Ab initio and LMO studies of integrated intensities of infrared absorption bands of polyatomic molecules. Part 1.—Method, application to cyanoacetylene and comparison with semi-empirical “all-valence” theories

Abstract

A general scheme is described for a non-empirical quantum chemical interpretation of the integrated intensities of infrared absorption bands. The localized molecular orbital (LMO) technique is used to calculate bond moment derivatives with respect to the normal coordinates. A second general decomposition scheme of (ab initio) calculated molecular dipole moments, into point charge, hybridization and homopolar terms, is also presented.

The theory is applied to the stretching modes of cyanoacetylene. The calculations are performed with three fundamentally different types of SCF-wavefunctions (INDO, INDO/D and STO-NG). The canonical molecular orbitals are localized with an INDO approximate charge density localization technique. Two different force fields were used in the calculation of the normal coordinates.

As opposed to the STO-NG functions, neither INDO, nor INDO/D functions yield satisfactory results for the calculated intensities. The influence of the force field on the calculated intensity seems to be of minor importance compared with the influence of the wavefunction.

Quantum chemical evidence is presented for the failure of the classical bond moment hypothesis for stretching modes. The dipole moment decomposition scheme provides sound evidence for the importance of the hybridization and homopolar terms in ab initio calculated intensities. LMO-analysis of the (∂µ/∂Q_i)₀ values led to an understanding of the combined effect of vibrational and electronic factors on the integrated intensity.