Spectroscopic and theoretical studies of some p-substituted α-methylthio-α-diethoxyphosphorylacetophenones

Abstract

The analysis of the IR carbonyl band of the α-methylthio-α-diethoxyphosphoryl p-substituted acetophenones p-Y-Ph-C(O)CH[SMe][P(O)(OEt) 2] (Y = OMe 1, H 2, F 3, Cl 4, Br 5 and NO 2 6), supported by HF/6-31G** ab initio calculations of the α-methylthio-α-diethoxyphosphoryl acetophenone 2, indicated the existence of a single stable c 1 conformer in gas phase and in solvents of increasing polarity, along with the presence of a second high energy conformation in gas phase. The c 1 conformer presents the (SMe) group in a syn-clinal ( gauche) geometry and the [P(O)(OEt 2)] group in a quasi-periplanar ( quasi-cis) geometry with respect to the carbonyl group, which is stabilized by the synergism of the n S/π* CO and σ C–S/ π* CO orbital interactions and O δ − ( PO ) ⋯ C δ + ( CO ) orbital and Coulombic interactions, acting to increase the carbonyl oxygen negative charge, and in turn facilitates the O δ − ( CO ) ⋯ P δ + ( PO ) electrostatic interaction. The negative and almost constant carbonyl frequency shifts (Δ ν) of ca. −11 cm −1 for compounds 1– 6 relative to the parent acetophenones 7– 12, in CCl 4, corroborate the prevalence of the electronic interactions over the (− I σ) inductive effect of the α-substituents for the title compounds. The X-ray diffraction analysis for 3 indicates that it exists in the solid state in the c 1 ′ conformation, which is stabilized by the intramolecular O δ− (1) [CO}⋯ P δ+ (5) [PO] and H δ+ (10) [SMe]⋯O δ− (6) [PO] orbital and electrostatic (hydrogen bond) interactions. Moreover, these molecules form dimers which are stabilized through intermolecular hydrogen bonds H δ+ (4) [CH]⋯O δ− (1) [CO], H δ+ (5′) [Ph]⋯ O δ− (6) [PO] and H δ+ (6′) [Ph]⋯ O δ− (6) [PO].