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/CCl 4 mixtures in the range of 1.0 g x m g 0.1, where x m 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 CCl 4 down to x m ∼ 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 x m ∼ 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 hydrogenbonded 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.

Raman noncoincidence effect on OH stretching profiles in liquid alcohols

“…For asymmetric bands, the M1 of the distributions must be taken into account. 9,32 In Fig. 3, we compare ISO, HV and IR OH stretching profiles, baseline corrected, for pure liquid 1-octanol at T D 20°C; the large Raman NCE is clearly evidenced.…”

Simultaneous photon absorption as a probe of molecular interaction and hydrogen-bond cooperativity in liquids

“…4͒, we obtain the root-mean-square coupling constant ͗␤ 2 ͘ 1/2 =46±1 cm −1 . This value may be compared to the Raman noncoincidence effect in liquid methanol, [17][18][19] the magnitude of which is an ͑orientationally weighted͒ ensemble average of the intermolecular coupling. [20][21][22] In liquid MeOH, the noncoincidence effect is 94 cm −1 , 18 about twice the value of the coupling estimated here from the simultaneous absorption of MeOH dissolved in MeOD.…”

“…This value may be compared to the Raman noncoincidence effect in liquid methanol, [17][18][19] the magnitude of which is an ͑orientationally weighted͒ ensemble average of the intermolecular coupling. [20][21][22] In liquid MeOH, the noncoincidence effect is 94 cm −1 , 18 about twice the value of the coupling estimated here from the simultaneous absorption of MeOH dissolved in MeOD. The discrepancy may be due to the fact that in dilute MeOH:MeOD an OH group involved in simultaneous absorption interacts with only one other OH group, whereas in liquid MeOH each OH group interacts with at least two other OH groups, 23,24 so that the Raman noncoincidence effect observed in liquid MeOH arises from multiple OH-OH couplings.…”

“…12,13,25 The information obtained from the simultaneous absorption spectrum is to some extent comparable to that derived from two-dimensional optical spectroscopy methods, [29][30][31] which can also be used to determine couplings between vibrations 14,32,33 and correlations between vibrational fre- quencies. However, two-dimensional vibrational spectroscopy requires that the coupled vibrations have different center frequencies, whereas simultaneous absorption, like the Raman noncoincidence effect, 17,18,20,21 can also provide information on interacting vibrations having the same center frequency.…”

Excess thermodynamic properties in liquid binary mixtures

“…the so-called non-coincidence effect (NCE). 1 NCE is related to the resonant vibrational energy transfer between identical vibrational modes of neighbouring molecules, which may be established in the presence of short-range order. 2,3 The results of that study allowed the authors to infer concentrationdependent structural changes of the mixture due to the existence of different polymeric species.…”