Conformational study of some 4’-substituted 2-(phenylselanyl)-2-(ethylsulfonyl)-acetophenones

The analysis of IR carbonyl bands of some 4’-substituted 2-(phenylselanyl)-2-(ethylsulfonyl)-acetophenones 1-5 bearing substituents NO2 1, Br 2, H 3, Me 4 and OMe 5, supported by B3LYP/6-31+G(d,p) and single point PCM calculations, along with NBO analysis (for 1, 3, 5) and X-ray diffraction (for 4) was performed. Theoretical data indicated the existence of two stable conformations c1 and c2. The former exhibits the highest νCO frequency (for 1-5) and corresponds to the most stable (for 1-5) and to the most polar one (for 2-4). The sum of the energy contributions of selected orbital interactions (NBO analysis) of 1, 3 and 5 is quite similar for both conformers. Nevertheless, adding the LPO(CO)→σ*C-H[CH2(Et)] and LPO(SO2)→σ*C-H(o-SePh) orbital interaction energies, the c1 conformer becomes significantly more stable than the c2 one. The occurrence of these hydrogen bonds plays an important role in determining the geometry of the c1 conformer. This geometry allows the oppositely charged atoms of the carbonyl and sulfonyl groups to assume interatomic distances shorter than the sum of the van der Waals radii that stabilize the referred conformer. Likewise, this geometry favours the Oδ-(CO)…Oδ-(SO2) short contact and the consequent Repulsive Field Effect that increases to a greater extent the νCO frequency of the c1 conformer with respect to that of the c2 one. Therefore the more intense higher frequency carbonyl doublet component in the IR spectrum in solution can be ascribed to the c1 conformer and, conversely, the less intense component at lower frequency to the c2 one. X-ray single crystal analysis of 4 indicates that this compound adopts the c1 geometry. The molecules in the solid are linked in centrosymmetrical pairs through C9-H10…O36i hydrogen bond interaction along with the LPSe⋯πPh interaction.