IR-spectroscopic investigations on solute-solvent interactions Part 1: Solvation of triethylphosphine oxide in aprotic and CH-acidic solvents

Abstract

A systematic study has been carried out on the solvent and concentration dependence of the P=O stretching vibrationv(PO) of the model compoundEt 3PO in various aprotic and several CH-acidic solvents. Evidence has been found for the existence of dipole-dipole complexes in concentrated solutions ofEt 3PO in solvents of low acceptor numbers. In dilute solutions, however, the P=O stretching band generally occurs as a single symmetrical peak which shows that only one kind of solvate species is present in solution. In CH-acidic solventsEt 3PO is present in the form of hydrogen bonded complexes. Informations about the structure of these complexes have been obtained by means of spectrophotometric titrations. It has been shown that the wavenumbersv°(PO) obtained by extrapolation ofv(PO) values to zero concentration are linearly related to the acceptor numbers of the solvents, previously derived from31P-chemical shift measurements. The existence of this linear relationships proves that both parameters are linearly related to the strength of the intermolecular interactions and are virtually free from unspecific contributions such as magnetic anisotropy or vibrational coupling effects. The results of the present study show that the P=O stretching vibration ofEt 3PO represents an ideal probe for the investigation of solvent effects and solvation mechanisms.