HeI photoelectron and ab initio study of retinal derivatives

The HeI photoelectron spectra of all-trans-retinal, 13-cis-retinal and 9-ci s-retinal have been observed in the gas phase by using an infrared heating device, which was developed in this laboratory. The first four ionization e nergies (I-obs) of all-trans-retinal have been determined to be 7.55, 8.40, 9.21, and 10.03 eV, and the ionization energies for the other two derivati ves were also equal to them within the experimental error. Ab initio calcul ations were performed to study the optimized structures of neutral and cati onic ground states of retinals and to elucidate the valence electronic stru ctures associated with the observed ionization energies. The structural cha nge from the neutral to the cation was considered, and the bond lengths and dihedral angles between the planes of the beta -ionone ring and the polyen e chain in each of retinals have been determined.