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 (B-1(2)) excited states of fulvene is well reproduced by calculated values. The second singlet state of fulvene, previously assigned as (1)A(1) on the basis of intensity, is incompatible with the calculated planar (1)A(1) 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 B-1(1) 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 (B-1(1)). Comparison of the separation of the (2)A(2) and B-2(1) states in the photoelectron spectra of the two compounds, with the threshold photoelectron spectrum of fulvene, shows that the B-2(1) state vibrational structure is largely lost for both molecules. A reconsideration of the interaction between the X(2)A(2) and A(2)B(1) ionic states has led to the identification of the (4)A(2) quartic state of fulvene. (c) 2025 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution-Non Commercial-NoDerivs 4.0 International (CC BY-NC-ND) license