Theoretical studies of the first strongly allowed singlet states of 3-desoxy analogs of previtamin D, vitamin D, and their E-isomers

The characteristics of the first singlet excited states (oscillator strength, Franck-Condon and 0-0 transitions) of isomeric seco-steroids, 3-desoxy-analogs of vitamin D (6), previtamin D (8), and their SE-isomers (7,9), have been calculated using the semiempirical QCFF method. The results obtained have been compared with the available experimental observations, as well as ground state ab initio calculations at the HF/6-31G level and agree well for vitamin D and its SE-isomer. Comparison of UV absorption characteristics of previtamin D with calculations of 3-desoxy-previtamin D conformers indicates the need for detailed modeling of the bandshape to attain good correlation with experiment. The failure of the semiempirical approach in the case of pre-vitamin D is also reflected in the poor agreement with ab initio results for the relative ground state conformer energies. This problem can be attributed to the importance of CH/pi interactions in the system which are followed via Mulliken population analysis. Using a Boltzmann conformational distribution based on the ab initio relative energies of 3-desoxy-previtamin D conformers in combination with QCFF/sol results on the absorption characteristics leads to good agreement with experiment. (C) 1999 Elsevier Science B.V. All rights reserved.