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

Abstract

The lowest two Rydberg and two π–π* valence excited singlet states of furan [referred to as A11(3s), B11(3p) and B12(V), A11(V′), respectively, at the C2v 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 B12(V) and A11(V′) valence states are unstable with respect to non-totally symmetric distortions at the C2v configuration. The symmetry breaking in the B12(V) state involves an in-plane coordinate of b2 symmetry. The relaxation process begins on the S2 adiabatic PES and, after passing through a conical intersection of the S2 and S1 PES, continues on the S1 surface, taking the system finally to the adiabatic minimum of S1 (1A2 state). The A11(V′) valence state is found to be unstable with respect to the out-of-plane bending coordinates of b1 and a2 symmetry. The resulting relaxed molecular structures have Cs and C2 symmetry, respectively. The present findings are analyzed in terms of a linear vibronic coupling model and spectroscopic implications are discussed.