Raman Measurements of the High-Temperature Heats of Vaporization of HF and H₂O and of the H-Bond Enthalpies of the Hydronium−Fluoride Complex in the 50 mol % HF−H₂O Solution

Heats of vaporization, Δ H v, of the monomers HF and H 2 O from 50 mol % aqueous HF were determined to 150 °C (HF to 300 °C) under saturation pressures using Inconel and Monel cells with sapphire windows. Heats were obtained from the temperature dependences of the integrated Raman intensities, as well as the peak heights, of the 3958 and 3652 cm -1 vapor bands from HF and H 2 O, respectively, ratioed against the 2329 cm -1 band from the N 2 internal standard. The Raman Δ H v for the HF at 20 °C, 9340 ± 220 cal/mol, agrees well with 20 °C calorimetric data, but it decreases to 6600 ± 200 cal/mol at 150 °C. The Raman Δ H v values for H 2 O are larger by the ≈Δ H of one H-bond; they are 11 800 ± 300 and 9350 ± 350 cal/mol at 25 and 150 °C, respectively. The H-bonded dimer, (HF) 2, contributes relatively little to the gas-phase spectrum below ≈150 °C, but its Raman intensity, centered near 3931 cm -1, rises rapidly between 150 and 300 °C. (HF) 2 vaporizes first, but its H-bond breaks almost instantaneously in the vapor to give the monomer, except that the dimer finally becomes more favorable at very high saturation pressures. Gaussian deconvolution of the broad, structured, nominal 3600 cm -1 contour from the liquid between −12 and ≈150 °C yielded an H-bond enthalpy of 2.8 ± 0.2 kcal/mol OH···F, which falls within the range of kcal/mol OH···F obtained from Young−Westerdahl analysis involving −Δ H INT = g B = Δ H v Raman (HF,solution) + Δ H v Raman (H 2 O,solution) − Δ H v (HF,pure liquid) − Δ H v (H 2 O,pure liquid). The 50 mol % solution is essentially a molten salt composed of very strong H 2 OH + ···F - complexes interbonded via peripheral H-bonds having ordinary, ≈2.9 Å, O−F distances. The Δ H of 2.5−2.8 kcal/mol refers to breakage of such peripheral H-bonds, and this breakage produces a pronounced Raman intensity increase near 3590 cm -1 due to dangling, non-H-bonded proton(s), schematically the left protons of H 2 OH +, or to the (above right of) F - . Three valence vibrations due to the O−H stretches of the H 3 O + ion were observed. However, a fourth, very broad, valence vibration from the H + ···F - stretching of the H 2 OH + ···F - complex was also observed, near 2900−3000 cm -1, which results from proton transfer in a double-well potential with a very short, ≈2.4 Å, O−F distance. A detailed energy analysis of the complete vaporization process also suggests that the hydronium−fluoride H-bond, between the H 2 OH + and the F - (above), has an unusually large energy, estimated to be at least ≈8 kcal/mol, thus breaking only upon vaporization, or at temperatures above those employed here for solution studies.