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Energetics of the Thermal Dimerization of Acenaphthylene to Heptacyclene
Abstract
The energetics of the thermal dimerization of acenaphthylene to give Z- or E-heptacyclene was investigated. The standard molar enthalpy of the formation of monoclinic Z- and E-heptacyclene isomers at 298.15 K was determined as Delta(f)H(m)o (E-C24H16, cr) = 269.3 +/- 5.6 kJ x mol(-1) and Delta(f)H(m)o (Z-C24H16, cr) = 317.7 +/- 5.6 kJ x mol(-1), respectively, by microcombustion calorimetry. The corresponding enthalpies of sublimation, Delta(sub)H(m)o (E-C24H16) = (149.0 +/- 3.1) kJ x mol(-1) and Delta(sub)H(m)o (Z-C24H16) = (128.5 +/- 2.3) kJ x mol(-1) were also obtained by Knudsen effusion and Calvet-drop microcalorimetry methods, leading to Delta(f)H(m)o (E-C24H16, g) = (418.3 +/- 6.4) kJ x mol(-1) and Delta(f)H(m)o (Z-C24H16, g) = (446.2 +/- 6.1) kJ x mol(-1), respectively. These results, in conjunction with the reported enthalpies of formation of solid and gaseous acenaphthylene, and the entropies of acenaphthylene and both hepatcyclene isomers obtained by the B3LYP/6-31G(d,p) method led to the conclusion that at 298.15 K the thermal dimerization of acenaphthylene is considerably exothermic and exergonic in the solid and gaseous states (although more favorable when the E isomer is the product), suggesting that the nonobservation of the reaction under these conditions is of kinetic nature. A full determination of the molecular and crystal structure of the E dimer by X-ray diffraction is reported for the first time. Finally, molecular dynamics computer simulations on acenaphthylene and the heptacyclene solids were carried out and the results discussed in light of the corresponding structural and Delta(sub)H(m)o data experimentally obtained.
... Pentaring more reactive than 6mem ring [389]. ...
... Symmetry forbidden [2+2] pericyclic reaction unstable [389]. ...
... While these edges are not able to crosslink with one another (e.g. [2+2] pericyclic reactions are symmetry forbidden [389]) [see Table 1(n) and Figure 23] they have significant reactivity with aryl σ-radical PAH [see Table 1(o) and Figure 23o)]. Significant bond energies of −44 kcal/mol have been calculated between phenyl radicals and ace-napthalene crosslinks, suggesting flame stability [478]. ...
The route by which gas-phase molecules in hydrocarbon flames form condensed-phase carbonaceous nanoparticles (incipient soot) is reviewed. These products of incomplete combustion are introduced as particulates and materials revealing both their useful applications and unwanted impacts as pollutants. Significant advances in experimental techniques in the last decade have allowed the gas phase precursors and the transformation from molecules to nanoparticles to be directly observed. These measurements combined with computational techniques allow for various mechanisms known to date to be compared and explored. Questions remain surrounding the various mechanisms that lead to nanoparticle formation. Mechanisms combining physical and chemical routes, so-called physically stabilised soot inception, are highlighted as a possible “middle way”.
... This [2+2] pericyclic reaction is not thermally allowed and requires optical excitation to form due to spin restrictions. 62 We have recently measured and calculated the band gap to be between 3.6-3.7 eV (≈350 nm deep UV). 59 From this systematic study, only σ-radicals (A) and localised π-radicals (B) are found to be capable of forming strong enough bonds to be long-lived in the flame. ...
This paper presents a systematic study of the reactivity of polycyclic aromatic hydrocarbons (PAH), identifying crosslinks that permit the combination of physical π-stacking interactions and covalent bonding. Hybrid density functional theory was used to identify the location of reactive sites on PAHs using the average local ionisation potential. The bond energies formed between these various reactive sites were then computed. σ-radicals were found to be the most reactive, forming bonds with other radicals and some reactive closed shell edge types. Partially saturated rim-based pentagonal rings were found to form localised π-radicals with high reactivity. This site, in addition to resonantly stabilised π-radicals, was found to be capable of bonding and stacking, which is explored for a variety of larger species. Localised π-radicals on rim-based pentagonal rings, in particular, were found to form strongly bound stacked complexes indicating a potentially important role in soot formation..
