The optical activity of the pyridine chromophore: MO analysis of the circular dichroism spectrum of 5α-cholest-2-eno[3,2-b]pyridine

Abstract

5α-Cholest-2-eno[3,2-b]pyridine (III), a chiral molecule containing the pyridine chromophore, was synthesized, and its c.d. spectrum measured between 195 and 300 nm. An ab initio computation of the optical activity was carried out on 5,6,7,8-tetrahydroquinoline, which has P chirality, and which was considered as the most important fragment in determining the optical activity of (III). Good agreement was found between the calculated values and the experimental c.d. spectrum of (III) for all accessible transitions. The low energy n→π* is red-shifted relative to the first π→π* transition and no crossing of the two transitions occurs. No substantial contribution to the optical activity is given by the n→π* transition at higher energy. The contributions of a one-electron mechanism to the optical activity of the n→π* and first π→π* transitions are briefly discussed.