Vibrational transition current density in (S)-methyl lactate: Visualizing the origin of the methine-stretching vibrational circular dichroism intensity

Vibrational transition current density (TCD) plots for the methine stretch of (S)-methyl-d(3) lactate-Cd-3 in four conformations are presented. A vibr ational TCD map is a vector-field plot over a grid of points of the integra nd of the electronic contribution to the velocity-form electric dipole tran sition moment. TCD plots allow visualization of the flow of electron densit y produced by nuclear motion, which can be used to identify angular and cir cular charge flow leading to the electronic magnetic dipole transition mome nt contribution to vibrational circular dichroism (VCD). For the methine st retch in (S)-methyl-d(3) lactate-Cd-3, nuclear motion is largely confined t o the two atoms of the methine bond, but the electron current density is di stributed throughout the molecule. The methine-stretching VCD intensity cal culated for the (S)-methyl-d(3) lactate-Cd-3 conformers is interpreted in t erms of the relative contributions from the nuclear electric- and magnetic- dipole transition moments, and from linear electron charge flow, circular e lectron charge flow about carbon and oxygen centers, and angular charge flo w across groups of atoms. In this example, we find no evidence to support t he existence of vibrational ring currents through hydrogen bonds, as propos ed earlier based on empirical evidence, although the empirical correlations of structure to VCD still hold for such intramolecularly hydrogen-bonded m olecules.