Vibrational and Electronic Circular Dichroism

Vibrational and electronic circular dichroism (VCD and ECD) spectra are used to determine the chirality of molecules. The VCD or ECD spectrum of a molecule is the mirror image of the VCD or ECD spectrum of its enantiomer, and the two spectra can therefore be easily distinguished. The assignment of chirality can be done by comparing the calculated spectrum [269, 270], for which the chirality is known, with the experimental spectrum. To calculate ECD spectral data requires a singlet TDDFT calculation in the Tamm-Dancoff approximation (see Density Functional Theory).

As well as information in the output file, the frequencies or energies and the intensities of the transitions are written to a spectrum file, called jobname_vcd.spm for VCD spectra and jobname_ecd_velocity.spm or jobname_ecd_length.spm for ECD spectra, depending on the formalism. The file can be read by the Spectrum Plot Panel in Maestro, which generates a simulated spectrum. As VCD calculations involve both the frequencies and a magnetic field, the thermochemical properties and the NMR shieldings are computed as a byproduct. It also follows that any changes to the settings for the frequencies affect the VCD spectrum.

Because conformers of a molecule have significantly different VCD and ECD spectra, it is important to compute a conformationally averaged spectrum when the molecule of interest has low-lying conformers. To generate and refine conformers, use MacroModel to perform the conformational search, refine the geometries of each conformer with Jaguar, then eliminate high-lying or redundant conformers with MacroModel. Then you can calculate their VCD or ECD spectra, and use the Spectrum Plot Panel to generate an averaged spectrum using Boltzmann weights based on Jaguar’s conformational energies. This entire procedure, including generation of an averaged spectrum, is automated in the Circular Dichroism Workflow—see Jaguar Spectroscopy Panel and Vibrational and Electronic Circular Dichroism Spectra. The Spectrum Plot Panel is described in Plotting Spectra.