On the CH bond dipole moment in alkanes

Abstract

The Thomas–Reiche–Kuhn sum rule, within the acceleration gauge for the transition dipole moment, is used to partition the total number of electrons in a molecule and to define atomic populations, which can be related to corresponding experimental estimates of atomic polar tensors from IR intensities. As a bond dipole moment can only be defined for diagonal atomic polar tensors, it is shown that the assumption of a C–H bond moment, transferable from molecule to molecule in the alkane series, is physically unreliable. From experimental IR intensities of methane we infer that, for the equilibrium geometry, μC–H =0.339 D, directed C−H+. Accordingly, it is argued that the theoretical bond dipole moments, estimated for the C–H bond in methane via localization procedures of SCF wave functions, are questionable, as they predict the opposite polarity. Finally, a resolution of the electric dipole moment into atomic contributions is suggested.