FOURIER-TRANSFORM INFRARED STUDY OF GAS-PHASE H-BONDING - ABSORPTIVITIES AND FORMATION EQUILIBRIUM-CONSTANTS OF FLUOROALCOHOL COMPLEXES

We have recorded gas-phase Fourier transform infrared spectra (4000-400 cm-1) of CF3CH2OH, (CF3)2CHOH and (CF3)3COH and of the complexes that these alcohols form with bases such as diethyl ether, acetone, tetrahydrofuran, ammonia and trimethyl- and triethylamine. The spectrum of each complex has been '' isolated'' from the spectra of mixtures of the corresponding alcohol (AH) and base (B) through spectral subtraction techniques. All efforts were made to obtain good estimates of the absorptivity of the complex leading to the determination of an accurate set of equilibrium constants for the corresponding formation reactions AH + B half arrow right over right arrow left AH ... B. A statistical procedure using a series of mixtures is proposed to calculate this absorptivity from their spectra, their total pressure and the sum of partial pressures p(AH) + P(B). The evaluated equilibrium constants quantitatively fit the Abboud-Bellon-Taft-Abraham bilinear model. We found: log K(c) = (-0.48 +/- 0.15) + (7.90 +/- 0.39) alpha2Hbeta2H with n = 13, r = 0.987, S.D. = 0.12 log units. The spectra of the complexes show, in addition to the typical DELTAnu(OH), DELTAdelta(OH) and DELTAgamma(OH) shifts, well defined features related to other molecular group vibrations [mainly nu(CF3), nu(CO), nu(CC) and gamma(CF3)], clearly different from those seen in the spectra of the pure alcohols. These features are understood in terms of conformational isomerism: the formation of intermolecular H-bonds implies the breaking of the intramolecular OH ... F bonds which stabilize the pure alcohols in definite conformations.