Hydrogen Bonding in Supercritical Ethanol Assessed by Infrared and Raman Spectroscopies

This present paper is aimed at discussing the state of aggregation in supercritical ethanol using infrared and Raman spectroscopies. A quantitative band shape analysis of the spectra associated with the OH stretching mode of ethanol has been done using IR and Raman activities determined by ab initio calculations on small oligomers. Such a methodology allows the quantification of the degree of hydrogen bonding (η), and both IR and Raman techniques lead to comparable results. This study shows that hydrogen bonds still exist in supercritical ethanol. The degree of hydrogen bonding (η) is found to be relatively constant above the critical density and strongly diminishes below it. The determined values of η are consistent with literature measurements reported using IR spectroscopy. The disagreement observed with NMR experiments has been critically discussed.