Photoelectron spectroscopy and ab-initio study of the conformation of ω-ethylthioacetophenones

Abstract

The He^I photoelectron spectra of ω-ethylthioacetophenones Y-C₆H₄COCH₂SC₂H₅(Y = NO₂, CN, H, Me, OMe) are assigned by means of the composite-molecule approach and ab-initio pseudopotential computations. The splitting-of the first band–deriving from sulphur lone-pair ionization–observed when Y is an electron-withdrawing substituent is ascribed to the presence of two groups of rotamers. On experimental basis it is concluded that, in the most abundant conformer, the sulphur lone pair is nearly parallel to the π-system, while it is stabilized by distortion from planarity and S→π* charge-transfer interaction in the other conformer. Theoretical results on conformational stability (while obtained by means of a quite crude approach) are in agreement with this interpretation.