The infrared spectra of dilute solutions of triethylphosphine oxide in the P—O stretch region has been measured for solutions in a wide range of protic and aprotic solvents, and in a range of mixed-solvent systems. Shifts of the ³¹P magnetic resonance have also been monitored for these mixed-solvent systems.

A linear dependence of ν_max for the infrared data against Δν(³¹P) for the n.m.r. data is established for most solvents. Using this correlation, the n.m.r. shifts for mixed-solvent systems can be quantitatively reproduced from the infrared results. In certain solvents, including methanol, the infrared spectra show the presence of two distinct solvates. From changes induced by added aprotic solvents, it is suggested that these species are di- and mono-hydrogen-bonded units, Et₃PO(HOMe)₂ and Et₃PO(HOMe). The ³¹P shift and single infrared band for aqueous solutions are well removed from the values for solutions in alcohols, and from the changes induced on adding aprotic cosolvents it is clear that at least four distinct species are involved. These are thought to be Et₃PO(H₂O)₃, Et₃PO(H₂O)₂, Et₃PO(H₂O) and Et₃PO with no hydrogen bonds.

We discuss the significance of Gutmann's solvent acceptor numbers (A.N.) in the light of these results, where A.N. is the ³¹P chemical shift from the value for Et₃PO in hexane. It is clear that in some cases A.N. refers to the average of a mixture of different solvates, and that in many cases the interactions involved are not 1:1 interactions as they are in the derivation of solvent donor numbers.