Conformational Study of some 4’-substituted 2-(phenylselanyl)-2-(ethylsulfanyl)-acetophenones

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 νCO IR analysis, B3LYP/6-31+G(d,p) and single point Polarisable Continuum Model (PCM) calculations along with NBO analysis for 1, 3, 5. Calculations for 1-5 indicate the existence of three stable conformers with respect to the α (S-C-C=O) and α’(Se-C-C=O) dihedral angles, i.e. c1 (α ≈ 37°; α’≈ -85°) as the least stable for 1-3, being the second one in stability for 4 and 5 (α ≈ 54°;α’≈ -78°); c2 (α ≈ 88°;α’ ≈ -39°) most stable for 1-4, being the stability of 5 slightly lower than that of c1; c3 (α ≈ -130° and α’≈100°) second in stability for 1-3 and the least stable for 4 and 5. The comparison between experimental IR and computed data allows to ascribe in solution the c1 conformer to the higher frequency carbonyl doublet component of smaller intensity while the c2 and c3 conformers to the more intense lower frequency component for 1-5. The summing up of the molar fraction of c2 and c3 conformers decreases from 95% to 63% going from 1 to 5 (in gas phase) while the population of the c1 conformer increases in the same direction. This trend compares well with the PCM calculations and with the IR data solution for the majority of the solvents for 1-5. The NBO analysis shows that the sum of the selected orbital delocalization energies for the c1, c2 and c3 conformers which is practically constant for1, 3 and 5 (ca. 100 kcal mol-1) do not match the computed stability order of the three conformers for the same compounds. The smallest stability of the c1 conformer for 1-3 derives from the small α dihedral angle which gets close the Oδ-…Sδ- atoms resulting in a strong electrostatic repulsion between them and to a destabilization of this conformer relative to the c2 and c3 conformers. The larger α dihedral angle for 4 and 5 decreases the Oδ-…Sδ- coulombic repulsion leading to an increase of the relative stability of the c1 conformer with respect to the c2 and c3conformers. Moreover the strong Repulsive Field Effect between the Cδ+=O δ- and Cδ+-Sδ- dipoles of the c1 conformer for 1-3 and into a lesser extent for 4 and 5implies in a increase to a larger extent of the C=O bod order and in its frequency for 1-3 with respect to 4 and 5. For the c2 conformer in spite that the O…Se contact is smaller than the ΣvdW radii for1-5, the negative charge at the selenium atom of the c2 conformer is significantly smaller than that of the sulfur of the c1 conformer. Thus this weak electrostatic repulsion does not act decreasing the summing up of the orbital interactions which stabilize the c2 conformer. The geometry of the c3 conformer for 1-5 shows that the Sδ-…Seδ- contact is the smallest with respect to the ΣvdW radii in comparison with the same contact for the c1 and c2 conformers. This electrostatic repulsion seems to be responsible for the c3 conformer to be the second in stability for 1-3, and the least stable for 4 and 5. Therefore the computed order of stability of the three conformers for 1-5 can be mainly explained on the grounds of short contact and atomic charges analyses as the summing up of the orbital delocalization energies are quite similar for the c1, c2 and c3 conformers.