Previtamin D conformations and the wavelength-dependent photoconversions of previtamin D

Conformational abundances calculated at different levels of theory have bee n used to derive spectral and photophysical properties of a previtamin D mo del compound. The individual conformer spectra of absorption are approximat ed by Gaussian functions using excited state characteristics calculated wit h the QCFF/sol method. The calculated total UV spectrum for 3-desoxy-previt amin D model compound is found to be in a good agreement with experiment. T he ratio of quantum yields for the cis-trans and ring-closure photochemical reactions of previtamin D is in excellent agreement with experiment in the long-wavelength spectral region, suggesting that lower-energy excitation p hotochemistry is conformationally controlled. We postulate that the observe d sudden change in the relative efficiency of cis-trans vs. ring-closure ph otoisomerization of previtamin D around 300 nm reflects the opening of new excited state dynamical channel, possibly involving intersystem crossing of the system to excited triplet states. (C) 2001 Elsevier Science B.V. All r ights reserved.