... Several issues related to the temperature and density dependence of the mean force constant are also addressed in this work. After the study of isotope effects using Molecular Dynamics simulations as a means to obtain auxiliary data, the investigations using Molecular Dynamics as the central predictive or interpretative tool rapidly gained center-stage status on a wide variety of molecular systems: policylclic aromatic compounds (heptacyclene isomers, [9]), organometallics (ferrocene and its derivatives, [10]), inorganic crystals (apatites,111213), ramified oligomers and polymers of controlled generation (dendrimers, [14]) and " last but indeed not the least " low melting temperature salts (room temperature ionic liquids,1516171819). In the case of heptacyclene molecules, the energetics of the thermal dimerization of acenaphthylene to give Z-or E-heptacyclene was investigated. ...
This communication describes the scientific research work of the author at Centro de Química Estrutural, Instituto Superior
Técnico, between 1996 and 2006, in the area of computational methods applied within the framework of statistical mechanics.
The first simulation methods to be introduced are those based on Monte Carlo algorithms, namely the extended version of the
Gibbs Ensemble Monte Carlo (GEMC) method. Several examples of the application of the method to the study of fluid phase equilibria
in model systems are discussed. The rest of the communication is dedicated to Molecular Dynamics techniques and their application
to the study of molecular systems. The case of ionic liquids, a class of compounds that attracted in recent years a lot of
attention from the scientific and technological communities, is particularly addressed. The diversity of the molecular systems
that were studied using computer simulations (and that can be described as the molecular portfolio of the author) is a measure
of the growing importance of these methods at the forefront of scientific research
The calculation of the heats of combustion ΔH°c and formation ΔH°f of organic molecules at standard conditions is presented using a commonly applicable computer algorithm based on the group-additivity method. This work is a continuation and extension of an earlier publication. The method rests on the complete breakdown of the molecules into their constituting atoms, these being further characterized by their immediate neighbor atoms. The group contributions are calculated by means of a fast Gauss–Seidel fitting calculus using the experimental data of 5030 molecules from literature. The applicability of this method has been tested by a subsequent ten-fold cross-validation procedure, which confirmed the extraordinary accuracy of the prediction of ΔH°c with a correlation coefficient R2 and a cross-validated correlation coefficient Q2 of 1, a standard deviation σ of 18.12 kJ/mol, a cross-validated standard deviation S of 19.16 kJ/mol, and a mean absolute deviation of 0.4%. The heat of formation ΔH°f has been calculated from ΔH°c using the standard enthalpies of combustion for the elements, yielding a correlation coefficient R2 for ΔH°f of 0.9979 and a corresponding standard deviation σ of 18.14 kJ/mol.
Traditional physical chemistry courses consist of two or three 45-hour semesters using traditional 1000+ page text books containing hundreds of exercises and problems. Typically physical chemistry courses include the study of key topics and a plethora of concepts, exploration of mathematical models of physical systems, and the use of increasingly sophisticated mathematical manipulations, all accompanied by an emphasis on scientific writing. Given the breadth of the discipline and its wide range of applications across other scientific disciplines we must ask two key questions. First, what of all available topics should an instructor include in a typical course? Second, what components of physical chemistry courses have the potential to lead to enhanced student learning and future professional growth? This paper will highlight some suggestions regarding the conundrum of too much too fast for the young physical chemistry student. Included will be examples showing project-based laboratory and lecture activities and the use of symbolic mathematics software as mechanisms for including more modern topics or more modern approaches to standard topics.
Motivated by the need of reliable experimental data for the assessment of theoretical predictions, this work proposes a dataset of enthalpies of sublimation determined for specific crystalline structures, for the validation of molecular force fields (FF). The selected data were used to explore the ability of the OPLS-AA parameterization to investigate the properties of solid materials in molecular dynamics simulations. Furthermore, several approaches to improve this parameterization were also considered. These modifications consisted in replacing the original FF atomic point charges (APC), by values calculated using quantum chemical methods, and by the implementation of a polarizable FF. The obtained results indicated that, in general, the best agreement between theoretical and experimental data is found when the OPLS-AA force field is used with the original APC or when these are replaced by ChelpG charges, computed at the MP2/aug-cc-pVDZ level of theory, for isolated molecules in the gaseous phase. If a good description of the energetic relations between the polymorphs of a compound is required, then, either the use of polarizable FF or the use of charges determined taking into account the vicinity of the molecules in the crystal (combining the ChelpG and MP2/cc-pVDZ methods) is recommended. Finally, it was concluded that density functional theory methods, like B3LYP or B3PW91, is not advisable for the evaluation of APC of organic compounds, for molecular dynamic simulations. Instead, the MP2 functional should be considered.
