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

• Published in: Journal of molecular structure. Theochem Vol. 467; no. 3; pp. 195 - 210
• Main Authors: Dmitrenko, Olga, Vivian, James T, Reischl, Wolfgang, Frederick, John H
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 30.07.1999
• Subjects: Ab initio calculations; Conformers; E-isomers; Previtamin D; QCFF semiempirical calculations; Singlet state; Vitamin D; QCFF semiempirical calculations; Vitamin D; Conformers; Previtamin D; E-isomers; Singlet state; Ab initio calculations
• ISSN: 0166-1280; 1872-7999
• DOI: 10.1016/S0166-1280(98)00489-8

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 5E-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 5E-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/π 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.