Spectroscopic and theoretical studies of some 2-(methoxy)-2-[(4-substituted)-phenylsulfany1]-(4′-substituted) acetophenones

The conformational analysis of some 2-(methoxy)-2-[(4-substituted)-phenylsulfany1]-(4'-substituted) acetophenones was performed through infrared (IR) spectroscopic analysis of the carbonyl stretching band (nu(co)), 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 vco fundamental transition band. Deuteration of these compounds (1a-5a) precluded the occurrence of FR, revealing a vco doublet for all compounds in all solvents used. The computational results indicated the existence of three conformers (c(1), c(2) and c(3)) for the whole series whose relative abundances varied with solvent permittivity. The higher vco frequency c(1) conformer was assigned to the higher frequency component of the carbonyl doublet, while both c(2) and c(3) 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 delta(CH) and one skeletal mode couples with the v(co) mode giving rise to the FR on the c(2) conformer in vacuum and on the c(1) one in non-polar solvents. The experimental data indicated a progressive increase in c(1) conformer stability with the increase of the solvent dielectric constant, which is in good agreement with the polarizable continuum model (PCM) calculations. The higher vco frequency and the stronger solvation of the c(1) conformer is a consequence of the repulsive field effect (RFE) originated by the alignment and closeness of the C delta+=O delta- and C delta+-O delta- dipoles. Finally, the balance between orbital and electrostatic interactions dictates the conformational preferences. X-ray single crystal analysis for compound 6 revealed the c(1) geometry in the solid state and its stabilization by C-H...O hydrogen bonds. (C) 2018 Published by Elsevier B.V.