On the disruptive effects of water on theH-bond network of methanol studied by the noncoincidence effect

Raman spectroscopy represents an invaluable tool to shed light into the nature of solvation processes. The interpretation of changes in band frequency, intensity and asymmetry observed upon dilution may reveal the nature of the intermolecular interactions in action, the chemical groups involved, the onset of structuring and disruptive effects such as those occurring in the formation of clusters and of self-assembled structures. Water, by virtue of its high dielectric constant and, additionally, its high performance as proton donor and acceptor, plays a central role in the study of solvation processes of several chemical compounds. In this contribution we report some preliminary results of Raman measurements of the (CO) band of methanol aqueous solutions at different concentrations with the aim of learning more about the progressive disruptive effects on the methanol H-bonded structure. The observables we are looking are the concentration dependences of the frequency shifts of the isotropic and anisotropic Raman profiles of the (CO) mode and then of the noncoincidence effect (NCE) for this mode [1]. The observation of different slopes of the frequency red shift concentration dependence for both these profiles confirms the idea that the hydration process of methanol passes through different regimes down to xmeth 0.2 where the solvation by H-bonds gets saturated [2]. A challenging scientific question regarding alcohol/water mixtures is that of the occurrence of the self-association which has been invoked in the interpretation of the low-frequency Raman spectra of ethanol/water mixtures [3] and of the neutron scattering results of methanol/water mixtures [4]. With this study we intend to confirm or disprove the existence of this phenomenon in methanol/water mixtures by determining their structures on the basis of the concentration dependence of NCE of the (CO) mode of methanol.