Vibrational overtone study of association in liquid methanol

Abstract

Publication costs assisted by Université de Montréal An attempt has been made to demonstrate the presence of oligomers and monomers in liquid methanol, by a study of the OH stretching fundamental, and its first and second overtones. The key to the solution of this problem is the previously made observation that a decrease in the intensity of the polymer overtones gives the bands of the less associated species a chance to appear.

On the Existence of Free OH Groups in Liquid Water

“…This implies that the polymer bands are too weak to be observed. (This conclusion is less surprising if we have examined the overtones of liquid methanol (22 (20) found for HDO in CCI, solution and this is what we find using the frequencies of the first and second overtones of the free bands as assigned above.…”

“…It is important in this respect to recognize the fact that the OH-stretching overtones of hydrogen bonded polymers are unusually weak compared to those of the corresponding free OH groups. This has been observed in various cases by Luck and Ditter (19), Burneau and Corset (20), and ourselves (21,22). This weakness is likely to be due to decreased electric anharmonicity (21) and it makes it possible to observe free and oligomer vibrations, which do not show up in the fundamental region.…”

“…exist even at liquid nitrogen temperature in glassy matrices (21), and that in the case of liquid methanol at room temperature we also have some free and oligomeric molecules (22).…”

“…This weakness is likely to be due to decreased electric anharmonicity (21) and it makes it possible to observe free and oligomer vibrations, which do not show up in the fundamental region. The contribution of the free and oligomer species to the observed intensity of the overtones increases rapidly with the order of the overtones (22).…”

“…We believe that these facts add weight to the assumption that free O H groups do exist in liquid water at least at room temperature and higher temperatures. Finding "species" in liquid water is not so surprising if we have studied liquid or dissolved alcohols (21,22) and amines (28) at low temperatures, where their bands appear clearly often even at the fundamental level. The proportion of free and oligomer species must be low, however, at least at room temperature, otherwise they would show up more strongly in the fundamental region.…”