Conformational study of some 4'-substituted 2-(phenylselanyl)-2-(ethylsulfanyl)-acetophenones

Abstract

The conformational analysis of some 4'-substituted 2-(phenylselanyl)-2-(ethylsulfanyl)-acetophenones bearing the substituents NO2 (1), Br (2), H (3), Me (4) and OMe (5) was performed by ν(CO) IR analysis, B3LYP/6-31+G(d,p) and single point polarisable continuum model (PCM) calculations, along with NBO analysis for 1, 3 and 5. Calculations for 1-5 indicate the existence of three stable conformations, c1, c2 and c3, whose stability depends on the balance between electrostatic and orbital interactions that are strictly related to the geometrical arrangement. The comparison between the experimental IR spectra in solution and the computed data in gas phase for 1-5 allows the c1 conformer to be assigned to the less intense component at higher frequency of the carbonyl doublet and both the c2 and c3 ones to the more intense lower frequency component. The sum of the calculated molar fraction of c2 and c3 conformers decreases from 95% to 63% on going from 1 to 5 (in gas phase), and this trend compares well with the PCM calculations and the IR experimental data for the majority of the solvents for all compounds 1-5. The NBO analysis for 1, 3 and 5 shows that the sum of the relevant orbital delocalization energies for the c1, c2 and c3 conformers is almost constant and does not match the computed stability order. The lowest stability of the c1 conformer for 1-3 can be related to the small value of the α dihedral angle that enables a strong electrostatic destabilizing repulsion between the O(CO)(δ-)…S(δ-) atoms. The relative stability of the c1 conformer increases for 4 and 5 as the α dihedral angle enlarges and the repulsion is minimized. Moreover, the strong repulsive field effect between the C(δ+)=O(δ-) and C(δ+)-S(δ-) dipoles exerted to a greater extent on the c1 conformers of 1-3 with respect to 4 and 5, causes a major increase of the corresponding C=O bond orders and related carbonyl frequencies. For the c2 conformer, the electrostatic destabilizing repulsion between the O(δ-)…Se(δ-) atoms is weaker than that involving the O(δ-)…S(δ-) atoms in the c1 conformer and therefore has negligible effects on the conformer stability that is mainly determined by the sum of the orbital interactions. The c3 conformer has the shortest S(δ-)…Se(δ-) contact for all compounds and thus the related electrostatic repulsion seems to be the most important factor that affects its stability. In conclusion, the computed order of stability of the three conformers for 1-5 depends on the electrostatic repulsions between close charged atoms rather than on the sum of the orbital delocalization energies that are quite similar for all the conformers.

Keywords: 4′-Substituted 2-(phenylselanyl)-2-(ethylsulfanyl)-acetophenones; Conformational analysis; Infrared spectroscopy; Solvent effect; Theoretical calculations.