COMPUTATIONAL EVALUATION OF THE COUPLED OSCILLATOR MODEL IN THE VIBRATIONAL CIRCULAR-DICHROISM OF SELECTED SMALL MOLECULES

A comparison is made between the predictions of the dynamic coupling or coupled oscillator model of vibrational circular dichroism (VCD) and those of the a priori magnetic field perturbation (MFP) theory as formulated by Stephens. For two independent formaldehyde molecules at reasonably large separations, both calculations are in agreement for the C=O VCD; but deviations develop at short separations. In addition, calculated results for seven different cyclopropane- or cyclobutane-based molecules made chiral by trans-dideuterio substitution are presented. For identical C-H and C-D stretch oscillators, the hydrocarbons studied gave MFP computed VCD spectra that are qualitatively predictable with the coupled oscillator model, but the MFP magnitudes were uniformly larger and exhibited a progressive degree of inequality. However, the coupled oscillator model failed for some oxygen-containing molecules studied, implying that its application is far from universal.