Ab Initio Study of 13C Shieldings for Linear π-Conjugated Systems. Theoretical Determination of the C12−C13 Conformation in the Chromophore of Rhodopsin

Abstract

The ab initio shielding calculations are carried out to investigate the conformation dependence of 13C chemical shifts for conjugated compounds like the chromophore of a visual pigment rhodopsin (Rh). First, the calculations are applied systematically to 10 diene derivatives to obtain basic and universal relationships between their conformation and the shieldings of unsaturated carbons. It is indicated that the conjugated carbons are classified into two types according to the profiles of conformation dependence of the shieldings. The shieldings of the carbons forming the rotating bond exhibit complicated angular dependence. It is rigorously evidenced that the behavior of such carbon shieldings can be understood by considering the effect of π-orbital modification, a new concept introduced here. On the other hand, the shieldings of the other carbons essentially follow well-known mechanisms including the steric and charge density effects. One of the most important findings is that the steric effects are refl...