Spectroscopic and theoretical studies of some 2‑(methoxy)‑2‑[(4‑substituted)‑phenylsulfanyl]‑(4'‑substituted) acetophenones.

Abstract

The conformational analysis of some 2‑(methoxy)‑2‑[(4‑substituted)‑phenylsulfanyl]‑(4'‑substituted) acetophenones was performed through infrared (IR) spectroscopic analysis of the carbonyl stretching band (νCO), supported by B3LYP/6-31+G(d,p) calculations and X-ray diffraction. Five (1-5) of the seven studied compounds (1-7) presented Fermi resonance (FR) on the νCO fundamental transition band. Deuteration of these compounds (1a-5a) precluded the occurrence of FR, revealing a νCO doublet for all compounds in all solvents used. The computational results indicated the existence of three conformers (c1, c2 and c3) for the whole series whose relative abundances varied with solvent permittivity. The higher νCO frequency c1 conformer was assigned to the higher frequency component of the carbonyl doublet, while both c2 and c3 were assigned to the lower frequency one. Anharmonic vibrational frequencies and Potential Energy Distribution (PED) calculations of compound 3 indicated that the combination band (cb) between the methyne δCH and one skeletal mode couples with the νCO mode giving rise to the FR on the c2 conformer in vacuum and on the c1 one in non-polar solvents. The experimental data indicated a progressive increase in c1 conformer stability with the increase of the solvent dielectric constant, which is in good agreement with the polarizable continuum model (PCM) calculations. The higher νCO frequency and the stronger solvation of the c1 conformer is a consequence of the repulsive field effect (RFE) originated by the alignment and closeness of the Cδ+Oδ- and Cδ+Oδ- dipoles. Finally, the balance between orbital and electrostatic interactions dictates the conformational preferences. X-ray single crystal analysis for compound 6 revealed the c1 geometry in the solid state and its stabilization by CH…O hydrogen bonds.