James E. Boggs

Force field, dipole moment derivatives, and vibronic constants of benzene from a combination of experimental and ab initio quantum chemical information

The quadratic and the most important cubic force constants of benzene have been determined from ab initio Hartree–Fock calculations with a double‐zeta basis set. Some constants have also been recalculated using other basis sets, including a polarized one. A few empirical scale factors, applied to the ab initio force field, allow the reproduction of a large number of observed vibrational frequencies, isotope shifts, and Coriolis constants within the uncertainties of experiment and the harmonic model. It is shown that the simultaneous utilization of ab initio and spectroscopical information is sufficient for the conclusive resolution of the uncertainties and alternatives in previous empirical force fields. The resulting scale factors can be used directly to obtain force fields for other aromatic hydrocarbons from ab initio calculations. Reproduction of the observed infrared intensities is only moderately successful, even with the polarized basis set. The calculated vibronic coupling constants show qualitati...

IUPAC Critical Evaluation of Thermochemical Properties of Selected Radicals. Part I

This is the first part of a series of articles reporting critically evaluated thermochemical properties of selected free radicals. The present article contains datasheets for 11 radicals: CH, CH2(triplet), CH2(singlet), CH3, CH2OH, CH3O, CH3CO, C2H5O, C6H5CH2, OH, and NH2. The thermochemical properties discussed are the enthalpy of formation, as well as the heat capacity, integrated heat capacity, and entropy of the radicals. One distinguishing feature of the present evaluation is the systematic utilization of available kinetic, spectroscopic and ion thermochemical data as well as high-level theoretical results.

Cubic force constants and equilibrium geometry of methane from Hartree–Fock and correlated wavefunctions

The ab initio harmonic and cubic force field of methane has been determined, both from Hartree–Fock and from highly correlated CEPA wavefunctions. The effect of correlation is small on the force field, particularly on the cubic constants. The ab initio cubic force constants were used to calculate the equilibrium CH bond length from the electron diffraction rg and from the spectroscopic r0 values. The weighted mean of the results is re=1.0862 ±0.0005 A. There remains a small but disturbing discrepancy between the best ab initio and the present corrected experimental results, both for the bond length and for the CH stretching force constants.

Collision Broadening of Rotational Absorption Lines. I. Theoretical Formulation

An impact theory approach to the pressure broadening of microwave absorption lines is presented. A time‐development matrix is evaluated to yield expressions which are averaged over all internal quantum states of the colliding molecule and the velocity distribution function to give half‐width and frequency‐shift parameters. The half‐width parameter is then evaluated for dipole—dipole, dipole—quadrupole, quadrupole—quadrupole, and dispersion interactions. The dependence of the linewidths on the multipole moments, temperature, and the rotational state of the colliding molecule is discussed. The results are compared with previous theoretical treatments.

THEORETICAL PREDICTION OF VIBRATIONAL SPECTRA. 1. THE In-PLANE FORCE FIELD AND VIBRATIONAL SPECTRA OF PYRIDINE.

Calcul du champ de force de vibration harmonique complet et des elements du champ de force cubique diagonal et semidiagonal pour la pyridine, au niveau Hartree-Fock 4-21

CH bond length variations due to the intramolecular environment: a comparison of the results obtained by the method of isolated CH stretching frequencies and by ab initio gradient calculations

Abstract Two approaches, one experimental and the other theoretical, to the elucidation of CH bond length variation due to the intramolecular environment are compared. The experimental method is based on the concept of isolated CH stretching frequencies while the theoretical method involves the ab initio gradient procedure. Provided molecules of not too dissimilar structure are compared agreement between the two methods is encouragingly satisfactory. The methods are to some extent complementary, the ν is CH method being especially applicable to compounds with heavy atoms (e.g. Br, Ge, etc.) and the ab initio method in cases where isotopic frequency data are lacking. The potential applications of this dual technique approach include assessment of the quality of microwave-based geometries, exploration of link between structure and reactivity and prediction of the structure of these basic units in much larger molecules.

A systematic study of the convergence and additivity of correlation and basis set effects on the force constants of small molecules: HF, HCN, and NH3

The quadratic and cubic force field, as well as the dominant quartic force constants of HF, HCN, and NH3 have been evaluated ab initio, using SCF, CI (singles and doubles) and cluster‐corrected CI wave functions. Five different basis sets have been used, ranging in quality from small double‐zeta to triple‐zeta with two sets of polarization functions. Our main conclusions are as follows: (1) Transferability of the correlation contribution between different basis sets is quite poor; in particular, basis sets without polarization functions yield exaggerated correlation contributions. (2) Basis set and correlation effects on the force constants can be separated into an intrinsic and a geometry contribution, with the latter dominating for stretchings. Using an accurate reference geometry (e.g., from a high‐quality calculation) in conjunction with a less accurate wave function for the evaluation of the force fields yields improved results. (3) Correlation contributions to cubic and quartic force constants, part...

The geometry of some amides obtained from ab initio calculations

Abstract Fully optimized geometries, obtained by a new ab initio gradient program, are reported for formamide, N-methylformamide, acetamide, and N-methylacetamide. Within the limits of comparing re with rs or rg structures, agreement with experiment is excellent. After a correction for systematic errors, true equilibrium structures are suggested for the four amides. The structural effects of N-methylation and C-methylation are discussed. It is believed that the re parameters obtained are as accurate as can be obtained by experiment and that the structural variations may be more reliable than the experimental values.

Theoretical prediction of vibrational spectra

The complete harmonic force field and the diagonal and first off-diagonal cubic constants of aniline have been calculated ab initio using a 4–21 basis set augmented by addition of d functions to the nitrogen atom. The force constants were then scaled using scale factors optimized previously to give the best fit to the similarly computed vibrational spectra of benzene and its deuterated isotopomers. The vibrational spectra of aniline, aniline-NHD, and aniline-ND2 were then calculated from this scaled quantum mechanical (SQM) force field and compared with experimentally observed spectra. Several corrections were made to previously proposed empirical spectral assignments. Because of computational difficulties, no definitive statement can be made about the torsion or inversion modes of the amino group. Aside from these and the C-H stretching frequencies for which the detailed assignment is still quite uncertain, the average deviation between the observed frequencies and those obtained entirely from the scaled...

Contributions of resonance, hybridization, and nonbonded interactions to the structure of butadiene

Abstract The fully optimized geometries of four rotational conformers of butadiene have been calculated ab initio in order to investigate the relative importance of resonance, hydridization and nonbonded interactions. The results indicate that the short central CC bond is primarily due to hybridization, although resonance makes a minor contribution. The resonance stabilization of the planar conformers is responsible for the relatively large barrier to internal rotation, whereas the nonbonded interactions are only important in destabilizing the sterically crowded cis conformer.