Investigation of intermolecular interactions in organic solutions by combining two-dimensional correlation Raman spectroscopy and DFT simulation: Example of methanol and Chloralkane

Abstract

Methanol (MeOH)-Dichloromethane (CH2Cl2) and methanol-Chloroform (CHCl3) binary solution systems under the different circumstances were studied using Raman spectroscopy combined with density functional theory (DFT) and two-dimensional correlation spectroscopy (2DCRS). Weak HBs interactions exist in the binary solutions, which can be reflected by the vibration of CH bond (in CH2Cl2 or CHCl3) and CO bond (in MeOH). We explore the influence of mole fractions and temperature of these two vibration modes and perform 2DCRS analysis to reveal the changes intermolecular interactions. The results showed that the Raman spectra of CH vibration of Chloralkane has a frequency shift as the molar fraction of methanol changes both in MeOHCH2Cl2 and MeOHCHCl3 binary solution. In MeOHCH2Cl2, both the bending and stretching vibration of CH move towards higher wavenumber as the MeOH mole fraction increases, but the bending vibration moved towards higher wavenumber and the stretching vibration moved towards lower wavenumber in MeOHCHCl3. This phenomenon is mainly due to the variation of intermolecular HBs and is affected by the vibrational coupling between different chemical bonds. The 2DCRS has a higher resolution, can be used to distinguish the weak peaks masked in one-dimensional spectra, showed that there is a correlation between the chemical bonds of MeOH and CH2Cl2, which proves the existence of HBs, and MeOH is more sensitive to the change of circumstance. The Raman intensity of CH2Cl2 increased more obvious than that of MeOH during the heating process in MeOHCH2Cl2 binary solution, and the full width at half maximum (FWHM) of the Raman peak in the solid phase changed more unregular than that in the liquid phase. Finally, the DFT simulation was used to optimize the geometrical structure under three modes, and vibration analysis and interaction energy calculation were performed. The HBs energies obtained in the three coordination modes are 9.68 kcal/mol, 5.04 kcal/mol and 11.31 kcal/mol, respectively, and the simulated vibration modes and Raman spectra are matched well with the experimental results.