interconversion pathways along the puckering transitions in cyclohexane were explored on the two-dimensional projection of the Cremer-Pople sphere using DFT methods and the CCSD(T), MP2, and dispersion-corrected DFT methods with various basis sets were assessed for the relative energies of local minima and transition states for the representative puckering transition pathways. The ωB97X-D/cc-pVTZ and ωB97X-D/def2-QZVP levels of theory well reproduced the relative energies with RMSD = 0.13 kcal/mol against the CCSD(T)/CBS-limit energies. The calculated activation parameters for chair to twist-boat interconversion of cyclohexane at the ωB97X-D/cc-pVTZ//(PCM) M06-2X/6-31+G(d) level of theory were consistent with the observed values.

Raman spectra of 1,1-difluoro-1-silacyclohexane as a liquid, and as a solid at 78 K were recorded and depolarization data obtained. The infrared spectra of the vapour, liquid and amorphous and crystalline solids have been studied. In the low temperature IR and Raman spectra eight and three bands, respectively, were shifted a few cm⁻¹ when the sample crystallized. No bands vanished after crystallization in agreement with the assumption that only one conformer (chair) was present in all the states of aggregation.
The compound exists in the stable chair conformation, whereas in the parent silacyclohexane a possible twist form should have more than 15 kJ mol⁻¹ higher energies than the chair, as derived from various calculations. The wavenumbers of the vibrational modes were calculated in the harmonic and anharmonic approximation employing B3LYP/cc-pVTZ calculations. The 27 A′ and 21 A″ fundamentals were assigned on the basis of the calculations, infrared vapour contours, Raman depolarization measurements and infrared and Raman band intensities. An average, relative deviation of 1.5% was found between the observed and the anharmonic wavenumbers for the 48 modes.

The molecular structures of axial and equatorial conformers of cyclo-C5H10SiHX (X = Cl, Br, I) as well as the thermodynamic equilibrium between these species was investigated by means of gas electron diffraction, dynamic nuclear magnetic resonance, temperature-dependent Raman spectroscopy, and quantum-chemical calculations applying CCSD(T), MP2, and DFT methods. According to the experimental and calculated results, all three compounds exist as a mixture of two chair conformers of the six-membered ring. The two chair forms of Cs symmetry differ in the axial or equatorial position of the X atom. In all cases, the axial conformer is preferred over the equatorial one. When the experimental uncertainties are taken into account, all of the experimental and theoretical results for the conformational energy (Eaxial – Eequatorial) fit into a remarkably narrow range of −0.50 ± 0.15 kcal mol–1. It was found by NBO analysis that the axial conformers are unfavorable in terms of steric energy and conjugation effects and that they are stabilized mainly by electrostatic interactions. The conformational energies for C6H11X and cyclo-C5H10SiHX (X = F, Cl, Br, I, At) were compared using CCSD(T) calculations. In both series, fluorine is predicted to have a lower conformational preference (cyclohexane equatorial, silacyclohexane axial) than Cl, Br, and I. It is predicted that astatine would behave very similarly to Cl, Br, and I within each series.

The molecular structure of axial and equatorial conformer of the 1-chloro-1-silacyclohexane molecule, CH2(CH2CH2)2SiH–Cl, as well as thermodynamic equilibrium between these species were investigated by means of gas-phase electron diffraction and quantum chemistry on the MP2(full)/AUG-cc-PVTZ level of theory. According to electron diffraction data, the compound exists in the gas-phase as a mixture of conformers possessing the chair conformation of the six-membered ring and Cs symmetry and differing in the axial and equatorial position of the Si-Cl bond at 352 K. NBO analysis revealed that axial conformer of 1-chloro-1-silacyclohexane molecule is an example of the stabilization of the form that is unfavorable from the point of view steric effects and effects of conjugations and that stabilization is achieved due to electrostatic interactions.

