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 vibrational TCD map is a vector-field plot over a grid of points of the integrand of the electronic contribution to the velocity-form electric dipole transition moment. TCD plots allow visualization of the flow of electron density produced by nuclear motion, which can be used to identify angular and circular charge flow leading to the electronic magnetic dipole transition moment contribution to vibrational circular dichroism (VCD). For the methine stretch in (S)-methyl-d(3) lactate-Cd-3, nuclear motion is largely confined to the two atoms of the methine bond, but the electron current density is distributed throughout the molecule. The methine-stretching VCD intensity calculated for the (S)-methyl-d(3) lactate-Cd-3 conformers is interpreted in terms of the relative contributions from the nuclear electric- and magnetic-dipole transition moments, and from linear electron charge flow, circular electron charge flow about carbon and oxygen centers, and angular charge flow across groups of atoms. In this example, we find no evidence to support the existence of vibrational ring currents through hydrogen bonds, as proposed earlier based on empirical evidence, although the empirical correlations of structure to VCD still hold for such intramolecularly hydrogen-bonded molecules.