The infrared spectra and enthalpies of strongly bound dimers of phosphinic acids in the gas phase. (CH 2Cl) 2POOH and (C 6H 5) 2POOH

Abstract

Infrared spectra of phosphinic acids R 2POOH (R=CH 2Cl, C 6H 5 and CH 3) have been recorded in the gas phase. The equilibrium between monomers and dimers of R 2POOH is studied in the temperature range of 400–650 K. The broad absorption band in the region 3600–900 cm −1 is assigned to the ν(OH) stretching vibration of the very strongly bound cyclic dimer. The characteristic ABC structure of the ν(OH) band, typical for the spectra of strongly hydrogen-bonded complexes in the solid phase, is observed in the gas phase spectra of dimers. Unlike the case of solid-phase spectra, the intensity of components decreases in the sequence A, B, C. The ν(OH) bands of free R 2POOH molecules (effective half-width is about 35 cm −1) appear in the range 3660–3630 cm −1 at temperatures 420–500 K. The dimerization constants, the spectral moments of monomer and dimer bands, and their intensities have been determined. The H-bond energies are estimated from the temperature dependence of equilibrium constants of dimerization. The values obtained for phosphinic acids are found to be equal to 25–50 kcal mol −1 per dimer. The spectral and energetic features of phosphinic acid dimers are typical for very strong hydrogen bonds with a partial covalent character.