A detailed theoretical study of the difluoromethane molecule

Citation

Carhart, Raymond E. (1973) A detailed theoretical study of the difluoromethane molecule. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/0SRB-RQ87. https://resolver.caltech.edu/CaltechTHESIS:03042011-095512877

Abstract

The first chapter describes our theoretical investigation of the potential energy surface of the difluoromethane molecule. The Hartree-Fock (HF) method, with a 73/3 gaussian basis contracted to the double-zeta level, was used, and in many cases, CNOO/2 calculations were included for comparison. The optimum HF geometry is found to be closer to experiment than that reported by other workers using a minimum (STO-3G) basis set, but it appears that our more flexible basis does little to improve the computed general harmonic force constants, the complete set of which is considered. The stretching constants are found to be in error by +20% to +35%, the bending constants by -4% to +45%. In comparison with HF, the CNOO/2 method grossly overestimates the stretching constants, but mimics rather well the bending and interaction constants. The theoretical (HF) normal modes and observed vibration frequencies are combined to give a set of semi-empirical force constants (SEFC's) which are used to predict the vibration frequencies of the deuterated difluoromethanes. The synthesis and IR spectrum analysis of these compounds is described, and the SEFC predictions are found to be superior to ones appearing previously in the literature. The Urey-Bradley potential (UBP) model, with 1/r^6 steric terms, is fit to the HF constants and SEFC's. A comparison of the two UBP models indicates that the HF method consistently over estimates all parameters but the F-F steric term, which it underestimates. Anharmonicity in the angular coordinates for large molecular distortions is investigated, and it is found that CNOO/2 mimics HF quite well, except that CNOO/2 under estimates the anharmonicity when the fluorines are quite close together. The UBP model derived from the HF force constants is found to account for most of the anharmonicity in the HF energy variation. The second chapter describes our investigation of the electronic structure of difluoromethane. The HF method, with the basis set discussed above, and certain configuration-interaction methods, were used. The localized (HF) molecular orbitals (LMO's) were obtained for the equilibrium geometry using a new, quadratically convergent approach which is useful for cases in which convergence of the Edmiston-Ruedenberg "two-by-two" method is slow. The LMO's are examined in detail, and several methods are used to show that the fluorine lone pairs are delocalized toward carbon, a delocalization which represents an important stabilization in the molecule. It is noted that this effect, which is most pronounced for lone pairs lying in the F-C-F plane, may be the molecular-orbital equivalent of the "double bond-no bond" resonance of valence-bond theory. An analysis of the LMO's for distorted geometries indicates that the "orbital following" concept does not apply to difluoromethane as the F-C-F angle is altered. An economical approximation to the generalized valence-bond (GVB) method is developed and is used to give a more detailed picture of the electron pairs in the molecule. The GVB-like pairs are localized, but in this case the localization is a result of the variation principle rather than a physically meaningless localization criterion. They are used to define (in an apprOXimate fashion) "naturally" localized Hartree- Fock orbitals (NLMO's) qualitatively similar to the LMO's. An analysis of the NLMO's supports the conclusions drawn from the LMO analysis concerning lone-pair delocalization and "orbital following".

Thesis Files