A study on the conformational space of the all-trans retinal deprotonated Schiff base

Abstract

The conformational space of the all-trans retinal deprotonated Schiff base (DSBT) molecule around its four internal backbone torsional angles was studied. A total of eight different conformers were detected from potential energy surface exploration corresponding to the four internal backbone torsional angles. The standard split-valence 6-311++G(d,p) basis set in combination with MP2 andB3LYP levels were carried out to analyze the structural characteristics (relative energies, dipole moments and bond length alternation) and spectral properties (electronic absorption spectrum and Raman spectrum) of the conformers. Results presented in this paper showed that the relative stability order of the conformers might be independent of the level of theory used. It was also reconfirmed that the TD-DFT method would systematically underestimate the excitation energy by 38.60–67.54kJmol−1; a trend had been reported in several previous studies. The characteristic peak nearby 1600cm−1 corresponding to the ECC mode could be notably observed from the predicted Raman spectrum calculated at both the MP2andthe B3LYP level of theories. Moreover, the spectral profile acquired based on the B3LYP level was more apparent and distinguishable than that from the MP2 method. It is suggested that B3LYP level may be a proper choice for theoretically study of Raman spectrum, especially concerning the polyene conjugated systems.