Excitation Spectra of Cation and Anion Radicals of Several Unsaturated Hydrocarbons: Symmetry Adapted Cluster-Configuration Interaction Theoretical Study

Abstract

Excitation spectra of cation and anion radicals of unsaturated hydrocarbons, hexatriene(±), octatetraene(±), cyclopentadiene(+), 1,3-cyclohexadiene(+), and naphthalene(±), were studied by the symmetry adapted cluster-configuration interaction (SAC-CI) method. The calculated results reasonably reproduced the experimental spectra observed by one of the authors (T.S.) and gave reasonable assignments. The two bands observed in the experimental spectra were assigned to π-π(SOMO) and π(SOMO)-π* for the cation radicals and π*(SOMO)-π* and π-π*(SOMO) for the anion radicals, in order of increasing energy, except for naphthalene(±). The four bands of naphthalene(+) originated from π-π(SOMO), π-π(SOMO), π(SOMO)-π*, and π(SOMO)-π* and those of naphthalene(-) originated from π*(SOMO)-π*, π*(SOMO)-π*, π-π*(SOMO), π*(SOMO)-vπ*, and π-π*(SOMO), in order of increasing energy. The SOMO orbitals were involved in the intense bands of both cation and anion radicals. Moreover, the ionization energies (IEs) and electron affinities (EAs) of these hydrocarbons were in good agreement with the experimental values, whereas the EAs of hexatriene and octatetraene were predicted to be negative and positive, respectively. The calculated IE + EA values were nearly constant for the three π-π* pairing states of hexatriene(±), octatetraene(±), and naphthalene(±), indicating that the pairing theorem is valid even at the SAC-CI level.