Theoretical study of the low-lying excited singlet states of furan

The lowest two Rydberg and two pi-pi* valence excited singlet states of furan [referred to as (1)A(2)(3s), B-1(1)(3p) and B-1(2)(V), (1)A(1)(V'), respectively, at the C-2v ground-state molecular configuration] have been studied using the equation-of-motion coupled-cluster singles and doubles method (EOM-CCSD). Full geometry optimizations with subsequent computation of harmonic vibrational frequencies have been performed in order to locate and characterize stationary points on the potential energy surfaces (PES). The latter optimization work was enabled by the availability of analytic energy gradient techniques for the EOM-CCSD approach. A major new finding is that both the B-1(2)(V) and (1)A(1)(V') valence states are unstable with respect to non-totally symmetric distortions at the C-2v configuration. The symmetry breaking in the B-1(2)(V) state involves an in-plane coordinate of b(2) symmetry. The relaxation process begins on the S-2 adiabatic PES and, after passing through a conical intersection of the S-2 and S-1 PES, continues on the S-1 surface, taking the system finally to the adiabatic minimum of S-1 ((1)A(2) state). The (1)A(1)(V') valence state is found to be unstable with respect to the out-of-plane bending coordinates of b(1) and a(2) symmetry. The resulting relaxed molecular structures have C-s and C-2 symmetry, respectively. The present findings are analyzed in terms of a linear vibronic coupling model and spectroscopic implications are discussed. (C) 2003 American Institute of Physics.