The structure and relative energies for the basic conformations of 1,3,5-trisilacyclohexane (1) have been calculated by several methods and their performance compared. It is found that HF ab initio calculations using the basis set 6-31G*, the SV(P) basis set used by TURBOMOLE and the MM3 force field produce mutually fairly consistent results. MM2 performs not as well as MM3, but in many cases MM2 performs better than 3-21G. Three semiempirical methods (AM1, MNDO, and PM3) were tested. None of them was found to produce reliable results.It is found by ab initio (6-31G* and SV(P)) and MM3 calculations that the dihedral angles for the chair conformation are 52.7–53.1°, which makes (1) more flattened than cyclohexane, and thus (1) does not exhibit behaviour similar to cyclohexasilane, which is less flattened than cyclohexane. The ring flattening of (1) is mainly caused by the intrinsically large SiCSi bond angle (114.1–114.8°). The twist conformation of (1) is found by the same calculations to be 2.1–3.1 kcal mol−1 higher in energy than the chair conformation, and the boat form is found to be 0.3–0.4 kcal mol−1 higher than the twist form. These values are much closer to the values for cyclohexasilane than to those for cyclohexane. The conformational energy surface of (1) has been calculated by using MM3. The energy barrier from the chair to the twist conformation of (1) is found to be 5.5 kcal mol−1.

The molecular structure of the axial and equatorial conformers of 1-iodo-1-silacyclohexane, CH2(CH2CH2)2SiH–I, as well as thermodynamic equilibrium between these species were investigated by means of gas-phase electron diffraction (GED) and quantum chemical calculations up to MP2(full)/SDB-AUG-CC-pVTZ level of theory (MP2). According to electron diffraction data, the vapor of this compound comprises a mixture of conformers with chair conformation and Cs symmetry differing in the axial and equatorial position of the Si–I bond (axial = 73(7) mol%/equatorial = 27(7) mol%) at T = 352 K. This corresponds to a free energy difference of A = −0.59(22) kcal mol−1. The observed gas-phase electron diffraction parameters are in good agreement with those obtained from theory. NBO analysis revealed that axial conformer of 1-iodo-1-silacyclohexane is an example for electrostatic stabilization of a conformer which is unfavorable in terms of steric and conjugation interaction.

The molecular structures of axial and equatorial conformers of 1-fluorosilacyclohexane, C5H10SiHF, as well as the thermodynamic equilibrium between these species were investigated by means of gas electron diffraction (GED), dynamic nuclear magnetic resonance, temperature-dependent Raman spectroscopy, and quantum chemical calculations (MP2, DFT, and composite methods). According to GED, the compound exists in the gas phase as a mixture of two conformers possessing the chair conformation of the six-membered ring and Cs symmetry and differing in the axial or equatorial position of the Si−F bond (axial = 63(8) mol %/equatorial = 37(8) mol %) at T = 293 K, corresponding to an A value of –0.31(20) kcal mol−1. Density functional theory (DFT) calculations were employed to obtain the minimal energy path of the conformational inversion. The MP2, G3B3, and CBS-QB3 methods were also employed to calculate the equilibrium geometries and energies of the local minima in the gas phase and in solution. The gas-phase results are in good agreement with the experiment, whereas a combined PCM/IPCM(B3LYP/6-311G(d)) approach overestimates the stabilization of the axial conformer by 0.3−0.4 kcal mol−1 in solution at 112 K. Temperature-dependent Raman spectroscopy in the temperature ranges of 210–300 K (neat liquid), 120–300 K (pentane solution), and 200–293 K (dichloromethane solution) also indicates that the axial conformer is favored over the equatorial one by 0.25(5), 0.22(5), and 0.28(5) kcal mol−1 (ΔH values), respectively.

