An IR study of hydrogen bonding in liquid and supercritical alcohols

IR spectra in the region of the OH stretching band of liquid-like and supercritical methanol (T-c = 239.4 degreesC, P-c = 80.8 bar), ethanol (240.9 degreesC, 61.4 bar), 2-propanol (235.2 degreesC, 47.6 bar), and 1-butanol (289.9 degreesC, 44.1 bar)'(data from ref 1, J. Chem. Eng. Data 1995, 40, 1025) have been recorded over a wide range of temperature and pressure up to 450 degreesC and 1000 bar. The spectra were obtained using a high-pressure, high-temperature IR cell and a specially developed measuring technique, in which the integrated intensity of the absorption band of the C-H stretching vibrations is used as an internal standard. This analysis allows the fraction of hydrogen-bonded and non-hydrogen-bonded hydroxyl groups in 2-propanol and I-butanol to be estimated over the whole range of experimental conditions as a function of both temperature and pressure. With this approach, it is unnecessary to know the density of the sample or even the path length at which the spectra were obtained. However, the approach is less reliable in the case of ethanol and cannot be applied to methanol because the spectra of these substances are complicated by rotational fine structure. An unexpected result of this study is the possibility that only one type of hydrogen-bonded species exists in ethanol, 2-propanol, and I-butanol when the mole fraction of hydrogen-bonded molecules is less than about 0.6. When the probability of hydrogen bonding is greater, evidence for a strong cooperative effect is seen.