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

Abstract The analysis of infrared (IR) carbonyl bands of some 4′-substituted 2-(phenylselanyl)-2-(ethylsulfonyl)-acetophenones 1 – 5 bearing substituents NO 2 1 , Br 2 , H 3 , Me 4 and OMe 5 , supported by B3LYP/6–31 + G(d,p) and single point polarizable continuum model calculations, along with natural bond orbital (NBO) analysis (for 1 , 3 , 5 ) and X-ray diffraction (for 4 ) was performed. Theoretical data indicated the existence of two stable conformations: c 1 and c 2 . The former exhibits the highest ν CO frequency 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 LP O(CO)  → σ ∗ C H[CH2(Et)] and LP O(SO2)  → σ ∗ C H( o SePh) orbital interaction energies, the c 1 conformer becomes significantly more stable than the c 2 one. The occurrence of these hydrogen bonds plays an important role in determining the geometry of the c 1 conformer. This geometry allows the oppositely charged O δ− (CO) ⋯S δ+ (SO2) and O δ− (SO2) ⋯C δ+ (CO) 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 O δ− (CO) ⋯O δ− (SO2) short contact and the consequent repulsive field effect that increases the ν CO frequency of the c 1 conformer to a greater extent with respect to that of the c 2 one. Therefore, the more intense higher frequency carbonyl doublet component in the IR spectrum in solution can be ascribed to the c 1 conformer and the less intense component at lower frequency to the c 2 one. X-ray single crystal analysis of 4 indicates that this compound adopts the c 1 geometry. The molecules in the solid are linked in centrosymmetrical pairs through C9 H10⋯O36 hydrogen bond interaction along with the LP Se ⋯π Ph interaction.