The formation of soluble 1:2 complexes within hydrophilic γ-cyclodextrin (γ-CD) thioethers allows to perform photodimerizations of aromatic guests under controlled, homogenous reaction conditions. The quantum yields for unsubstituted anthracene, acenaphthylene, and coumarin complexed in these γ-CD thioethers were found to be up to 10 times higher than in the non-complexed state. The configuration of the photoproduct reflected the configuration of the dimeric inclusion complex of the guest. Anti-parallel orientation of acenaphthylene within the CD cavity led to the exclusive formation of the anti photo-dimer in quantitative yield. Parallel orientation of coumarin within the complex of a CD thioether led to the formation of the syn head-to-head dimer. The degree of complexation of coumarin could be increased by employing the salting out effect.
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.
Two new programs have been developed for searching the Cambridge Structural Database (CSD) and visualizing database entries:
ConQuest
and
Mercury
. The former is a new search interface to the CSD, the latter is a high-performance crystal-structure visualizer with extensive facilities for exploring networks of intermolecular contacts. Particular emphasis has been placed on making the programs as intuitive as possible. Both
ConQuest
and
Mercury
run under Windows and various types of Unix, including Linux.
The standard molar enthalpies of sublimation of M(η5-C5H5)Cl2 (M = Ti, V, Zr, Nb, Hf, and W) compounds, at 298.15 K, were determined by the Knudsen effusion method to be: 124.4±2.9 kJ⋅mol−1 (M = Ti), 108.5±4.6 kJ⋅mol−1 (M = Zr), 110.2±2.9 kJ⋅mol−1 (M = Hf), 140.1±7.4 kJ⋅mol−1 (M = V), 127.4±4.4 kJ⋅mol−1 (M = Nb), and 120.7±8.6 kJ⋅mol−1 (M = W). The results obtained show a tendency to linearly decrease with increasing metal atomic radius.
Orbital symmetry controls in an easily discernible manner the feasibility and stereochemical consequences of every concerted reaction.
The application of a micro rotating-bomb combustion calorimeter to chlorine-containing organic compounds was tested by determining the standard massic energy of combustion and the standard molar enthalpy of formation of 4-chlorobenzoic acid at 298.15 K. Experiments with samples of masses in the range 25.5 mg to 39.5 mg led to Δcuo = − (19563.45 ± 4.88)J · g − 1and ΔfHmo(C7H5O2Cl, cr) = − (429.9 ± 2.2)kJ · mol − 1in good agreement with the corresponding values recommended in the literature.
The enthalpy of sublimation of diphenylacetylene at 298.15 K, ΔgcrH⊖m(C2(C6H5)2] = 95.1 ± 1.1 kJmol−1, was derived from vapour pressure-temperature data, obtained with two different Knudsen effusion apparatus, and from heat capacity measurements obtained by differential scanning calorimetry. The molybdenum-diphenylacetylene bond dissociation enthalpy in Mo(η5-C5H5)2[C2(C6H5)2] was reevaluated as 115 ± 26 kJ mol−1, on the basis of the new value for ΔcrgH⊖m[C2(C6H5)2].
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.
The vapour pressure and heat of sublimation of ferrocene have been determined in the region 20-30 °C, by the effusion and thermistor manometer methods. The most reliable values are log 10 p mm = 10.68-3830/T(°K), vapour pressure at 25°C = 0.00684 mm, ΔH (25°C) = 17,530±100 cal mole -1. This value for the heat of sublimation is compared with values previously reported in the literature and with the maximum permissible value deduced from the entropy of sublimation. It is shown (a) that previous values are impossibly high, (6) that under conditions where the ratio of the mean free path to the diameter of the hole (λ/d) varies from 1 to 5, the effusion technique combined with the Knudsen or Clausing methods of deducing the pressure results in an impossibly high value for ΔH sub., (c) that the introduction of the correction factor based on Hiby and Pahl's treatment of effusion, results in a realistic value for ΔH sub., (d) that the thermistor gauge technique gives reliable value for ΔH sub.
