Noncoincidence Effect of Vibrational Bands of Methanol/CCl₄ Mixtures and Its Relation with Concentration-Dependent Liquid Structures

Abstract

The concentration dependence of the Raman noncoincidence effect (NCE) of the C−O and O−H stretching bands of methanol is investigated in methanol/CCl4 mixtures in the range of 1.0 ≥ xm ≥ 0.1, where xm is the mole fraction of methanol, by performing Raman spectroscopic measurements and molecular dynamics (MD) simulations. Band asymmetry observed for both bands is carefully taken into account. The experimental and simulation results are in satisfactory agreement with each other. For the C−O stretching band, it is observed that the magnitude of the negative NCE gets larger upon dilution in CCl4 down to xm ∼ 0.2, contrary to the expectation of becoming smaller from simple guess that the NCE arises from intermolecular vibrational resonant interactions between methanol molecules, which, on average, get separated from each other upon dilution. For the O−H stretching band, the magnitude of the positive NCE remains almost the same upon dilution down to xm ∼ 0.3. These apparently peculiar experimental results are reasonably explained by the MD simulations on the basis of the transition dipole coupling (TDC) mechanism of intermolecular resonant vibrational interactions and the simulated hydrogen-bonded liquid structures. In the case of the C−O stretching band, the negative NCE arises mainly from positive vibrational coupling between hydrogen-bonded pairs of molecules, which is partially canceled by negative vibrational coupling between molecules in different hydrogen-bonded chains. In the case of the O−H stretching band, the positive NCE arises predominantly from negative vibrational coupling within hydrogen-bonded chains. As a result, a locally anisotropic change in the liquid structure that occurs upon dilution, in which, around each molecule, intermolecular distances do not change very much along hydrogen-bond directions but do change significantly in other directions, gives rise to the apparently peculiar behavior of the NCE described above.