The energetics of a double bond rotation in neutral and protonated polyene Schiff bases is investigated by means of quantum chemical methods. In the case of small neutral and protonated imines the performance of various quantum chemical methods including closed-shell and open-shell Hartree-Fock (HF), closed-shell and open-shell density functional theory (DFT), complete active space self-consistent field (CAS-SCF), and unrestricted natural orbital complete active space (UNO-CAS) is compared to one another. The efficiency of these methods is carefully investigated, and in the protonated case comparison of theoretical and experimental vibrational spectra is also used for the verification of the methods. These evaluations show that the rotation of the double bond neighboring the imine group can be well described at the B3LYP/6-31G(d) or UB3LYP/ 6-31G(d) levels in the case of protonated or neutral Schiff bases, respectively. The calculated rotational barriers in the case of the protonated and neutral N-methylretinal Schiff base are 23.5 and 35.7 kcal/mol, respectively. The results are interpreted by various mesomeric structures of the trans and rotated species.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Surfaces, Coatings and Films
- Materials Chemistry