He I and He II photoelectron spectra of γ-thionobutyrolactones and γ-butyrolactones

The valence electronic structures of the compounds γ-thionobutyrolactone (X=S, Y=O, R = R″ = H), γ-butyrolactone (X=O, Y=O, R = R″ = H) and their respective 3-and 5-methyl-substituted derivatives have been investigated by photoelectron spectroscopy. Two distinct bands are observed in the ionization potential (IP) range 8-11 eV in the spectra of all these compounds, with the first ionization band exhibiting clear vibrational progressions. Variation in the He I/He II relative intensities in the spectra of the γ-thionobutyrolactones suggests the assignment of the first band to the lone-pair n_s orbital, the second band to the π*_OCS orbital and the third band to the π_OCS orbital. The first two bands in the spectra of the γ-butyrolactones, on the other hand, are ascribed to the n_O(C=O) and the ethereal n_O(π) orbitals, respectively. The assignments are aided by the results from semi-empirical PM3 and ab initio calculations using the 6-31G** basis sets. Correlation with the IPs of thioformaldehyde, cyclopentanone and tetrahydrofuran indicates that the interaction between the carbonyl group and the ring oxygen atom is mainly inductive in nature. In the γ-thionobutyrolactones, the conjugative interaction is more significant. Methyl substitution causes mainly inductive effects on the parent heterocyclics.