Creation of intensity theory in vibrational spectroscopy: Key role of ab initio quantum mechanical calculations

Abstract

The development of effective theoretical approaches for analytic evaluation of first, second, and third derivatives of molecular properties, in particular energy, dipole moment, and polarizability, has contributed to increased accuracy of ab initio methods in predicting vibrational spectral parameters. It has become possible to devise and test theoretical approaches for analysis of vibrational intensities using consistent and reliable results from high-level ab initio calculations. There is a second important side of the application of ab initio molecular orbital (MO) calculations to the study of vibrational intensities. As for many other physical and chemical phenomena, the quantum mechanical studies provide essential information about the fine mechanisms and factors determining the observed physical quantities, in our case the intensities in infrared and Raman spectra of molecules. On this basis, new theoretical formulations for infrared and Raman intensities were recently developed. Infrared intensities are transformed into quantities termed effective bond charges that are derived from dipole moment derivatives with respect to atomic Cartesian coordinates. Raman intensities are reduced to effective induced bond charges following an analogous approach. In the present work we review the relation between advances in analytic derivative ab initio quantum mechanical calculations and the development of methods for analysis and interpretation of experimental or ab initio dipole and polarizability derivatives. Brief presentation of the effective bond charge and effective induced bond charge formulations is given. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 331–339, 1998