HE-I AND HE-II PHOTOELECTRON-SPECTRA OF GAMMA-THIONOBUTYROLACTONES AND GAMMA-BUTYROLACTONES

The valence electronic structures of the compounds gamma-thionobutyrol actone (X=S, Y=O, R=R ''=H), gamma-butyrolactone (X = O, Y = O, R = R '' = H) and their respective 3- and 5-methyl-substituted derivatives h ave been investigated by photoelectron spectroscopy. Two distinct band s are observed in the ionization potential (IP) range 8-11 eV in the s pectra of all these compounds, with the first ionization band exhibiti ng clear vibrational progressions. Variation in the He I/He II relativ e intensities in the spectra of the gamma-thionobutyrolactones suggest s the assignment of the first band to the lone-pair ns orbital, the se cond band to the pi(OCS) orbital and the third band to the pi(OCS) or bital. The first two bands in the spectra of the gamma-butyrolactones, on the other hand, are ascribed to the n(O(C=O)) and the ethereal n(O )(pi) 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, cyclopentano ne and tetrahydrofuran indicates that the interaction between the carb onyl group and the ring oxygen atom is mainly inductive in nature. In the gamma-thionobutyrolactones, the conjugative interaction is more si gnificant. Methyl substitution causes mainly inductive effects on the parent heterocyclics. (C) 1998 Elsevier Science B.V.