Vibrational relaxation processes in isotropic molecular liquids. A critical comparison

Abstract

Dynamic processes in the liquid state can be examined from different points of view; the rapid development of highly sophisticated time-resolved experiments and very complex computer simulation techniques have certainly improved the accuracy of some labratory results, but they are often not followed by an adequate attempt at interpretation. In any case, the extent of the subject and the enormous spread of experimental data do not help systematic analysis.This review attempts to contribute to a critical examination of a particular aspect of the dynamics of the liquid phase. Through a study of vibrational relaxation in isotropic molecular liquids it is possible to gain information about the molecular environment and the main intermolecular forces operative in this state of matter. It is therefore possible to formulate hypotheses about the force potentials and to describe the dynamic regime of the liquid system under consideration.(1) diatomic and/or highly symmetric molecules (e.g. HCl, 0, , CH,);(2) derivatives of hydrocarbons, especially methane (e.g. CH,NO, , CHCl, , CH,CI,);(3) aromatic and cyclic aliphatic molecules (e.g. benzene and its derivatives, cyclohexane, pyridine); (4) hydrogen bonded (with OH and/or HCO groups).This paper concerns the comparison between vibrational motions that have similar normal coordinate descrip tions; for this reason, we shall limit our considerations to the second and third groups in the above list. Hydrogenbonded and highly symmetrical systems are excluded because they need specific considerations.The currently used basic nomenclature of this field will be illustrated, in addition to the physical meaning of the dynamic parameters usually obtained by steady-state (continuous Raman and infrared), time-resolved spectroscopy and computer simulation techniques; these different experimental data will be compared, when available. We shall also outline and discuss current theoretical models, and attempt to emphasize their potential and limitations.At present, we may single out four groups of molecules that have been investigated by these methods: VIBRATIONAL RELAXATION IN LIQUIDS: THE PROBLEMSince the early 1960s, the study of vibrational relaxation in condensed phases has been considered a way of investigating dynamic processes,' but only with the advent of lasers as spectroscopic sources have frequency-and time-resolved experiments been performed; lineshape and coherent picosecond excitation studies allowed accurate information to be obtained on the dynamics in the solid and liquid states. Since that time, many papers2-14 have proved useful in the development of theoretical models and experimental results; for a detailed analysis of the whole question, one may refer to the above publications and references therein. For the purpose of this paper, the basic definitions of the main physical entities and variables involved in the subject must be clarified, avoiding the conflicts and ambiguities often present in the literature. DEPHASING: DEFINITIONSDephasing is important as probe of the...

Probing self‐associated structures of the solute molecule, acrylonitrile, the solvent molecule 2Cl‐phenol and their binary complexes via concentration‐dependent Raman study and DFT calculation

“…1, in neat form shows two components, one at ¾1029 cm 1 due to free 2Cl-phenol molecules and the other one at ¾1034 cm 1 due to self-associated 2Cl-phenol molecules. On diluting 2Cl-phenol with acrylonitrile, the peak position of the ¾1029 cm 1 band shifts by ¾0.5 cm 1 , whereas there is a corresponding decrease of ¾0.2 cm 1 in going from neat liquid to mole fraction 0.1 of Cl-phenol (C D 0.9) for the 1034 cm 1 Further, on mixing more and more of acrylonitrile, leading to decrease in the mole fraction of 2Cl-phenol in the mixture, the ¾1034 cm 1 band increases in intensity at the cost of loss in the intensity of the ¾1029 cm 1 band (Fig. 1).…”

“…Viscosity may be another factor that causes variation in linewidth. 1 By taking into account the microviscosity instead of dynamic viscosity of the solute as well as of the mixture at different concentrations, using the model proposed recently by Ojha et al, 16 experimentally measured linewidth corrected for slit function, is represented by the following relationship.…”

“…A large amount of work has been done over the past two decades and a detailed account of such studies has been presented by Morresi et al 1 In another review, Asthana and Kiefer 2 have also discussed several interesting examples of hydrogen-bonded systems. A direct visualization of vibrational dephasing of a vibrational mode can essentially be achieved through time-resolved studies.…”

“…Linewidth and peak position variation of 1029 cm1 band of 2Cl-phenol at different mole fractions of 2Cl-phenol in the binary mixture acrylonitrile C 2Cl-phenol along with the linear fit for the peak position and the fitted curve for the linewidth using Eqn(2). Linewidth variation of ¾1027 cm 1 and ¾1035 cm 1 band of 2Cl-phenol at different mole fractions of 2Cl-phenol in the binary mixture acrylonitrile C 2Cl-phenol along with the fitted curve for the linewidth using Eqn(2).…”

Raman and DFT study of hydrogen‐bonded 2‐ and 3‐chloropyridine with methanol

“…It is due to the complexity in the liquid phase that many analytical models had been proposed 22 -27 to investigate the dephasing mechanism and to explain the variation in linewidth, peak position and intensity with concentration. Many such theoretical models have been well reviewed by Morresi et al 28 The Raman linewidth contains dynamical information about the vibrational relaxation (dephasing) and reorientational motion in molecular liquids and solids, 29 whereas the peak position gives information on the static interactions basically related to the force constant, electronic structure and bonding. The integrated intensity of the Raman band, when measured carefully with respect to a stable reference mode recorded at similar experimental conditions, gives definite information about the amount of association.…”