The crystallographic problem: The production, and the visibility in the published literature, of thermal ellipsoid plots for small-molecule crystallographic studies remains an important method for assessing the quality of reported results. Since the mid 1960s, the program ORTEP (Johnson, 1965) has been perhaps the most popular computer program for generating thermal ellipsoid drawings for publication. The recently released update of ORTEP-III (Johnson & Burnett, 1996) has some additional features over the earlier versions, but still relies on fixed-format input files. Many users will find this very inconvenient, and will prefer to obtain drawings directly from their crystallographic coordinate files. This new version of ORTEP-3 for Windows provides all the facilities of ORTEP-III, but with a modern Graphical User Interface (GUI). Method of solution: A Microsoft-Windows GUI has been added to ORTEP-III. All the facilities of ORTEP-III are retained, and a number of extra features have been added. The GUI is effectively an editor that writes ORTEP-III input files, but the user need not have any knowledge of the inner workings of ORTEP. The main features of this program are: (i) ORTEP-3 for Windows can directly read many of the common crystallographic ASCII file formats. Currently supported formats are SHELX (Sheldrick, 1993), GX (Mallinson & Muir, 1985), GIF (Hall, Allen & Brown, 1991), SPF (Spek, 1990), CRYSTALS (Watkin, Prout, Carruthers & Betteridge, 1996), CSD-XR and CSD-FDAT. In addition, ORTEP-3 for Windows will accept any legal ORTEP-III instruction file. (ii) Covalent radii for the first 94 elements are stored internally, and may be modified by the user. All bonds are calculated automatically, and any individual bonds may be selected for removal, or for a special representation. (iii) The graphical representations of thermal ellipsoids for any element or selected sets of atoms can be individually set. All the possible graphical representations of thermal ellipsoids in ORTEP-III are also available in ORTEP-3 for Windows. (iv) A mouse labelling routine is provided by the GUI. Any number of selected atoms may be labelled, and any available Windows font may be used for the labels. The font attributes, e.g. italic, bold, colour, point size etc. can also be selected via a standard Windows dialog box. (v) As well as HPGL and PostScript Graphics graphic metafiles, it is also possible to get high quality graphics output by printing directly to an attached printer. The screen display may be saved as BMP or PCX format metafiles, and may also be copied to the clip-board for subsequent use by other Windows programs, e.g. word processing or graphics processing programs. Colour is available for all these output modes. (vi) A simple text editor is provided, so that input files may be modified without leaving the program. (vii) Symmetry expansion of the asymmetric unit to give complete connected fragments may be carried out automatically. (viii) Unit-cell packing diagrams are produced automatically. (ix) A number of options are provided to control the view direction. The molecular view may be rotated or translated by button commands from the tool bar, and views normal to crystallographic planes may also be obtained. Software environment and program specification: The program will read several common crystallographic file formats which hold information on the anisotropic displacement parameters. The operation of the program is carried out via standard self-explanatory MS-Windows menu items and dialog boxes. Hard-copy output is either by HPGL or Encapsulated PostScript metafiles, or by directly printing the graphics screen. Hardware environment: The program is implemented for IBM PC compatible computers running MS-Windows versions 3.1x, Windows 95 or Windows NT. At least a 486-66 machine is recommended with 8 Mbytes of RAM, and at least 5 Mbytes of disk space. Documentation and availability: The executable program, together with full documentation, is available free for academic users from http://www.chem. gla.ac.uk/̃louis/ortep3. Although the program is written in Fortran77, a large number of nonstandard FTN77 calls are used to create the GUI. For this reason, the source code is not available.
A micro-combustion calorimeter suitable for samples of mass about 10 mg to 50 mg is described. Calibration with benzoic acid led to the energy equivalent ε° = (1809.82 ± 0.28) J·K−1. The calorimeter was tested by determining the standard molar enthalpies of formation at the temperature 298.15 K of salicylic acid: ΔfHm°{o-C6H4(OH)(CO2H), cr} = −(587.8 ± 1.5) kJ mol−1and of urea: ΔfHm°(H2NCONH2, cr) = − (333.4 ± 0.6) kJ mol−1.
From sublimation enthalpies, ΔHv(s, 298.15 K), for a number of planar aromatic hydrocarbons, structural bond contributions have been derived. The close additivity of the sublimation enthalpy is demonstrated for these compounds, which are characterized by similar electronic configuration, molecular rigidity, and mobility. The non-coplanarity of a few phenyl-substituted aromatic compounds is discussed.
