The singlet excited states of fulvene and of its 6,6-dimethyl derivative: A combined study of their energy levels by absorption spectroscopy, configuration interaction, and density functional calculations

The ultraviolet and vacuum ultraviolet (VUV) photo absorption spectra of fulvene were reconsidered by a combination of configuration interaction and density functional methods and extended to the newly acquired VUV photo absorption spectrum of the 6,6-dimethylfulvene derivative, where several Rydberg states have been identified. Singlet states of fulvene were studied using multi-root multi-reference configuration interaction with the H2C unit either coplanar with, or perpendicular to the ring. In contrast to ethylene, the lowest states are coplanar. The vibrational structure of the lowest (1B2) excited states of fulvene is well reproduced by calculated values. The second singlet state of fulvene, previously assigned as 1A1 on the basis of intensity, is incompatible with the calculated planar 1A1 state, which itself is a saddle point. This state shows significant quartic character on bending, and the best interpretation of the observed UV band of fulvene is of a bent form. The UV spectral state with low onset intensity is probably a 1B1 state of CS symmetry. Theoretical Rydberg states were determined for fulvene; the closest fit to the two known Rydberg states is to the 3p and 4p states (1B1). Comparison of the separation of the 2A2 and 2B1 states in the photoelectron spectra of the two compounds, with the threshold photoelectron spectrum of fulvene, shows that the 2B1 state vibrational structure is largely lost for both molecules. A reconsideration of the interaction between the X2A2 and A2B1 ionic states has led to the identification of the 4A2 quartic state of fulvene.