Influence of the Methyl Groups on the Structure, Charge Distribution, and Proton Affinity of the Retinal Schiff Base

The effect of the methyl substitutions at different carbon atoms of the polyene chain of the retinal Schiff base has been studied on the structure, charge distribution, and proton affinity (PA) in a Schiff base model with the same number of conjugated double bonds (six conjugated double bonds including the Schiff base group). The methyl groups were added to the nitrogen atom and/or different carbon atoms along the main chain, and the results were compared to those of the retinal Schiff base. To study the effect of the polarization functions on hydrogen atoms, all of the structures were studied using 6-31G* and 6-31G** basis sets at the Becke3LYP (B3LYP) level of theory. For each optimized model, the PA was then calculated on the basis of the difference of the total energy of the protonated and unprotonated species. The influence of the zero-point energy corrections on the PA values was also examined at the same level of theory. The results show that, for all species, the application of the larger basis se...