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 laboratory 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.

At present, we may single out four groups of molecues that have been investigated by these methods:

(1) diatomic and/or highly symmetric molecules (e.g. HCI, O2, CH4);

(2) derivatives of hydrocarbons, especially methane (e.g. CH3NO2, CHCl3, CH2Cl2);

(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 descriptions; for this reason, we shall limit our considerations to the second and third groups in the above list. Hydrogen-bonded 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.

Vibrational relaxation processes in isotropic molecular liquids. A critical comparison

The effect of the time delay between the picosecond Raman pump and the femtosecond Stokes probe pulse on the Raman gain line shape in femtosecond broadband stimulated Raman spectroscopy (FSRS) is presented Experimental data are obtained for cyclohexane to investigate the dependence of the FSRS line shape on this time delay Theoretical simulations of the line shapes as a function of the time delay using the coupled wave theory agree well with experimental data, recovering broad line shapes at positive time delays and narrower bands with small Raman loss side wings at negative time delays The analysis yields the lower bounds of the vibrational dephasing times of 20 ps and 065 ps for the 802 and 1027 cm−1 modes for cyclohexane, respectively The theoretical description and simulation using the coupled wave theory are also consistent with the observed Raman gain intensity profile over time delay, reaching the maximum at a slightly negative time delay (∼−1 ps), and show that the coupled wave theory is a g

Dependence of line shapes in femtosecond broadband stimulated Raman spectroscopy on pump-probe time delay.

Dielectric spectra from 200 MHz up to 3 THz were determined to study the fast dynamics of dilute water+1,4-dioxane. epsilon(nu) could be fitted by a collision induced oscillator at high frequencies plus two Debye relaxations in the microwave region. Isotope substitution was used to assign water and dioxane modes. The presence of the cooperative hydrogen-bond network relaxation down to a water mole fraction of 0.005 suggests a microheterogeneous structure of the mixtures even at low water content. The collision mode of dioxane at approximately 2 THz grows upon water addition, revealing the presence of H2O molecules in dioxane-rich domains.

Picosecond dynamics and microheterogenity of water+dioxane mixtures.

The Raman band of the amide I vibrational mode of N, N-dimethylformamide was studied in the neat liquid and as a function of solvent concentration. The non-coincidence effect (anisotropy shift) is explained in terms of transition dipole-transition dipole interactions. The vibrational relaxation rate has been shown to be linearly dependent on the product of the density, viscosity and a dispersion energy-dependent term, for which a theoretical basis is outlined.

Raman scattering and vibrational relaxation studies of N, N‐dimethylformamide

Molecular dynamics simulations for the six‐center Lennard‐Jones model of C6H12 are reported. The potential parameters have been adjusted to fit cyclohexane properties along the saturated vapor pressure curve. The behavior of cyclohexane molecules in the bulk liquid and in the plastic crystal state is studied. Satisfactory agreements between experimental data and calculated heat capacities, diffusion coefficients, rotational and angular velocity relaxation times, as well as radial distribution functions, are reported.

Molecular dynamics simulation of liquid-solid phase transition of cyclohexane. I

Reorientational correlation functions have been obtained from the analysis of the totally symmetric vibrations ν4, ν5 and ν2 of liquid cyclohexane. For the interpretation of the data, the Kubo line shape theory was assumed to be valid. The most reliable results were obtained from the ν4 vibration and indicated that reorientational relaxation occurs close to the Debye limit of rotational diffusion. The data are compared with NMR results given in the literature. The effects of rotational-vibrational coupling are discussed.

Raman studies of reorientational relaxation of cyclohexane in the liquid state

The Raman spectra of the degenerate bands of cyclohexane and cyclohexane-d12 have been recorded at medium resolution in the gas phase. The computer simulation of the band contours has allowed to determine the Coriolis coupling constants ζ19, ζ20, ζ21, ζ22, ζ23, and ζ24.

T. Bien

Papers

Raman studies of reorientational relaxation of cyclohexane in the liquid state

Band contours of the degenerate fundamental bands of cyclohexane and cyclohexane-d12 determination of the coriolis constants