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 excit ed singlet states S-0 and S-1 of the protonated Schiff base 2,4-pentad ien-1-iminium cation (CH2=CH-CH=CH-CH=NH2)(+) are presented. To compar e different multireference CI approaches that differ in their choice o f reference configurations and basis sets, potential energy surfaces w ith respect to two dihedral angles have been calculated for the states S-0 and S-1. The study reveals that the characteristic features of th e two potential energy surfaces, i.e., the appearance of two minima an d four maxima in the case of the S-0 surface and of three maxima and t wo minima in the case of the S-1 surface, are correctly predicted at a ll applied levels of theory: The energies of torsional barriers and of higher energy maxima, however, depend considerably on the applied qua ntum chemical procedure. Ground state calculations are also compared t o the results of Moller-Plesset perturbation theory calculations up to the MP4 level.