MRCI calculations of the ground and excited state potential energy surfaces of the 2,4-pentadien-1-iminium cation

Quantum chemical calculations of the electronic ground and first excited singlet states S-0 and S-1 of the protonated Schiff base 2,4-pentadien-1-iminium cation (CH2=CH-CH=CH-CH=NH2)(+) are presented. To compare different multireference CI approaches that differ in their choice of reference configurations and basis sets, potential energy surfaces with respect to two dihedral angles have been calculated for the states S-0 and S-1. The study reveals that the characteristic features of the two potential energy surfaces, i.e., the appearance of two minima and four maxima in the case of the S-0 surface and of three maxima and two minima in the case of the S-1 surface, are correctly predicted at all applied levels of theory: The energies of torsional barriers and of higher energy maxima, however, depend considerably on the applied quantum chemical procedure. Ground state calculations are also compared to the results of Moller-Plesset perturbation theory calculations up to the MP4 level.