The structures and relative energies for the basic conformations of silacyclohexane 1 have been calculated using HF, RI-MP2, RI-DFT and MM3 methods. All methods predict the chair form to be the dominant conformal ion and all of them predict structures which are in good agreement with experimental data. The conformational energy surface of 1 has been calculated using MM3. It is found that there are two symmetric lowest energy pathways for the chair-to-chair inversion. Each of them consists of two sofa-like transition states, two twist forms with C1 symmetry (twist-C1), two boat forms with Si in a gunnel position (C1 symmetry), and one twist form with C2 symmetry (twist-C2). All methods calculate the relative energy to increase in the order chair < twist-C2 < twist-C1 < boat. At the MP2 level of theory and using TZVP and TZVPP (Si atoms) basis sets the relative energies are calculated to be 3.76, 4.80, and 5.47 kcal mol-1 for the twist-C2, twist-C1, and boat conformations, respectively. The energy barrier from the chair to the twisted conformations of 1 is found to be 6.6 and 5.7 kcal mol-1 from MM3 and RI-DFT calculations, respectively. The boat form with Si at the prow (Cs symmetry) does not correspond to a local minimum nor a saddle point on the MM3 energy surface, whereas a RI-DFT optimization under Cs symmetry constraint resulted in a local minimum. In both cases its energy is above that of the chair-to-twist-C1 transition state, however, and it is clearly not a part of the chair-to-chair inversion.

The electron-diffraction data for cyclohexane and perdeuterated cyclohexane were analyzed and the results were compared. It was found that rg(C-C) = 1.535 Å (±0.002) for both compounds; rg (C-H) = 1.116 Å (±0.004) and rg(C-D) = 1.109 Å (±0.003). Observed hydrogen isotope effects in mean amplitudes agree very well with calculated ones. The C-C bond distance of cyclohexane is in good agreement with some of the previous studies, which is of importance in view of a recent scaling controversy involving this parameter.

Raman spectra of silacyclohexane as a liquid was recorded and depolarization data obtained. The infrared spectra of the vapour and liquid have been studied.The compound exists in the stable chair conformation, whereas a possible twist form should have more than 15 kJ mol−1 higher energies for silacyclohexane derived from numerous calculations. The wavenumbers of the vibrational modes were derived in the harmonic and anharmonic approximation in B3LYP/cc-pVTZ calculations. The 27 A′ and 21 A″ fundamentals were assigned on the basis of the calculations, on infrared vapour contours, Raman depolarization measurements and infrared and Raman band intensities. An average, relative deviation of ca 0.77% was found between the observed and the anharmonic wavenumbers for the 48 modes.

The rotational spectra of cyclohexane-1,1-d2, cyclohexane-13C-1,1-d2, cyclohexane-d1 (equatorial and axial), and cyclohexane-1,1,2,2,3,3-d6 have been measured between 8 and 26 GHz with a pulsed microwave Fourier transform spectrometer. From their analysis the rotational constants and the quartic centrifugal distortion constants have been determined. The complete substitution structure has been deduced from these rotational constants. Measurements of the Stark effect have provided information on the dipole moment induced by deuterium substitution.

