Thomas-Reiche-Kuhn populations in alkanes

Atomic populations in a molecule have been defined via the 'Thomas-Reiche-Kuhn sum rule for oscillator strengths written within the acceleration gauge. These atomic populations are related to nuclear electric shieldings, i.e., to geometrical derivatives of electric dipole moment, and can therefore be connected with observable infrared intensities. A number of relationships can be considered to test a priori the quality of calculated electronic charges and to assess their physical meaning. It is shown via extended numerical tests on the first members of the alkane series that the Thomas-Reiche-Kuhn populations are consistent with a (small) polarity C+-H- of carbon-hydrogen bond in methane, for which a bond dipole moment can be exactly defined. Although the idea of bond dipole cannot be extended to the C-H fragments belonging to other alkane molecules in the absence of local C-3v symmetry, the calculations prove that the same electron charge polarization should characterize the whole homologous series. (C) 1999 Elsevier Science B.V. All rights reserved.