Effects of Point Charges on the Excitation Energies of Protonated Schiff Base of Retinal.

Effects of point charges on the excitation energies of protonated Schiff base of retinal (PSBR) in gas phase have been investigated by means of time-dependent density functional theory (TD-DFT) method in order to shed light on the electrostatic effects on the absorption spectra of PSBR. The positive and negative charges around PSBR were tested for 11-cis of PSBR. The TD-DFT calculations indicated that the first and second excitation energies are blue-shifted by the existence of the negative charge around the N-H site of PSBR, whereas those are red-shifted by positive charge. On the other hand, the spectral feature in β -ionone ring was much different from that of the N-H site: the first and second excitation energies are red-shifted by the negative charge. The spectral feature is drastically changed at the polyene part of PSBR. It was found that the excitation energies of PSBR are affected strongly by the electrostatic environment. The effects of electrostatic interaction on the absorption spectra of PSBR are discussed on the basis of theoretical results. [ABSTRACT FROM AUTHOR]