Effect of protonation on the ground state properties of retinal analogs: an ab-initio study

Abstract

The ground state properties (bond lengths, bond orders, net atomic charge distribution) of various cis and trans isomers of retinal analogs have been studied at abinitio SCF and correlated levels. The effect of protonation on the properties of the retinal Schiff base analogs has also been studied. Convergence in various properties has been observed with increasing chain length justifying the use of smaller analogs mimicking retinal. Convergence is, however, slower for the protonated retinal Schiff base analogs. The protonated retinal Schiff base analogs show an increased conjugation reflected in the bond orders, bond lengths, and in the decrease in the HOMO–LUMO gap. The nodal characteristics of the HOMO of the protonated retinal Schiff base analog also indicate a strong conjugation in the vicinity of the protonated nitrogen. Based on the results of calculations at the correlated level, the sudden polarization charge transfer mechanism is shown to be an artifact of HF-based methods used in the calculations.