All-trans- and 11-ds-retinal, their AT-methyl Schiff base and TV-methyl protonated Schiff base derivatives: a comparative ab initio study

摘要

initio calculations have been performed on all-frans-retinal, 1, ll-c2>12-s-cw-retinal, 2, and ll-cii-12-s-fra/w-retinal, 3, as well as their corresponding A-methyl Schiff bases and protonated //-methyl Schiff base derivatives. Geometries pre-qplimized at the RHF/3-21G level were fully optimized with the RHF/6-31G** basis set. The stabilities in order of increasing energy are 1, 2 and 3 for the aldehydes and Schiff bases, but 1,3 and 2 for the protonated species. Geometries are in excellent agreement with available X-ray results. Minute details are reproduced with surprising accuracy, except for the large dihedral angles at the C6-C7 bond (and the C12-C13 bond for the 11-cis isomers) where packing effects may play an important role. Geometry changes from the aldehydes to the corresponding Schiff bases are negligible, while protonation is accompanied by the loss of double bond fixation and an increased tendency towards planarization. The geometry about the C6-C7 bond is distorted 6-s-cisin the aldehydes and the Schiff basest while in the protonated Schiff bases an additional almost planar 6-s-trans conformation is found. The out-of-plane deformation about the C12-C13 bond is 49° in 2,5° in 3 and 0° in the protonated Schiff base of 3.