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 strengt h, Franck-Condon and 0-0 transitions) of isomeric seco-steroids, 3-desoxy-a nalogs 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 f or vitamin D and its SE-isomer. Comparison of UV absorption characteristics of previtamin D with calculations of 3-desoxy-previtamin D conformers indi cates the need for detailed modeling of the bandshape to attain good correl ation with experiment. The failure of the semiempirical approach in the cas e of pre-vitamin D is also reflected in the poor agreement with ab initio r esults for the relative ground state conformer energies. This problem can b e attributed to the importance of CH/pi interactions in the system which ar e followed via Mulliken population analysis. Using a Boltzmann conformation al distribution based on the ab initio relative energies of 3-desoxy-previt amin D conformers in combination with QCFF/sol results on the absorption ch aracteristics leads to good agreement with experiment. (C) 1999 Elsevier Sc ience B.V. All rights reserved.