The intermolecular interactions of methanol diluted in protic and aprotic solvent probed by polarized Raman spectroscopy and HNMR

Abstract

Polarized Raman spectra of the VV component (R∥) and the VH component (R⊥), 2D-COS synchronous and asynchronous spectra, and HNMR of CH3OH/CH3CN and CH3OH/H2O binary mixture were collected at different volume fractions. The intermolecular interactions of methanol diluted in protic (H2O) and aprotic solvent (CH3CN) were compared and discussed. The experiments showed that the frequency difference ΔνAIS = ν⊥-ν∥ of C-O stretching was −4 for pure methanol and 0 at very dilute concentrations. During the dilution process, the ν∥ of C-O stretching downshifted by approximately 17 cm−1 for CH3OH/H2O binary mixture, while down shifted 5 cm−1 for the CH3OH/CH3CN mixture and 12 cm−1 for the CH3OH/CHCl3 mixture. The dilution process was identified as a cyclic cluster depolymerization process. The difference in amplitude of frequency downshift could be explained by the different properties of the hydrogen bond. Different H-bond cyclic clusters were proposed for different volume fractions of the CH3OH/H2O mixture. Additionally, the chemical shift movement of δOH with concentration also supports the cluster depolymerization process, which is consistent with the results of polarization Raman spectroscopy. The establishment of the coupling-induced spectral splitting theory will play an important role in determining the aggregation behavior of biomolecules and the structure of supramolecular self-assembly.