ALL-TRANS-RETINAL AND 11-CIS-RETINAL, THEIR N-METHYL SCHIFF-BASE AND N-METHYL PROTONATED SCHIFF-BASE DERIVATIVES - A COMPARATIVE AB-INITIO STUDY

Ab initio calculations have been performed on all-trans-retinal, 1, 11 -cis-12-s-cis-retinal, 2, and 11-cis-12-s-trans-retinal, 3, as well as their corresponding N-methyl Schiff bases and protonated N-methyl Sch iff base derivatives. Geometries preoptimized at the RHF/3-21G level w ere 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 Schif f bases, but 1, 3 and 2 for the protonated species. Geometries are in excellent agreement with available X-ray results. Minute details are r eproduced with surprising accuracy, except for the large dihedral angl es at the C6-C7 bond (and the C12-C13 bond for the Il-cis isomers) whe re packing effects may play an important role. Geometry changes from t he aldehydes to the corresponding Schiff bases are negligible, while p rotonation is accompanied by the loss of double bond fixation and an i ncreased tendency towards planarization. The geometry about the C6-C7 bond is distorted 6-s-cis in the aldehydes and the Schiff bases, 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 degrees in 2, 5 degrees in 3 and 0 degrees in the protonate d Schiff base of 3.