The ultraviolet and vacuum ultraviolet absorption spectrum of gamma-pyrone; the singlet states studied by configuration interaction and density functional calculations

A synchrotron based vacuum ultraviolet absorption spectrum for γ-pyrone has been interpreted in terms of singlet excited electronic states, using a variety of coupled cluster, configuration interaction, and density functional calculations. The extremely weak spectral onset at 3.557 eV shows 8 vibrational peaks and following previous analyses is attributed to a forbidden ¹A₂ state. A contrasting broad peak with maximum at 5.381 eV has a relatively high cross-section of 30 Mb; this arises from three overlapping states, where a ¹A₁ state dominates over progressively weaker ¹B₂ and ¹B₁ states. After fitting the second band to a polynomial Gaussian function, and plotting the regular residuals (RR), over 20 vibrational peaks were revealed. We have had limited success in analyzing this fine structure. However, the small separation between these three states clearly shows that their vibrational satellites must overlap. Singlet valence and Rydberg state vibrational profiles were determined by configuration interaction using the CAM-B3LYP density functional. Vibrational analysis, using both Franck-Condon and Herzberg-Teller procedures showed that both procedures contributed to the profiles. Theoretical Rydberg states were evaluated by a highly focused CI procedure. Super-position of the lowest photoelectron spectral band on the VUV spectrum near 6.4 eV, shows that the 3s and 3p Rydberg states based on the ²B₂ ionic state are present; those based on the other low-lying ionic state (X²B₁) are destroyed by broadening; this is a dramatic extension of the broadening previously witnessed in our studies of halogenobenzenes.