Charles W. Bock

An alternative approach to the problem of assessing destabilization energies (strain energies) in cyclic hydrocarbons

Abstract Reaction involving no hypothetical structures having averaged group energies or particular bond energy assignments are described, that provide an alternative basis for evaluating strain energies of cyclic hydrocarbons. These reactions match CC bonds in the sense of having equal number of sp3-sp3, sp3-sp2, sp2-sp2, etc. bonds in reactants and products, while simultaneously matching various CH bonds as closely as possible. Such reactions single out those structural features that lead to destabilizations of strained ring systems. Theoretical and experimental molecular indices are introduced as a measure of these destabilizing effects, and are compared ring strain energies.

An alternative approach to the problem of assessing stabilization energies in cyclic conjugated hydrocarbons

Reactions are described that provide an alternative basis for evaluating stabilization energies of cyclic conjugated hydrocarbons. These reactions involve no changes in hybridization of carbon atoms and minimal changes in the nature of the carbon-hydrogen bonds. Such reactions exemplify those structural features that lead to stabilization. Theoretical and experimental molecular indices are introduced as a measure of these stabilizing effects. Calculated values of these indices are compared to experimental results.

Molecular structure of nitrobenzene in the planar and orthogonal conformations - A concerted study by electron diffraction, X-ray crystallography, and molecular orbital calculations

The molecular structure and ring distortions of nitrobenzene have been determined by gas-phase electron diffraction and ab initio molecular orbital (MO) calculations as well as from the structures of six derivatives studied by X-ray crystallography. The experimental value of the ring angle at the ipso position isα = 123.4 ± 0.3° in the free molecule; this is about 1.5° less than the hitherto reported values. Regression analysis of the ring angles in the six derivatives studied by X-ray crystallography yieldsα = 122.7(1)° for nitrobenzene in a crystalline environment. The small difference in the two values of a is interpreted as an effect of intermolecular interactions in the crystal. The value produced by the MO calculations,α = 122.3° at the 6–31G* (5D) level, is smaller than either of the experimental results. As regards the ring angles at the meta and para positions, the three techniques of structure determination consistently indicate that these are larger than 120° by a few tenths of a degree. Other important geometrical parameters from the electron diffraction study are 〈rg(C-C)〉 = 1.399 ± 0.003 Å,rg(C-N) = 1.486 ± 0.004 Å,rg(N-O) = 1.223 ± 0.003 Å, and A sO-N-O = 125.3 ± 0.2°. X-ray diffraction experiments on 3,5-dimethyl-4-nitrobenzoic acid and 3,5-dimethylbenzoic acid and ab initio MO calculations provide solid evidence that the geometry of nitrobenzene is little affected when the nitrogroup is twisted by 90° out of the planar equilibrium conformation. This indicates that the extent of π-electron transfer from the benzene ring to the nitro group is small. The barrier to rotation is estimated to be 17 ± 4 kJ mol−1 from the electron diffraction data.

The distortion of the ring in monosubstituted benzene derivatives: A molecular orbital study

Abstract Ab initio calculations using the 6-31G basis set have been carried out on benzene and the monosubstituted derivatives with CH3, NH2, OH, F, NO, CHCH2, CCH, CCF COO− O−, and with NO2, CHO, CHNH, COOH, CFO, CN, NC and NH3+, as substituent groups. The C- and H-atoms of the ring and the substituent group atom directly attached to it were assumed to lie in the same plane, and a particular orientation was assumed for certain groups, otherwise full geometry optimization was employed. Trends in the following parameters are discussed — the ipso angle, the lengths of the CC bonds which include the ipso angle, the CC and CH bond lengths, nonbonded C ⋯ C and H ⋯ H distances, the ring area, and the tilt of the unsymmetrical substituent groups with respect to the ring axis. Additional calculations at the 6–31G* level on benzene and the F, CN and NH2 derivatives show the trends to be unaffected by the inclusion of polarization functions on the heavy atoms. In selected cases the calculated geometries are compared with microwave and X-ray diffraction results. Comparison is also made with the Mulliken population analysis of Hehre, Radom and Pople (1972) who used the STO-3G basis set and standard geometry. The difference in energy between that for the optimized structure and that for a reference structure with optimized benzene ring geometry and the optimized geometry for the attachment and substituent group has been calculated for the F, NO2, OH, CN, CHCH2 and O− derivatives. The small values, less than 1 kcal mol−1, except for O−, suggest that the actual physical state might well be a mixture of structures having slightly different ring geometries.

Homodesmotic reactions for the assessment of stabilization energies in benzenoid and other conjugated cyclic hydrocarbons

New homodesmotic reactions are designed that provide and alternative basis for evaluating stabilization energies of benzenoid and other conjugated cyclic hydrocarbons. As in previous cases, carbon–carbon bonds are matched in the sense of having equal numbers of Csp2–Csp2, Csp2Csp2, Csp2–Csp3, etc. bonds in reactants and products, while simultaneously the various carbon–hydrogen bonds are matched as closely as possible. By minimizing extraneous energy contributions to the reaction heat arising from changes in hybridization and C–H binding, such reactions single out those structural features resulting in stabilization. These new reactions have the advantage that experimental ΔHf° data is currently available for the necessary reactant molecules, thus allowing an explict evaluation of the homodesmotic stablization energy to be made, which is compared to quantum theoretical calculations wherever possible.

