The aggregation structure of a methanol/CHCl3 binary mixture investigated by polarized Raman spectroscopy and HNMR

The structural evolution of methanol in the protic solvent chloroform was studied by polarized Raman spectroscopy. Negative noncoincidence effects were detected for the full range of concentrations of the methanol/CHCl3 binary mixture, that is, nu(perpendicular to) < nu(parallel to) for the C-O stretching vibration. With the dilution of methanol/CHCl3 by CHCl3, the vibration frequency of the C-O bond moves to a lower wavenumber for both parallel nu(parallel to) and vertical nu(perpendicular to) Raman spectra, contrary to the phenomenon observed for the methanol/CCl4 mixture. The absolute value of the frequency difference vertical bar nu(perpendicular to) - nu(parallel to)vertical bar increased with volume fraction dilution from 1 to 0.3 and then sharply decreased from 0.3 to 0.03. Aggregation-induced split (AIS) theory was applied to explain these phenomena. PCM models of the most likely different structures of H-bond combined cyclic multimers, CHCl3 H-bonded multimers, and CHCl3 H-bonded monomers were established for different concentration stages. The DFT calculations based on these models of different concentrations can aptly reproduce the noncoincidence effect and concentration-dependent phenomenon. (C) 2021 Elsevier B.V. All rights reserved.

Automated summary

The structural evolution of methanol in chloroform solvent was studied and a negative noncoincidence effect was observed. As methanol/CHCl3 was diluted by CHCl3, the vibration frequency of the C-O bond moved to lower wavenumbers, and the absolute value of the frequency difference also changed.