EXPERIMENTAL AND THEORETICAL-STUDY OF THE INTRAMOLECULAR INTERACTIONSDETERMINING THE CONFORMATION OF BETA-CARBONYL SULFOXIDES

Information on the geometrical and electronic structures of alpha-meth ylsulfinylacetophenone, C6H5C(O)CH2S(O)CH3 2, have been obtained from X-ray diffraction analysis, UV photoelectron spectroscopy and ab initi o 6-31G*calculations. A comparison of the results with those obtained from the spectra and the computations on alpha-methylthioacetophenone , C6H5C(O)CH2SCH3 1 and alpha-methylsulfonylacetophenone, C6H5C(O)CH2S O2CH3 3, together with previous results on; beta-keto sulfides and bet a-keto sulfones indicates that the CH2-S(O) bond in is quasi-cis to th e carbonyl group in the gas and solid phase, at variance with the othe r beta-carbonyl thioderivatives which adopt a gauche conformation. Eig envector analysis, electron charge distribution at various atoms and/o r groups and geometric parameters indicate that the cis conformation o f 2 is stabilized by a strong non-bonded interaction between the negat ively charged carbonyl oxygen and the positively charged sulfur atom f rom which it is separated by a distance (2.8-2.9 Angstrom much shorter than the sum of the the van der Waals radii. The predominant charge t ransfer interaction in 3 and related sulfones occurs in the opposite d irection (O-SO2-->C-CO). The inversion of We direction of the charge t ransfer (and the change of the cis/gauche orientation of the thio grou p) from sulfone to sulfoxide is associated with an increase of electro n affinity of the thio group in the latter, and could explain its smal ler thermal stability. Ab initio 3-21G calculations on several confor mations of the bis-thioderivatives C6H5C(O)CH(SCH3)S(O)CH3 4, C6H5C(O) CH(SR)SO(2)R (R = Me 5 and Ph 6) and C6H5C(O)CH(SOCH3)SO2CH3 7, togeth er with X-ray diffraction (4, 6 and 7) and photoelectron spectroscopy (4) analyses confirmed the cis (SOR) and gauche (SR and SO(2)R) prefer red orientation of the thio groups with respect to the carbonyl group as observed in the monosubstituted derivatives. In 4 and 7 the S-SO at om is about 30 degrees out of the cis plane [O(1)-C(2)C(3)].