SIR97 is the integration of two programs, SIR92 and CAOS, the first devoted to the solution of crystal structures by direct methods, the second to refinement via least-squares-Fourier procedures. Several new features have been introduced in SIR97 with respect to the previous version, SIR92: greater automatization, increased efficiency of the direct methods section, and a powerful graphics interface. The program also provides publication tables and CIF files. © 1999 International Union of Crystallography Printed in Great Britain - all rights reserved.
The photophysical properties of acenaphthylene (ACN), fluoranthene, rubicene and decacyclene were investigated in toluene by electronic spectroscopy and photoacoustic calorimetry. The photophysical parameters of the related molecules pyracyclene (PYR), periflanthene (PF) and 1,16-benzoperiflanthene, taken from the literature, were included in the discussion. A Siebrand plot was used to analyse the energy dependence of the rate constant kic of internal conversion. With the exception of ACN and PF and possibly PYR, the molecules follow the energy gap law, demonstrating that internal conversion should be promoted by CH modes. For ACN and PF, deviations to much larger values of kic were observed. This effect could be caused by a strong displacement of the So and S1 potential energy curves, which is related to the large differences in the CC bond lengths in the two states.
The mechanism of photodimerization of acenaphthylene (ACN) has been investigated in order to elucidate the roles of the singlet and the triplet excited states of ACN in the formation of the Z- and E-dimers in several solvents. The quantum yields and the ratio of the produced Z- to E-dimer were determined under irradiation of ACN at 435.8 nm in several solvents (1,2-dichloroethane, acetonitrile, cyclohexane, benzene, methanol, and DMF) over a wide concentration range (2.0 × 10-4−2.0 M) in the absence of additives and in the presence of 1,2-dibromoethane, a heavy-atom-containing solvent; Eosin-Y, a triplet sensitizer (irradiated at 546.1 nm); and ferrocene, a triplet quencher. On irradiation of ACN in dilute solution, the initially generated S1 state crosses over to the T1 state, though with low quantum yield, before reaction with the ground state ACN can occur due to the very short lifetime of the S1 state, and the resultant T1 state reacts with ground state ACN to give a mixture of the Z- and E-dimers in a ratio that depends on the solvent employed. In more concentrated solution, the S1 state reacts with ACN before intersystem crossing to the T1 state can occur, affording exclusively the Z-dimer.
The quantum yields of dimerization of acenaphthylene were measured at 365 nm as a function of ethyl iodide concentration, substrate concentration, and quencher concentration. The product distributions in each case were also determined. Ethyl iodide appears to have a considerable effect on T1 → S1, thus leading to dimerization predominantly from the triplet state. A smaller but significant effect on S0 ← T1 also results from the presence of ethyl iodide producing a decrease in the dimerization efficiency at high concentrations of heavy-atom solvent. A mechanism consistent with the data has been proposed and the triplet rate constants for dimerization and the various modes of deactivation have been determined.
Acenaphthylene is photodimerized by visible and near-ultraviolet light. In solution, the quantum yield increases with increasing concentration and is depressed by dissolved O 2. Only the cis dimer is formed in O 2-saturated solutions. In O 2-free solutions, both dimers are formed; the trans dimer is the dominant species in dilute solutions and the cis dimer in concentrated solutions. The effects of the nature of the solvent and of temperature were studied. The results are compatible with a relatively simple mechanism. The dimers are efficiently photodissociated, re-forming acenaphthylene. The quantum yield of photodissociation does not depend on the concentrations of dimer or of O 2.
The low-temperature heat capacities from 5 K to 330 or 350 K of acenaphthene (C12H10) and acenaphthytene (C12H8) have been determined by adiabatic calorimetry. Although the heat capacity of acenaphthene is typically sigmoidal, that of acenaphthylene has two transitions, one designated crystal phase III/crystal phase II (a rounded bump with a maximum at 116.6 K), and the other crystal phase II/crystal phase I (a very sharp spike located at 127.1 K). Severe hysteresis was noted on cooling through the regions of the two transitions, but optimized cooling permitted reproducible results for the thermodynamics of both transitions. The total Delta(trs)S degrees is 12.13 J.K-1.mol(-1) and approximates R ln 4. A comparison between the calorimetric and statistically estimated standard entropies in the ideal gas state for both compounds is made. Although a good agreement for acenaphthene is found at 300 K, evidence of residual entropy is found for acenaphthylene, estimated as about 10.33 J.K-1.mol(-1) (R ln 4). A correlation of these data based on neutron diffraction, NMR, and other studies will be shown. The thermodynamic functions C-p(J.K-1.mol(-1)), S degrees(J.K-1.mol(-1)), and phi degrees(J.K-1.mol(-1)) at 298.15 K are for C(12)H(8)183.59, 206.23, and 105.86 and for C12H10 191.38, 188.82, and 93.97.
