Phosphine oxides as NMR and IR spectroscopic probes for the estimation of the geometry and energy of PO⋯H–A hydrogen bonds

Abstract

In this work we evaluate the possibility of using the NMR and IR spectral properties of the PO group to estimate the geometry and strength of hydrogen bonds which it forms with OH-, NH- and CH-acids. The results of the DFT study of 70 hydrogen-bonded 1 : 1 complexes of a model trimethylphosphine oxide, Me₃PO, with various proton donors in the gas phase and in aprotic medium (modelled as a polarizable continuum) are presented. Four types of hydrogen bonds with the general formula Me₃PO⋯H–A were considered, where the A atom is O, C, and N (neutral or cationic acids). Within the selected set of complexes the hydrogen bond energy varies over a wide range (ca. 0–85 kJ mol⁻¹). We show that it is possible to use simple correlations to estimate the energy and geometry of OHO, NHO and CHO hydrogen bonds from the changes of isotropic ³¹P NMR chemical shifts and harmonic PO stretching vibration frequencies upon complexation. Such correlations also could be used to estimate the proton-donating ability (and Brønsted acidity; pK_a) of OH acids.