Knowledge Bank

Vibrational Circular Dichroism (VCD) Spectra are reported for a variety of chiral molecules isolated in argon matrices at $\sim 18\circ K$. These are the first observations of VCD of matrix-isolated molecules. Spectra are limited to the C-H stretching region $(2800-3100cm-1)$. In all cases, VCD spectra are substantially more structured than the corresponding room-temperature spectra, as a result of much narrower line widths. Further, VCD magnitudes are greater due to decreased cancellation of overlapping transitions with oppositely signed VCD. Where practicable, experimental data are compared with theoretical calculations. Particular attention is focussed on 3-methyl cyclohexanone, whose VCD has been the subject of several theoretical analyses, using the ``fixed partial charge'' model. In all cases where comparison is feasible, observed anisotropy ratios are an order of magnitude greater than those predicted. Earlier conclusions regarding the validity of theoretical models for vibrational rotational strengths therefore require substantial re-evaluation.