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

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 NO2 6), supported by HF/6-31G** ab initio calculations of the α-methylthio-α-diethoxyphosphoryl acetophenone 2, indicated the existence of a single stable c1 conformer in gas phase and in solvents of increasing polarity, along with the presence of a second high energy conformation in gas phase. The c1 conformer presents the (SMe) group in a syn-clinal (gauche) geometry and the [P(O)(OEt2)] group in a quasi-periplanar (quasi-cis) geometry with respect to the carbonyl group, which is stabilized by the synergism of the nS/π*CO and σC-S/π*CO orbital interactions and Oδ-(PO)...Cδ+(CO) orbital and Coulombic interactions, acting to increase the carbonyl oxygen negative charge, and in turn facilitates the Oδ-(CO)…Pδ+(PO) electrostatic interaction. The negative and almost constant carbonyl frequency shifts (Δν) of ca. -11 cm-1 for compounds 1-6 relative to the parent acetophenones 7-14, in CCl4, corroborate the prevalence of the electronic interactions over the (-Iσ) 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 c1’ conformation, which is stabilized by the intramolecular Oδ-(1)[CO}…Pδ+(5)[PO] and Hδ+(10)[SMe]…Oδ-(6)[PO] orbital and electrostatic (hydrogen bond) interactions. Moreover, these molecules form dimers which are stabilized through intermolecular hydrogen bonds Hδ+(4)[CH]…Oδ-(1)[CO], Hδ+(5’)[Ph]…Oδ-(6)[PO] and Hδ+(6’)[Ph]…Oδ-(6)[PO]