Conformational analysis and electronic interactions of some 2-ethylsulfinyl-(4′-substituted)-phenylacetates

The analysis of the infrared carbonyl bands of some 2-ethylsulfinyl-(4′-substituted)-phenylacetates bearing the substituents NO2(1), Cl(2), Br(3), H(4), Me(5) and OMe(6), along with B3LYP/6-31G(d,p), SM5.42R solvation model calculations and natural bond orbital (NBO) analysis was carried out. Theoretical data indicated the existence in the gas phase of three stable gauche conformers, whose relative abundances in condensed phase change at different extent in the series 1-6, depending on the solvent permittivity. The comparison between the IR spectra in solvents of increasing permittivity (from CCl4 to CH3CN) and the calculated SM5.42R data in solution allows a precise assignment of the experimental band components to the distinct conformers. The sum of the main relevant NBO orbital interactions energies does not match the calculated B3LYP relative energies of the three stable conformers of 1-6 in the gas phase. On the contrary, the short contact analysis indicates that the electrostatic interactions largely control their relative stability and play a basic role to determine the calculated νCO frequencies order. Moreover, as these contacts dictate the specific geometry assumed by each conformer, the electrostatic interactions determine their different solvation properties too, thus accounting for the experimental stability observed in solution.