04. Kazarian, Sergei G.; Gupta, Ram B.; Clarke, Matthew J.; Johnston, Keith P.; Poliakoff, Martyn. How is hydrogen-bonding influenced by solvent density? The spectroscopic study and modeling of the interaction between a proton donor and acceptor from the gas phase to supercritical fluid states. J. Am. Chem. Soc. (1993), 115(24), 11099-109.

Abstract

FTIR spectroscopy is used to study the effects of solvent d. on the H-bonding equil. between perfluoro-CMe3 alc. (PFTB), (CF3)3COH, and di-Me ether (DME), Me2O, in soln. in SF6 (Tc, 45.5°; Pc, 540 psi; rc 5.03 mol/L). The interaction of PFTB and DME is quite strong, and thus it was possible to use rather more dil. solns. than in previous studies of H-bonding in supercrit. fluids. Both PFTB and DME are highly volatile so the equil. could be studied over the full range of densities of SF6 from the pure gas phase (i.e. in the absence of SF6) through the supercrit. region to liq.-like densities .apprx.10 mol/L (1.5 gm/L) and at 20-65°. Both qual. and quant. measurements were made at const. temp., const. pressure, and const. d. The expts. introduce a no. of innovative features both in methodol. and in data manipulation. The modified lattice-fluid hydrogen-bonding model (MLFHB) was used to calc. the effects of d. on the percent of free (uncomplexed) PFTB in the soln. and on the value of the equil. const. Kc. Qual. studies show explicitly and without any spectroscopic assumptions that increasing d. causes an increase in the concn. of free PFTB and a concomitant decrease in the concn. of the H-bonded PFTB/DME complex. More detailed measurements have allowed these changes to be quantified and modeled; particularly interesting are (a) the variation of Kc with temp. at const. pressure (4.4 MPa), where the rapid increase in solvent d. near the crit. temp. cancels almost completely the effects of lowering the temp. and (b) the isothermal dependence of Kc with d., including the unusual behavior at 50° in the d. range .apprx.3-6 mol/L of SF6 (0.45-0.9 gm/L), behavior which is not obsd. at 60°. Thus unusual behavior provides good evidence of enhanced solute-solute interactions toward the solvent crit. point, as is further demonstrated with a simplified model.

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