WinGX is a suite of Microsoft Windows™ programs for the processing, solution, refinement and publication of single-crystal diffraction data.
Die Photodimerisation des Acenaphthylens (1) kann durch Lösungsmittel und Zusatz von Cyclooctatraen (COT) als Löschsubstanz so gelenkt werden, daß entweder bevorzugt das anti -Dimere (2) oder ausschließlich das syn-Dimere (3) entsteht. Bei der unsensibilisierten Dimerisation konkurrieren zwei Reaktionen. Die eine liefert 2 und 3 und ist durch COT hemmbar, die andere wird so nicht gehemmt und führt nur zu 3. Die Konkurrenz beider Reaktionen wird vom Lösungsmittel gesteuert. — Bei der durch Bengalrosa sensibilisierten und bei der durch COT hemmbaren, unsensibilisierten Photodimerisation von 1 werden zur Cyclisierung gleiche Übergangszustände durchlaufen. Das Ausbeuteverhältnis von 2 und 3 hängt hier linear vom Kirkwood-Onsager-Parameter der Lösungsmittel ab.
A heat-flow Calvet microcalorimeter was adapted for the measurement of sublimation enthalpies by the vacuum-drop method, with samples of masses in the range 1 mg to 5 mg. The electrically calibrated apparatus was tested by determining the enthalpies of sublimation of benzoic acid and ferrocene, at T = 298.15 K. The obtained results, ΔcrgHmo(C7H6O2) = (88.3 ± 0.5)kJ · mol − 1and ΔcrgHmo(C10H10Fe) = (73.3 ± 0.1)kJ · mol − 1, are in excellent agreement with the corresponding values recommended in the literature. Subsequent application of the apparatus to the determination of the enthalpy of sublimation of η5-bis-pentamethylcyclopentadyenyl iron, at T = 298.15 K, led to ΔcrgHmo(C20H30Fe) = (96.8 ± 0.6)kJ · mol − 1.
The parametrization and testing of the OPLS all-atom force field for organic molecules and peptides are described. Parameters for both torsional and nonbonded energetics have been derived, while the bond stretching and angle bending parameters have been adopted mostly from the AMBER all-atom force field. The torsional parameters were determined by fitting to rotational energy profiles obtained from ab initio molecular orbital calculations at the RHF/6-31G*//RHF/6-31G* level for more than 50 organic molecules and ions. The quality of the fits was high with average errors for conformational energies of less than 0.2 kcal/mol. The force-field results for molecular structures are also demonstrated to closely match the ab initio predictions. The nonbonded parameters were developed in conjunction with Monte Carlo statistical mechanics simulations by computing thermodynamic and structural properties for 34 pure organic liquids including alkanes, alkenes, alcohols, ethers, acetals, thiols, sulfides, disulfides, aldehydes, ketones, and amides. Average errors in comparison with experimental data are 2% for heats of vaporization and densities. The Monte Carlo simulations included sampling all internal and intermolecular degrees of freedom. It is found that such non-polar and monofunctional systems do not show significant condensed-phase effects on internal energies in going from the gas phase to the pure liquids.
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.
Heat capacity measurements from 5 to 350[deg] K and vapor pressure measurements on bis(benzene)chromium together with the published frequency assignments permit a correlation between the third-law and spectroscopic entropies. Neither thermal anomalies nor zero point entropy were found. The agreement is consistent with free rotation about the ring to metal bonds and D6h symmetry for the molecule as proposed by Cotton. The thermodynamic functions for the crystal at 298.15[deg]K are 53.52, 54.07, 26.77, and -27.30 cal [middle dot] mole-1 [deg]K-1 for the heat capacity, entropy, enthalpy function, and Gibbs function, respectively. The vapor pressure over the range 310 to 365[deg]K is represented by log10p (mm) = 27.42-5451/T-5.535 log10T, (T in [deg]K). Peer Reviewed http://deepblue.lib.umich.edu/bitstream/2027.42/32975/1/0000359.pdf
A correlation-energy formula due to Colle and Salvetti [Theor. Chim. Acta 37, 329 (1975)], in which the correlation energy density is expressed in terms of the electron density and a Laplacian of the second-order Hartree-Fock density matrix, is restated as a formula involving the density and local kinetic-energy density. On insertion of gradient expansions for the local kinetic-energy density, density-functional formulas for the correlation energy and correlation potential are then obtained. Through numerical calculations on a number of atoms, positive ions, and molecules, of both open- and closed-shell type, it is demonstrated that these formulas, like the original Colle-Salvetti formulas, give correlation energies within a few percent.
