Rotational circular dichroism of diamagnetic and paramagnetic molecules. A computational study.

Abstract

Rotational circular dichroism (RCD) has not been observed yet, but it is expected to deliver information about chiral molecules useful in many branches of chemistry. In the past, rather weak RCD intensities were predicted for model diamagnetic molecules and a limited number of rotational transitions. Here, we review quantum-mechanical foundations and simulate entire spectral profiles, including larger molecules, open-shell molecular radicals, and high-momentum rotational bands. Contribution of the electric quadrupolar moment was considered, but it turned out that it does not contribute to field-free RCD. Two conformers of a model dipeptide provided clearly distinct spectra. The dissymmetry Kuhn parameter gK predicted for the diamagnetic molecules even for high-J transitions rarely exceeded 10-5, and the simulated RCD spectra were often biased to one sign. In the radicals, the coupling of the rotational angular momentum with the spin for some transitions raised gK to about 10-2, and the RCD pattern was more conservative. In the resultant spectra, many transitions had negligible intensities due to small populations of the involved states, and a convolution with a spectral function made the typical RCD/absorption ratios about 100-times smaller (gK ∼ 10-4). This is still comparable with values typical for electronic or vibrational circular dichroism, and paramagnetic RCD is thus likely to be measurable relatively easily.