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 C_S 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.