The infrared spectra and enthalpies of strongly bound dimers of phosphinic acids in the gas phase.: (CH2Cl)2POOH and (C6H5)2POOH

Infrared spectra of phosphinic acids R2POOH (R = CH2Cl, C6H5 and CH3) have been recorded in the gas phase. The equilibrium between monomers and dimers of R2POOH 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 v(OH) stretching vibration of the very strongly bound cyclic dimer. The characteristic ABC structure of the v(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 v(OH) bands of free R2POOH 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. (C) 2002 Elsevier Science B.V. All rights reserved.