SKELETAL VIBRATIONAL CIRCULAR-DICHROISM OF A SERIES OF BICYCLIC DILACTONES - THE FINGERPRINT REGION

The vibrational circular dichroism (VCD) spectra of 2,5-dioxabicyclo[2 .2.1]heptane-3,6-dione 1 (R = H) and the 1,4-dialkyl derivatives, 1 (R = Me) and 1 (R = t-Bu), were calculated by the ab initio magnetic fie ld perturbation (MFP) procedure with a B3LYP/6-31G force field, and t he VCD spectra of the dimethyl and di-t-butyl derivatives were measure d in the region 800-1500 cm(-1). While the absolute configurations of 1 (R = Me) and 1 (R = t-Bu) could be assigned unambiguously by compari son of the experimental and predicted spectra, there is little obvious correspondence between the spectra of the series of compounds. The VC D spectra are analysed on the basis of operation of a coupled oscillat or mechanism involving motions of the polar bonds. In the case of the parent dihydro system, bridgehead C-H bond motions appear to dominate contributions to the VCD of vibrational transitions into which they ar e mixed.