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Abstract:This chapter deals with the theories of vibrational line profiles and anomalous (negative) frequency non-coincidence effects in condensed media. First, a novel approach to the line profile analysis is described. It is based on a new, flexible time-correlation function (TCF), which has an analytical counterpart in the frequency domain. Using this function one can fit vibrational line profiles obtaining dynamical information at the same time. Numerous applications of this TCF are considered, including analyses of both line profiles and dynamics. Second, direct methods of estimation of repulsion contributions to frequency shifts and non-coincidence effects are described, and model calculations enabling one to separate the contributions of repulsion and attraction forces resulting in frequency non-coincidences are presented

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Raman spectroscopy has been used to investigate picosecond vibrational dynamics of the molecular glass-former phenyl salicylate (salol). The study has concentrated on the elucidation and comparison of molecular dynamics of salol subjected to different local environments, namely in bulk, in a dilute CCl4 solution, and under spatial confinement in nanoporous sol–gel glasses. Contrary to most of the previous picosecond dynamics studies performed by means of Raman line profile analysis, we have employed in this paper a novel approach that enables the extraction of valuable short-time dynamics information through spectra fitting in the frequency domain. As a result, strongly overlapping vibrational lines can be put in focus. The analysis has shown a systematic dependence of vibrational relaxation on the local environment around a salol molecule. The magnitude of the vibrational relaxation time τV decreases in the sequence dilute solution→bulk liquid→confined liquid, indicating the modification of interactions when passing from the dilute solution to confined geometries. This fact has been rationalized by invoking the enhancement of the orientational order of salol molecules under spatial confinement. Similar conclusions can be drawn from the analysis of changes of the frequency modulation time τω upon confinement. The changes in second moments of vibrations as well as the observed blueshifts of the Raman peaks in confinement suggest the strengthening of repulsion between salol molecules in the restricted space.

Dimethyl sulfoxide (DMSO) has been known for almost 150 years, but its properties as a solvent and reaction medium are far from being understood. In particular, association equilibriums in liquid DMSO have been characterized in just four papers, and the enthalpy of its self-association is unknown. The aims of this paper are to study self-association equilibriums in neat liquid DMSO at various temperatures by means of Raman spectroscopy, to find the enthalpy of self-association, and to solve the problem of hydrogen bonding in this liquid. Time correlation functions of vibrational dephasing and reorientation of coexisting monomers and dimers studied by the C-S-C and C-H symmetric vibrations indicate that external perturbations and vibrational and reorientational dynamics of these particles occur on different time scales. No signatures of H-bonding in DMSO are found. The association constants vary from 0.20 L mol(-1) (23 °C) to 0.081 L mol(-1) (100 °C). Their temperature dependence gives the enthalpy of association of DMSO as -11.7 ± 0.9 kJ mol(-1).

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The picosecond dynamics of molten alkali halides is discussed, and the low-frequency Raman spectra of molten LiCl, CsCl, and the LiCl–CsCl eutectic are fitted to the model enabling to obtain the times of vibrational dephasing, τV and vibrational frequency modulation τω. In terms of the Wilmshurst criterion [J. Chem. Phys. 39, 1779 (1963)] and using the data of NMR studies and molecular dynamics simulations, a conclusion is drawn that molten alkali halides cannot contain long-lived stable complexes with lifetimes greater than 10−8 s. The low-frequency Raman spectra of molten alkali halides and their mixtures probe the presence of instantaneous spatial configurations of MXn−n+1 type, where M+ is the alkali metal cation and X− is the halide anion existing in melts during the time intervals equal to the time of duration of collision of oppositely charged ions τd, which is less than 0.5 ps. This time is sufficient to a collision complex to execute several (at least one) vibrations. Vibrational dephasing and modulation processes elapse during this same time, thereby indicating the instantaneous nature of configurations in question. To discern between short-lived and long-lived complexes, we propose relations between the minimal damping time of the probe oscillator set equal to the half-period of vibration T/2, τV, τω, and τd, as well as the time between collisions τBC. The duration of an act resulting in the vibrational phase shift (or energy transfer) must be equal to (or longer than) the half-period of vibration of the probe oscillator, τV⩾T/2. The modulation time may vary from this same half-period of vibration or the time between collisions τBC to very long times, τω⩾T/2, τω⩾τBC. For short-lived complexes, the longest of two characteristic times describing the phase decay cannot exceed possible duration of collision, τω⩽τd, τV⩽τd. Cs-containing configurations follow this definition and therefore should be considered instantaneous short-lived collision complexes: their τV≈T/2∼0.1 ps, and τω≈τBC∼0.03 ps. Li-containing configurations appear to be relatively long-lived: their lifetimes could be associated with τω∼0.17 ps, which is several times longer than any other shortest possible characteristic time in the system (τBC∼0.026 ps or T/2∼0.05 ps). In light of these conclusions, an a priori assumption of autocomplex MX4n−4 anions and Mn+ cations as being structural elements of molten halides made in the so-called autocomplex model by Smirnov, Shabanov, and Khaimenov [Elektrohim. 2, 1240 (1966)] is discussed, and the autocomplexes are identified as instantaneous short-lived configurations detectable by the Raman method.