Spectroscopic and theoretical studies of some alpha-ethylsulfinyl ortho-substituted acetophenones

The preferred conformations of the α-ethylsulfinyl ortho-substituted acetophenones X_PhC(O)CH2S(O)Et (X = OMe 1, Me 2, F 3, Cl 4, Br 5, NO2 6) are determined by νCO IR analysis, ab initio HF/6-31G** computations (1-4 and 6) and X-ray diffraction analysis (6). The matching between the IR frequency and intensity of the carbonyl doublet components, in carbon tetrachloride and the results of the calculations indicates that the lower energy component corresponds to the cis2 (anti) conformer for 1, 3-5 and cis2 (syn) for 2. The higher frequency component is related to the gauche3 conformer anti for 3 and syn for 2, 4 and 5, and to the gauche4 (anti) conformer for 1. For 6, gauche conformers correspond to both doublet components. The cis2 conformer population prevails over the gauche one for 1-3 (ca.80%) but not for 4 and 5 (ca.30%). For 6, the lower frequency gauche3 conformer population (85%) is significantly larger than that for the gauche2 one. These trends are discussed in terms of strong stabilising charge transfer and electrostatic interactions between pairs of oppositely charged atoms and their variation with the electronic and steric properties of the X substituent. The X-ray diffraction analysis shows that 6, in the solid state, is in the cis1 conformation which is stabilised through dipole moment coupling and intermolecular hydrogen bonds. In the gas phase and in the crystal, the ortho-nitrophenyl plane is almost perpendicular to the acetyl plane allowing a strong stabilising interaction.