The ground state ((1)A(1)) and the lowest triplet state (B-3(1)) of the phenyl cation C6H5+ revisited

Ionization of the phenyl radical C6H5.((2)A(1)) is examined by high level theoretical methods. A systematic comparison of MCSCF, B3LYP, and CCSD(T) results shows that the ground state of the phenyl cation C6H5+ is definitely the singlet ((1)A(1)) state and that the associated adiabatic ionization energy for the transition C6H5.((2)A(1)) --> C6H5+((1)A(1)) is estimated to be 8.1 +/- 0.1 eV. The lowest triplet state C6H5+(B-3(1)) is approximately 0.8 eV higher than the singlet ground state in energy. However, theoretical geometries of the phenyl radical and cations suggest that formation of the triplet cation is favored in photoionization experiments because the Franck-Condon factors are much larger for the transition C6H5.((2)A(1)) --> C6H5+(B-3(1)) than for that of the singlet ground state C6H5+((1)A(1)). By evaluating the Franck-Condon factors for both photoionization processes [i.e., ((2)A(1)) --> C6H5+((1)A(1)) and ((2)A(1)) --> C6H5+(B-3(1))] of C6H5. and C6D5., attempts were made to assign the reported photoelectron spectra. (C) 1997 American Institute of Physics.