ABINITIO STUDY ON THE EXCITATION-ENERGIES OF THE PROTONATED SCHIFF-BASE OF 11-CIS-RETINAL

Ab initio computational results from MRSD-Cl with split-valence basis set plus d orbitals predict that the first excited of the protonated Schiff base of retinal (PSBR) is the ionic, strongly allowed, 1Bu state. Our best estimate for the excitation energy of this state of the isolated PSBR cation is 2.7 ± 0.5 eV, in agreement with the solution spectra. The higher singlet excited states of PSBR were found to be mixed in bonding character, and only a vague line can be drawn between the ionic and covalent states. In contrast to the conventional assignment of the spectra of PSBR, which assumes the second excited state to be covalent 1Ag, this state was found to be mostly ionic. It is the third excited state that has a wave function closer to that of the covalent 1Ag state, although the two-electron excitation from π6 to π7* only makes a small contribution to its wave function. The fourth excited state calculated was also a covalent state and 1Ag-like. The wave functions of the two ionic excited states computed are essentially monoconfigurational, and those of the two covalent excited states are both multiconfigurational. The ground-state charge density concentrates on the N16, C15, C13, and C11 atoms. It is found that there is a large decrease of bond alternation from the ground state to the 1Bu state. The overall overlap population becomes smaller and smaller for the higher excited states. Comparison of the excited states of PSBR and dodecahexaene at the π SDTQQS-CI/STO-3G level proves that the nature of the excited states of PSBR is very different from that of dodecahexaene in this approximation. © 1990 American Chemical Society.