The structure of nitrobenzene and the interpretation of the vibrational frequencies of the CNO2 moiety on the basis of ab initio calculations

Abstract The vibrational spectra of nitrobenzene and its para - d 1 , d 5 , 16 O 18 O, 18 O 2 and 15 N isotopic modifications are evaluated using the RHF/6-31G* ab initio harmonic force field. A rigorous interpretation of the experimental CNO 2 moiety bands is carried out. Systematic deficiencies of the SCF method are effectively removed by applying scale factors optimized previously for a number of aliphatic nitro compounds. Fully optimized geometries are also reported for planar and orthogonal nitrobenzene conformations at the RHF and MP2 computational levels using the standard 6-31G* and 6-31G** basis sets. Theoretical geometries and barriers to internal rotation are compared with available experimental data. The calculations suggest that steric factors affect appreciably the structural parameters of the CNO 2 fragment in the equilibrium planar conformation and consequently the potential function for internal rotation in nitrobenzene.

A Molecular orbital study of the rotation about the CC bond in styrene

Abstract The geometry and energy of styrene have been calculated using the 6-31G basis set as a function of the C β C 2 C 1 C 2 dihedral angle-Φ = 0°(cis), 15°, 30°, 60° and 90° — assuming that the vinyl and phenyl groups remain planar, but otherwise with full geometry optimization. Similar calculations have been carried out for 1,3-butadiene and 3-methylene-1,4-pentadiene (MPD) where rotation about 180° generates a different and not the same conformer. The torsional potential energy curve for styrene has a very flat minimum Φ = 0, i.e. the cis structure is the most stable, whereas butadiene and MPD have minima in the region Φ = 37° to 40°, indicative of more stable gauche structures. For styrene the barrier height Φ = 90° is 131.1 KJ mol −1 . These results provide strong support for the potential function obtained by Hollas and Ridley from single level vibronic fluorescence and other spectroscopic data. The distortion of the benzene ring brought about the vinyl group substitution is discussed, also the variation of the C/C and H/C bond lenghts with Φ and the change in charge on the vinyl group and the polarity of the various bonds in the conversion of the cis into the 90° gauche conformer. The stabilization energy for styrene relative to that for benzene has been evaluated according to various criteria, and, in addition, the energy associated with the distortion of the ring.

A computational investigation of the nitrogen-boron interaction in o-(N,N-dialkylaminomethyl)arylboronate systems.

o-(N,N-Dialkylaminomethyl)arylboronate systems are an important class of compounds in diol-sensor development. We report results from a computational investigation of fourteen o-(N,N-dialkylaminomethyl)arylboronates using second-order Møller-Plesset (MP2) perturbation theory. Geometry optimizations were performed at the MP2/cc-pVDZ level and followed by single-point calculations at the MP2/aug-cc-pVDZ(cc-pVTZ) levels. These results are compared to those from density functional theory (DFT) at the PBE1PBE(PBE1PBE-D)/6-311++G(d,p)(aug-cc-pVDZ) levels, as well as to experiment. Results from continuum PCM and CPCM solvation models were employed to assess the effects of a bulk aqueous environment. Although the behavior of o-(N,N-dialkylaminomethyl) free acid and ester proved to be complicated, we were able to extract some important trends from our calculations: (1) for the free acids the intramolecular hydrogen-bonded B-O-H···N seven-membered ring conformers 12 and 16 are found to be slightly lower in energy than the dative-bonded N→B five-membered ring conformers 10 and 14 while conformers 13 and 17, with no direct boron-nitrogen interaction, are significantly higher in energy than 12 and 16; (2) for the esters where no intramolecular B-O-H···N bonded form is possible, the N→B conformers 18 and 21 are significantly lower in energy than the no-interaction forms 20 and 23; (3) H(2)O insertion reactions into the N→B structures 10, 14, 18, and 21 leading to the seven-membered intermolecular hydrogen-bonded B···OH(2)···N ring structures 11, 15, 19, and 22 are all energetically favorable.

Mutagenicity of aminoazobenzene dyes and related structures: a QSAR/QPAR investigation

Abstract Quantitative structure–activity/property–activity relationships (QSAR/QPARs) are developed that correlate the observed mutagenic activity of 43 aminoazobenzene derivatives with a variety of molecular descriptors calculated using quantum-chemical semiempirical methodology. Models based on multilinear regression techniques and artificial neural networks are presented that account for more than 80% of the variation in the reported relative mutagenicity of these compounds.

A molecular orbital study of nitrogen inversion in aniline with extensive geometry optimization

The geometry and energy of aniline have been calculated using the 6-31G and 6-31G*(5D) basis sets for the planar structure and various pyramidal structures, assuming that the ring and the N-atom bonded to it lie in the same plane, but otherwise with full geometry optimization. With the 6-31G basis set the planar structure is predicated to be the most stable, whereas the inclusion of polarization functions in the 6-31G*(5D) basis set finds a pyramidal structure with the out-of-plane angle φ=42.3° to be most stable and the energy barrier to inversion via the planar transition state to be 1.59±0.02 kcal mol−1, in close agreement with experiment. Completing the optimization, allowing the N-atom and the C- and H-atoms of the ring to take up equilibrium out-of-plane positions increases the calculated energy carrier to inversion by less than 0.1 kcal mol−1 to 1.66 kcal mol−1. The ring adopts a very shallow inverted boat-type conformation with ∠N7-C1⋯C4 = 2.0°.