THEORY OF VIBRATIONAL ROTATIONAL STRENGTHS - COMPARISON OF A PRIORI THEORY AND APPROXIMATE MODELS

The atomic polar tensor (APT), fixed partial charge (FPC), and coupled oscillator (CO) equations for vibrational rotational strengths have been used in the analysis of vibrational circular dichroism (VCD) spectra. The accuracy of these equations is examined by comparison of their predictions for a diverse set of eight, small, chiral molecules to those previously obtained using Stephens' a priori theory of vibrational rotational strengths. The atomic polar and axial tensors central to the latter were evaluated ab initio at the SCF level of approximation, using large basis sets and (in the case of the atomic axial tensors) the distributed origin gauge. Rotational strengths predicted by the APT, FPC, and CO equations are in each case in very poor overall agreement with the predictions of the a priori theory. The causes of the differences are analyzed. The continued use of the APT, FPC, and CO equations is not supported and conclusions arrived at previously resulting from the analysis of VCD spectra using these equations should be reexamined. © 1990, American Chemical Society. All rights reserved.