Infrared intensities and the polar properties of bonds

Abstract

The derivatives of a molecular electric dipole moment with respect to symmetry coordinates can be rigorously evaluated from vibrational band intensities; any such derivative can be expressed as the sum of a zero-order term and a polarization term, the zero-order term being unambiguously defined in terms of polar parameters of the valence bonds. The estimation of the polarization term by ab initio molecular-orbital calculations is discussed. It is shown that the estimate will be basis-independent only if (i) the polarization term is calculated from properly normalized eigenvectors and (ii) the wavefunction is constrained to give correctly the overall dipole moment derivatives. The calculation is applied to the CH₄ molecule and it is shown that the infrared intensities for this molecule are consistent with a bond moment µ^B of 0.71 D and a derivative of µ^B with respect to the corresponding bond length 1.86 D Å^–1, both quantities being positive in the sense C^–H⁺.