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

Henrique J Traesel; Paulo R Olivato; Daniel N S Rodrigues; Jéssica Valença; Alessandro Rodrigues; Julio Zukerman-Schpector; Maurizio Dal Colle

doi:10.1016/j.saa.2018.11.010

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

Spectrochim Acta A Mol Biomol Spectrosc. 2019 Mar 5:210:82-97. doi: 10.1016/j.saa.2018.11.010. Epub 2018 Nov 12.

Authors

Henrique J Traesel¹, Paulo R Olivato¹, Daniel N S Rodrigues², Jéssica Valença¹, Alessandro Rodrigues³, Julio Zukerman-Schpector⁴, Maurizio Dal Colle⁵

Affiliations

¹ Conformational Analysis and Electronic Interactions Laboratory, Institute of Chemistry, University of São Paulo, CP 26077, 05513-970 São Paulo, SP, Brazil.
² Conformational Analysis and Electronic Interactions Laboratory, Institute of Chemistry, University of São Paulo, CP 26077, 05513-970 São Paulo, SP, Brazil. Electronic address: dannoppper@gmail.com.
³ Department of Chemistry, Federal University of São Paulo, UNIFESP, 09972-270 Diadema, SP, Brazil.
⁴ Department of Chemistry, Federal University of São Carlos, CP 676, 13565-905 São Carlos, SP, Brazil.
⁵ Dipartimento di Scienze Chimiche e Farmaceutiche, Università di Ferrara, 44121 Ferrara, Italy.

PMID: 30447629
DOI: 10.1016/j.saa.2018.11.010

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 (ν_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 ν_CO fundamental transition band. Deuteration of these compounds (1a-5a) precluded the occurrence of FR, revealing a ν_CO 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 ν_CO 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 δ_CH and one skeletal mode couples with the ν_CO 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 ν_CO 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 C^δ+O^δ- and C^δ+O^δ- 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.

Keywords: 2‑(Methoxy)‑2‑[(4‑substituted)‑phenylsulfanyl]‑(4′‑substituted) acetophenones; Conformational analysis; Fermi resonance; Infrared spectroscopy; Solvent effect; Theoretical calculations; X-ray diffraction.