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

In this work we evaluate the possibility of using the NMR and IR spectral properties of the PO 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 PO 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.