C–H···O Interaction in Methanol–Water Solution Revealed from Raman Spectroscopy and Theoretical Calculations

Abstract

A combination of temperature-dependent Raman spectroscopy and quantum chemistry calculation was employed to investigate the blue shift of CH stretching vibration in methanol-water mixtures. It shows that the conventional O-H···O hydrogen bonds do not fully dominate the origin of the C-H blue shift and the weak C-H···O interactions also contribute to it. This is consistent with the temperature-dependent results, which reveal that the C-H···O interaction is enhanced upon increasing the temperature, leading to further C-H blue shift in observed spectra at high temperature. This behavior is in contrast with the general trend that the conventional O-H···O hydrogen bond is destroyed by the temperature. The results will shed new light onto the nature of the C-H···O interaction and be helpful to understand hydrophilic and hydrophobic interactions of amphiphilic molecules in different environments.

Site-specific X-ray induced dynamics in liquid methanol

“…The population of lone pairs around O-H is also higher, due to the strong O-HÁ Á ÁO hydrogen bonds formed in methanol. 40 C-HÁ Á ÁO hydrogen bonds have been observed for methanol in water, 41 where the -CH 3 group can only interact with an oxygen lone pair. In pure methanol this is not the case, since the hydroxyl groups preferentially interact with each other, thereby creating hydrophobic domains.…”

Rapid, accurate, and precise concentration measurements of a methanol–water mixture using Raman spectroscopy

“…It is likely that our observed deviation from the true methanol mass fraction is a result of real changes in the mixture. Indeed, it has been shown that water-methanol [10,12,21,22] and waterethanol [22][23][24] mixtures form different water-methanol complexes at different concentrations, and that these complexes display fundamentally different Raman spectra from that of the pure compound. Thus, fundamentally, the spectrum of a water-methanol mixture cannot be perfectly modeled as a linear combination of the spectra of the two components.…”

“…Since a Raman measurement only requires optical access to the sample, and can be completed using relatively inexpensive lasers and spectrometers, it is ideally suited for probing the precise concentrations of mixtures in a laboratory, field, or industrial setting. While Raman spectroscopy has indeed been demonstrated for sensing the concentrations of solutions [4][5][6][7][8], liquid mixtures [9][10][11][12], and gas mixtures [13], the accuracy is typically observed to be on the level of a few percent, the precision is often not quantified, and long-term systematic uncertainties are not investigated. Moreover, demonstrations are typically completed in a laboratory setting and with bulky, expensive equipment.…”

Probe of Alcohol Structures in the Gas and Liquid States Using C–H Stretching Raman Spectroscopy

“…Among the blue-shifted hydrogen bonds, the C–H···O is the most common one. In methanol-water solution, the frequency of CH 3 stretching vibration was shifted to higher wavenumbers due to the increase in water concentration, as shown in Figure 12 [ 71 ]. This is consistent with the expected behavior for the formation of a blue-shifted hydrogen bond between the methyl group of methanol and the O atom of water (C–H···O).…”

“…This is consistent with the expected behavior for the formation of a blue-shifted hydrogen bond between the methyl group of methanol and the O atom of water (C–H···O). However, the theoretical calculation from a cluster model indicated that this C–H blue shift can also be indirectly caused by a normal hydrogen bond O–H···O in methanol-water solution [ 71 ]. To distinguish the contribution of O–H···O from that of C–H···O and provide evidence for presence of C–H···O in methanol-water solution, the temperature-dependent C–H spectra are recorded in the range of 5–75 °C at the concentration of 1% solution ( Figure 13 ).…”