Examination of charge alternation in CH4 and CH3F from ab initio LCAO SCF MO wavefunctions and their localized bond orbitals

Abstract

The question of whether or not fluorine substitution produces charge alternation is examined for CH4 and CH3F. Two sets of ab initio LCAO SCF MO wavefunctions (one a 3 G STO based one, the other a double zeta based one) are analyzed via charge density, localized CH bond moment, and population analysis calculations. Although both sets of wavefunctions show a slightly more negative H region in CH3F relative to CH4, in qualitative agreement with earlier work by Pople et al., the differences are small, and their sources are not clear. For example, in the 3 G calculations the CH localized orbital is the essential source of the increased density in CH3F, while for the double zeta calculations the increased density is due to the tail of an F lone-pair orbital trans to the CH bond. Consideration of details of these studies as well as those from large STO based SCF MO wavefunctions by Arrighini et al., suggests that one will need very accurate wavefunctions to resolve the problem unambiguously.