a b s t r a c t Two new 1,1-disubstitued silacyclohexanes, C 5 H 10 SiFCH 3 (1) and C 5 H 10 SiCF 3 CH 3 (2) were synthesized. The molecular structure of their axial and equatorial conformers as well as the thermodynamic equilib-rium between these species was investigated by means of gas electron diffraction (GED), dynamic nuclear magnetic resonance (DNMR), temperature-dependent Raman spectroscopy, and quantum chemical cal-culations (CCSD(T), MP2, and DFT methods). 1a, 2a and 1e, 2e are used to denote the conformers having the CH 3 group in axial and equatorial positions, respectively. According to GED, both compounds exist as a mixture of two conformers possessing the chair conformation of the six-membered ring and C s symme-try and differing in the axial or equatorial position of the two substituents (axial-CH 3 :equatorial-CH 3 ratio of 45(6)%:55(6)% and 51(5)%:49(5)% was found for 1 and 2, respectively). Hence, G ax ÀG eq = 0.11(13) kcal -mol –1 for 1, whereas 2a and 2e have virtually the same free energy. Low-temperature 19 F NMR experi-ments resulted in G ax ÀG eq = 0.26(2) kcal mol –1 at 125 K for 1 and G ax ÀG eq = 0.36(2) kcal mol –1 at 118 K for 2. Temperature-dependent Raman spectroscopy in the temperature range of 210–300 K of the neat liquids and their solutions in THF and hexane indicates that 1e and 2e are favoured over 1a and 2a by 0.50(15) and 0.73(15) kcal mol –1 , respectively (DH values). The Raman results seem not to depend on the polarity of the medium. CCSD(T)/CBS calculations at the NMR temperatures predict G ax ÀG eq = 0.28 and 0.36 kcal mol –1 for 1 and 2, respectively, and are thus in excellent agreement with the DNMR results. The agreement of CCSD(T)/CBS with the GED results is slightly worse, predicting G ax ÀG eq = 0.31 and 0.22 kcal mol –1 for 1 and 2, respectively. The CCSD(T)/CBS calculations are also in slight disagreement with the Raman results, predicting DH values of 0.25 and 0.48 kcal mol –1 for 1 and 2, respectively. The CCSD(T)/CBS calculations of both mono-and disubstituted silacyclohexanes with F, CH 3 , and CF 3 substit-uents revealed a remarkable good additivity of substituent effects, which is not shown by the analogous cyclohexanes.

Raman spectra of liquid cyclohexane, C6H12, and deuterated cyclohexane, C6D12, were recorded with both parallel and perpendicular polarizations. The observed vibrational wavenumbers, depolarization ratios, and their intensities were measured and compared with the corresponding predicted values as well as the experimental values previously reported. The conformational energetics were obtained with the Møller-Plesset perturbation method to the second order [MP2(full)] as well as with density functional theory by the B3LYP method utilizing a variety of basis sets. The average ab initio predicted difference in energy between the more stable chair form (D3d) and the less stable twisted-boat form (D2) is 2213 cm-1 (26.47 kJ/mol), with a similar value of 2223 cm-1 (26.59 kJ/mol) from the density function theory calculations. By using two dihedral angles as variables, we calculated the chair-boat interconversion pathway for cyclohexane at the MP2(full)/6-31G(d) level. The harmonic force constants, Raman intensities, depolarization values, and the potential energy distribution were predicted from both MP2(full) and B3LYP calculations with the 6-31G(d) basis set and compared with the experimental values for the chair form when available. The 'adjusted' r 0 structural parameters were obtained from MP2/6-311+G(d,p) calculations and previously reported microwave rotational constants of five isotopomers of cyclohexane: i.e. 1,1-d2, 13C-1,1-d 2, 1,1,2,2,3,3-d6, and d1 (equatorial and axial). The determined distances in Å are: r(CC) = 1.536(3), r(CH) ax = 1.098(1); r(CH)eq = 1.095(1); and the angles in degrees: <CCHax = 108.8(3); <CCHeq = 110.2(3); <CCC = 111.1(3); and <HCH = 107.6(3) with dihedral angle <CCCC = 55.7(3). These values are compared with those previously reported and it is found that the difference in the r0 distances (0.003 Å) between the two CH values is much smaller than the difference (0.008 Å) previously reported for the rs values.

A series of ab initio computations have been performed to estimate the CCSD(T) complete basis set limit of the axial/equatorial energy difference for the chair conformation of 1-fluoro, 1-chloro, 1-methyl, 1-hydroxy, and 1-methoxysilacyclohexanes. The equatorial conformation is more stable than the axial by 0.21 kcal mol−1 for 1-methylsilacyclohexane, while the axial position is more stable for 1-fluoro, 1-chloro, and 1-methoxysilacyclohexane (by 0.09, 0.40, and 0.15 kcal mol−1, respectively). The axial and equatorial orientations of 1-hydroxysilacyclohexane are nearly electronically isoenergetic (equatorial favored by 0.03 kcal mol−1). Zero point vibrational energy corrections have very little effect on the relative energies (less than 0.08 kcal mol−1). These results suggest the presence of an interesting stereoelectronic effect, similar to the anomeric effect, in these silacyclohexanes even without an additional heteroatom, like O, in the ring. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007

