Conformation and absolute configuration of 11,12-cyclopropyl retinal analogs - an RHF/DFT/CIS study
Ab initio RHF and DFT/B3LYP calculations at the 6-31G** level have been performed to study possible conformations of the cyclopropyl retinal Schiff base analog 3 of known absolute configuration. In both the free base and the protonated form, the geometries are determined on the diene side by optimum conjugative interaction with the three-membered ring, on the triene side by repulsive interaction with the 9-methyl group. There are three low energy conformations, in which the seven-membered ring is either in a chair or in a twist-chair conformation. To decide between these alternatives, chiroptical parameters were calculated employing the GAUSSIAN/CIS routines and compared with the CD spectrum obtained by Nakanishi et al. Of the energy-minimized geometries only two fit the experimental data. In both, the dihedral angle C12-C13, which is indicative of the relative orientation of the two chromophores, is positive.
retinal analogs retinal gaussian/cis ab initio calculations