We propose a simple analytic representation of the correlation energy εc for a uniform electron gas, as a function of density parameter rs and relative spin polarization zeta. Within the random-phase approximation (RPA), this representation allows for the r-3/4s behavior as rs-->∞. Close agreement with numerical RPA values for εc(rs,0), εc(rs,1), and the spin stiffness alphac(rs)=∂2εc(rs, zeta=0)/deltazeta2, and recovery of the correct rslnrs term for rs-->0, indicate the appropriateness of the chosen analytic form. Beyond RPA, different parameters for the same analytic form are found by fitting to the Green's-function Monte Carlo data of Ceperley and Alder [Phys. Rev. Lett. 45, 566 (1980)], taking into account data uncertainties that have been ignored in earlier fits by Vosko, Wilk, and Nusair (VWN) [Can. J. Phys. 58, 1200 (1980)] or by Perdew and Zunger (PZ) [Phys. Rev. B 23, 5048 (1981)]. While we confirm the practical accuracy of the VWN and PZ representations, we eliminate some minor problems with these forms. We study the zeta-dependent coefficients in the high- and low-density expansions, and the rs-dependent spin susceptibility. We also present a conjecture for the exact low-density limit. The correlation potential musigmac(rs,zeta) is evaluated for use in self-consistent density-functional calculations.
In this work, the aromaticity of pyracylene (2) was investigated from an energetic point of view. The standard enthalpy of hydrogenation of acenaphthylene (1) to acenaphthene (3) at 298.15 K was determined to be minus sign(114.5 +/- 4.2) kJ x mol(-1) in toluene solution and minus sign(107.9 +/- 4.2) kJ x mol(-1) in the gas phase, by combining results of combustion and reaction-solution calorimetry. A direct calorimetric measurement of the standard enthalpy of hydrogenation of pyracylene (2) to pyracene (4) in toluene at 298.15 K gave -(249.9 plus minus 4.6) kJ x mol(-1). The corresponding enthalpy of hydrogenation in the gas phase, computed from the Delta(f)H(o)m(cr) and DeltaH(o)m(sub) values obtained in this work for 2 and 4, was -(236.0 +/- 7.0) kJ x mol(-1). Molecular mechanics calculations (MM3) led to Delta(hyd)H(o)m(1,g) = -110.9 kJ x mol(-1) and Delta(hyd)H(o)m(2,g) = -249.3 kJ x mol(-1) at 298.15 K. Density functional theory calculations [B3LYP/6-311+G(3d,2p)//B3LYP/6-31G(d)] provided Delta(hyd)H(o)m(2,g) = -(244.6 +/- 8.9) kJ x mol(-1) at 298.15 K. The results are put in perspective with discussions concerning the "aromaticity" of pyracylene. It is concluded that, on energetic grounds, pyracylene is a borderline case in terms of aromaticity/antiaromaticity character.
The photochemical dimerisation of acenaphthylene in the presence of various cation-exchanged bentonite clays has been studied. While the cis-dimer is the predominant product when smaller cations are present in the clay interlayer, the presence of heavier atoms in the clay favors the trans-dimer. Synthetic anionic hydrotalcite clays are also found to be efficient for the efficient dimerisation of acenaphthylene.
Selective excitation of charge-transfer complexes of indene or acenaphthylene with various electron acceptors does or does not afford final net reaction products, depending on the free energy of the resulting radical ion pairs over the ground state, -deltaGBET, with threshold values. A similar factor governs the efficiency of the reaction on direct excitation of either the donor or the acceptor of their components, except that it does not fall to nil below the threshold and the reaction affords higher quantum yields than the selective excitation of the charge-transfer complex.
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