The structural differences and the relative energies for the axial and equatorial forms of 1-methyl-1-silacyclohexane have been obtained from the rotational spectra of the normal, 29 Si, and all 13 C isotopologues of the axial and equatorial forms, observed by molecular beam Fourier transform microwave spectroscopy. The two species appear to have the same energy, within uncertainty limits, ∆E) 0.0 (0.2 kcal/mol. Structural parameters (r 0 and r s) are given for the two forms. The main structural differences are discussed. Potential barriers for the methyl group internal rotations (V 3) have been determined for both conformers to be 1.26(1) and 1.48(2) kcal/mol for the axial and equatorial species, respectively.

Relative energies for local minima and transition states on the potential energy surface (PES) of silacylohexane have been calculated at the B3LYP/6-311+G(d,p) level of theory. STQN(path), and intrinsic reaction path (IRC) calculations were used to calculate the lowest-energy pathway for the chair-to-chair interconversion. Both methods gave identical results. An earlier description of the PES of silacyclohexane, derived with the MM3 force field, was confirmed. A different path for the ring inversion of silacyclohexane, proposed in a recent work, is shown to be incorrect. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006

Despite the remarkable thermochemical accuracy of Kohn–Sham density-functional theories with gradient corrections for exchange-correlation [see, for example, A. D. Becke, J. Chem. Phys. 96, 2155 (1992)], we believe that further improvements are unlikely unless exact-exchange information is considered. Arguments to support this view are presented, and a semiempirical exchange-correlation functional containing local-spin-density, gradient, and exact-exchange terms is tested on 56 atomization energies, 42 ionization potentials, 8 proton affinities, and 10 total atomic energies of first- and second-row systems. This functional performs significantly better than previous functionals with gradient corrections only, and fits experimental atomization energies with an impressively small average absolute deviation of 2.4 kcal/mol.

In this paper, the N(+)-H···N, N(+)-H···O, and O-H···O(-) charge-assisted intramolecular hydrogen bonds (CAHBs) are investigated using different theoretical approaches. Monocharged cyclohexyldiamines (CHDA), aminocyclohexanols (ACHO), and cyclohexanediols (CHDO) are used as model compounds. Geometry optimizations at the MP2/aug-cc-pVDZ level are used to find the equilibrium structures for all possible H-bonded conformers. CAHBs are characterized geometrically and spectroscopically, and their energy is evaluated by means of homodesmic reactions. By comparison with the neutral forms, the presence of the charge is found to have a deep influence on the geometric and energetic H-bond parameters. In addition, these parameters are strongly dependent on the type of the groups involved as well as on their relative position in the cyclohexyl ring. For the systems under study, the H-bond energies vary from -23 to -113 kJ mol(-1), being classified from moderate to strong H-bonds. These H-bonds are also characterized by the application of the NBO and AIM theories. NBO analysis reveals that the energy corresponding to the charge transfer between the lone-pairs of the electron donor group and the antibonding orbitals of the acceptor group represents an important contribution in the H-bond stabilization. From the application of the AIM theory it is possible to see that these H-bonds possess some covalence which varies according to the type and relative position of the intervenient groups.

