Stereochemical and electronic interaction studies of 4'-substituted 2-(phenylselanyl)-2-(ethylsulfinyl)-acetophenones

Infrared carbonyl band analysis, supported by B3LYP/6-31+G(d,p) and single-point PCM calculations, natural bond orbital (NBO) analysis and X-ray diffraction were carried out for the diastereoisomers of a selection of 4’-substituted 2-(phenylselanyl)-2-(ethylsulfinyl)-acetophenones bearing the substituents NO2 1a, Br 2a, H 3a, Me 4a and OMe 5a for the CRSR/CSSS enantiomeric pair and Br 2b and Me 4b for the CRSS/CSSR pair. For the gas phase, the theoretical data indicated the existence of three stable conformations for the CRSR series and only two for the CRSS series. For the whole CRSR series 1a–5a, the most stable c1 and the intermediate c2 conformers have similar νCO frequencies, lowered by about 20 cm-1 with respect to the less stable c3. Likewise, the less stable c2 conformers for the CRSS series 2b and 4b exhibit the higher νCO frequencies. The single-point PCM calculations show that the relative abundance of both the less stable c3 CRSR and c2 CRSS conformers, in the gas phase, progressively increases as the dielectric constant of the media increases. The balance between the electrostatic and orbital interactions controls the calculated stability for compounds 1a–5a (CRSR/ CSSS), along with the νCO frequency order of the three conformers. Conversely, the larger stabilisation of the c1 conformer with respect to the c2 one in the 2b and 4b (CRSS/CSSR) compounds depends mainly on the orbital interactions LP(S)→σ*C-Se, LPO(S-O)→σ*C-H(CH2Me) and LPO(S-O)→σ*C-H(o-PhSe), which are present uniquely in the former conformer. The X-ray single crystal analysis indicates that the major component of 3a (CRSR) adopts the same geometry as the c2 conformer in the gas phase. Similarly, the solid-state structure of 2b matches that of the c2 conformer. The molecules in the solid are linked in centrosymmetrical pairs through π…π interactions between Se-bound phenyl rings, along with phenyl-C–H…O(carbonyl) and phenyl-C–H…O(sulphinyl) interactions.