MEASUREMENT AND MODEL-CALCULATIONS OF THE VIBRATIONAL CIRCULAR-DICHROISM SPECTRUM OF 6,8-DIOXABICYCLO[3.2.1]OCTANE

The vibrational circular dichroism (VCD) and absorption spectra of 6,8-dioxabicyclo[3.2.1]octane between 800 and 1500 cm-1 are reported. Also presented here for the same region are the complete assignment of the vibrational bands, and the calculated absorption and VCD intensities, based on a previously obtained harmonic ab initio 3-21G force field which was scaled and refined to fit 195 observed frequencies of the title compound and five analogues. Relative absorption intensities are predicted successfully by the atomic polar tensor expression (APT), but not with the fixed partial charge model (FPC). APT also gives correct signs and reasonable absolute VCD intensities for most bands. FPC reproduces most VCD signs but underestimates the intensities considerably, particularly for modes that involve C-O stretching. The low FPC absorption and VCD intensities are substantially improved by adding one parameter, transferred from 2-methyloxetane, that introduces electronic charge flow along the C-O bonds. These model calculations suggest that significant contributions to the vibrational dipole moment derivatives are generated by electronic charge redistributions which can be adequately modeled by a transferred charge flow parameter.