We combine femtosecond time-resolved rotational coherence spectroscopy with high-level ab initio theory to obtain accurate structural information for the nonpolar molecules cyclohexane (C(6)H(12)) and cyclohexane-d(12) (C(6)D(12)). We measured the rotational B(0) and centrifugal distortion constants D(J), D(JK) of the v = 0 states of C(6)H(12) and C(6)D(12) to high accuracy, for example, B(0)(C(6)H(12)) = 4306.08(5) MHz, as well as B(v) for the vibrationally excited states ν(32), ν(6), ν(16) and ν(24) of C(6)H(12) and additionally ν(15) for C(6)D(12). To successfully reproduce the experimental RCS transient, the overtone and combination levels 2ν(32), 3ν(32), ν(32) + ν(6), and ν(32) + ν(16) had to be included in the RCS model calculations. The experimental rotational constants are compared to those obtained at the second-order Møller-Plesset (MP2) level. Combining the experimental and calculated rotational constants with the calculated equilibrium bond lengths and angles allows determination of accurate semiexperimental equilibrium structure parameters, for example, r(e)(C-C) = 1.526 ± 0.001 Å, r(e)(C-H(axial)) = 1.098 ± 0.001 Å, and r(e)(C-H(equatorial)) = 1.093 ± 0.001 Å. The equilibrium C-C bond length of C(6)H(12) is only 0.004 Å longer than that of ethane. The effect of ring strain due to the unfavorable gauche interactions is mainly manifested as small deviations from the C-C-C, C-C-H(axial), and C-C-H(equatorial) angles from the tetrahedral value.

Density functional methods were evaluated in their ability to predict relative conformational energies of a test set of monosubstituted cyclohexanes and six-membered heterocycles. It is shown that while popular density functionals like B3LYP are unreliable for predicting accurate conformational energies for the axial/equatorial equilibrium of monosubstituted cyclohexanes, 1-silacyclohexanes and tetrahydropyrans, density functionals that take into account dispersion interactions like M06-2X and B2PLYP-D result in energy differences close to CCSD(T)/CBS results. Using the M06-2X density functional, we have then investigated the conformational properties of a large number of monosubstituted silacyclohexanes, with the number of silicon atoms ranging from 1 to 6. Our calculations suggest remarkably different conformational properties when compared to cyclohexane. The carbon/silicon exchange in a cyclohexane ring often has systematic, yet counterintuitive effects on the conformational properties. Dispersion interactions are shown to be especially important for accurate relative energy calculations of polysilacyclohexanes.

A perturbation theory is developed for treating a system of n electrons in which the Hartree-Fock solution appears as the zero-order approximation. It is shown by this development that the first order correction for the energy and the charge density of the system is zero. The expression for the second-order correction for the energy greatly simplifies because of the special property of the zero-order solution. It is pointed out that the development of the higher approximation involves only calculations based on a definite one-body problem.

The features of blue- and red-shifted electron acceptor-donor (ACH/B) hydrogen bonds have been compared by using quantum chemical calculations. The geometry, the interaction energy and the vibrational frequencies of both blue- (ACH=F3CH, Cl3CH with B=FCD3) and red-shifted (ACH=F3CH, Cl3CH with B=NH3 and ACH=CH3CCH with B=FCD3, NH3) complexes were obtained by using ab initio MP2(Full)/6-31+G(d,p) calculations with the a priori basis-set superposition error (BSSE) correction method. One-dimensional potential energy and dipole moment functions of the dimensionless normal coordinate Q1, corresponding to the CH stretching mode of ACH, have been compared for both types of complexes. Contributions of separate components of the interaction energy to the frequency shift and the effect of electron charge transfer were examined for a set of intermolecular distances by using the symmetry-adapted perturbation theory (SAPT) approach and natural bond orbitals (NBO) population analysis.

