31P NMR chemical shift of phosphine oxides measures the total strength of multiple anticooperative H-bonds formed between the PO group and proton donors

Abstract

In this work, experimental ³¹P NMR chemical shifts of triphenylphosphine oxide are used to evaluate the strength of PO⋯H–O hydrogen bonds. Complexes featuring either one or two H-bonds (1 : 1 and 1 : 2 complexes, respectively) formed between Ph₃PO and 24 substituted phenols are investigated by low-temperature liquid-state NMR spectroscopy in CDF₃/CDF₂Cl solution. We demonstrate that the ³¹P NMR chemical shift changes upon complexation, ΔδP, correlate well with the total strength of all hydrogen bonds formed by the PO group. This allows one to use ΔδP as a tool for estimating overall binding energies in compounds containing phosphine oxides. Additionally, using DFT calculations with implicit (PCM) and explicit (2–3 solvent molecules) solvation models we reached a semi-quantitative agreement between calculated and experimental ΔδP values. The anticooperativity effects in 1 : 2 complexes were estimated to be ca. 15–20% both in experiment and calculations.