A series of MP2 and CCSD(T) computations have been carried out with correlation consistent basis sets as large as aug-cc-pV5Z to determine the intrinsic equatorial-axial conformational preference of CH(3)-, F-, OCH(3)-, and OH-substituted cyclohexane and tetrahydropyran rings. The high-accuracy relative electronic energies reported here shed new light on the intrinsic energetics of these cyclic prototypes for the anomeric effect. At the CCSD(T) complete basis set (CBS) limit, the energy of the equatorial conformation relative to the axial position (DeltaE (CBS)(CCSD(T))) is -1.75, -0.20, -0.21, and -0.56 kcal mol(-1) in methyl-, fluoro-, methoxy-, and hydroxycyclohexane, respectively, while DeltaE(CBS)(CCSD(T) is -2.83, +2.45, +1.27, and +0.86 kcal mol(-1) for 2-methyl-, 2-fluoro-, 2-methoxy-, and 2-hydroxytetrahydropyran, respectively. Note that the equatorial and axial conformers are nearly electronically isoenergetic in both fluoro- and methoxycyclohexane. For all eight cyclic species, a zero-point vibrational energy correction decreases Delta by a few tenths of a kilocalorie per mole. Relative energies obtained with popular methods and basis sets are unreliable, including Hartree-Fock theory, the B3LYP density functional, and the 6-31G and 6-311G families of split-valence basis sets. Even with the massive pentuple-zeta basis sets, the HF and B3LYP methods substantially overestimate the stability of the equatorial conformers (by as much as 0.99 and 0.73 kcal mol(-1), respectively, for 2-methoxytetrahydropyran). Only because of a consistent cancellation of errors do these popular approaches sometimes provide reasonable estimates of the anomeric effect.

The molecular structure of axial and equatorial conformers of 1-trifluoromethyl-1-silacyclohexane, (C5H10SiHCF3), as well as the thermodynamic equilibrium between these species was investigated by means of gas electron diffraction (GED), dynamic nuclear magnetic resonance (DNMR) spectroscopy, and quantum chemical calculations (B3LYP, MP2, and CBS-QB3). According to GED, the compound exists as a mixture of two Cs symmetry conformers possessing the chair conformation of the six-membered ring and differing in the axial or equatorial position of the CF3 group (axial=58(12) mol%/equatorial=42(12) mol%) at T=293 K. This result is in a good agreement with the theoretical prediction. This is, however, in sharp contrast to the conformational properties of the cyclohexane analogue. The main structural feature for both conformers is the unusually long exocyclic bond length Si--C 1.934(10) A. A low-temperature 19F NMR experiment results in an axial/equatorial ratio of 17(2) mol%:83(2) mol% at 113 K and a DeltaG (not equal) of 5.5(2) kcal mol-1. CBS-QB3 calculations in the gas-phase and solvation effect calculations using the PCM(B3LYP/6-311G*) and IPCM(B3LYP/6-311G*) models were applied to estimate the axial/equatorial ratio in the 100-300 K temperature range, which showed excellent agreement with the experimental results. The minimum energy pathways for the chair-to-chair inversion of trifluoromethylsilacyclohexane and methylsilacyclohexane were also calculated using the STQN(Path) method.

Relative Energies, Stereoelectronic Interactions, and Conformational Interconversion in Silacycloalkanes

- A Bodi

A. Bodi, Comment on "Relative Energies, Stereoelectronic Interactions,
and Conformational Interconversion in Silacycloalkanes", Int. J. Quantum Chem., 106, 1975-1978,
2006.
14

- Quantum-Chemical Spectroscopy
- Calculations

Spectroscopy, and Quantum-Chemical Calculations, Organometallics, 32, p. 6996−7005, 2013.
(doi: dx.doi.org/10.1021/om4005725)

Vibrational spectra, conformations, quantum chemical calculations and spectral assignments of 1-chloro-1-silacyclohexane

- V Aleksa
- G A Guirgis
- A Horn
- P Klaeboe
- R J Liberatore
- C J Nielsen

V. Aleksa, G. A. Guirgis, A. Horn, P. Klaeboe, R. J. Liberatore, C. J. Nielsen, Vibrational spectra,
conformations, quantum chemical calculations and spectral assignments of 1-chloro-1-silacyclohexane,
J.
Mol.
Struct.,
61,
p.
167-175,
2012.
(doi:
https://doi.org/10.1016/j.molstruc.2011.12.032)

Lopes jesus, J. S. Redinha, charge-assisted intramolecular hydrogen bonds in disubstituted cyclohexane derivatives

- A J