Level-dependent damping in intermolecular vibrations: Linear spectroscopy

Assessing the role of moment of inertia in optical Kerr effect spectroscopy

Optical Kerr effect (OKE) spectroscopy allows for the acquisition of high-quality, Bose-Einstein-corrected, low-frequency Raman spectra in liquids. However, the assignment of a molecular interpretation to these spectra remains an open problem. To address this issue, here we present an OKE study of benzene and four of its isotopologues. Our results indicate that hindered rotations are the major contributor to the OKE reduced spectral density (RSD) over the entire intermolecular spectral region (0-250 cm(-1)). We also have found that isotopologues with six (13)C atoms have RSDs that are enhanced at frequencies below 30 cm(-1). We further demonstrate that the collective orientational correlation time of these liquids scales with the inverse square root of the tumbling moment of inertia, indicating that there is strong translation-rotation coupling in benzene.

Ultrafast dynamics in complex fluids observed through the ultrafast optically-heterodyne-detected optical-Kerr-effect (OHD-OKE)

The ultrafast molecular dynamics of complex fluids have been recorded using the optically-heterodyne-detected optical-Kerr-effect (OHD-OKE). The OHD-OKE method is reviewed and some recent refinements to the method are described. Applications to a range of complex fluids, including microemulsions, polymer melts and solutions, liquid crystal and ionic liquids are surveyed. The level of detail attainable with the OHD-OKE method in these complex fluids is discussed. The prospects for future experiments are discussed.

Temperature-Dependent Optical Kerr Effect Spectroscopy of Aromatic Liquids

Ultrafast optical Kerr effect (OKE) spectroscopy has been used to study the temperature-dependent dynamics of five aromatic liquids: benzene, benzene-d(6), hexafluorobenzene, mesitylene, and 1,3,5-trifluorobenzene. The intermediate response time of all of the liquids was found to scale with the collective orientational correlation time, as has been observed for other simple liquids. The spectra of hexafluorobenzene, 1,3,5-trifluorobenzene, and mesitylene are qualitatively different from those of the other liquids and exhibit different behavior with temperature. These spectra allow us to assess the influence of different molecular parameters on the shape of the OKE spectrum. On the basis of these data, we propose a model that links the differences in the OKE spectra to corresponding differences in the local ordering of the liquids.

Temperature- and solvation-dependent dynamics of liquid sulfur dioxide studied through the ultrafast optical Kerr effect

The ultrafast dynamics of liquid sulphur dioxide have been studied over a wide temperature range and in solution. The optically heterodyne-detected and spatially masked optical Kerr effect (OKE) has been used to record the anisotropic and isotropic third-order responses, respectively. Analysis of the anisotropic response reveals two components, an ultrafast nonexponential relaxation and a slower exponential relaxation. The slower component is well described by the Stokes-Einstein-Debye equation for diffusive orientational relaxation. The simple form of the temperature dependence and the agreement between collective (OKE) and single molecule (e.g., NMR) measurements of the orientational relaxation time suggests that orientational pair correlation is not significant in this liquid. The relative contributions of intermolecular interaction-induced and single-molecule orientational dynamics to the ultrafast part of the spectral density are discussed. Single-molecule librational-orientational dynamics appear to dominate the ultrafast OKE response of liquid SO2. The temperature-dependent OKE data are transformed to the frequency domain to yield the Raman spectral density for the low-frequency intermolecular modes. These are bimodal with the lowest-frequency component arising from diffusive orientational relaxation and a higher-frequency component connected with the ultrafast time-domain response. This component is characterized by a shift to higher frequency at lower temperature. This result is analyzed in terms of a harmonic librational oscillator model, which describes the data accurately. The observed spectral shifts with temperature are ascribed to increasing intermolecular interactions with increasing liquid density. Overall, the dynamics of liquid SO2 are found to be well described in terms of molecular orientational relaxation which is controlled over every relevant time range by intermolecular interactions.

Why Does the Intermolecular Dynamics of Liquid Biphenyl so Closely Resemble that of Liquid Benzene? Molecular Dynamics Simulation of the Optical-Kerr-Effect Spectra

The combination of optical-Kerr-effect (OKE) spectroscopy and molecular dynamics simulations has provided us with a newfound ability to delve into the librational dynamics of liquids, revealing, in the process, some surprising commonalities among aromatic liquids. Benzene and biphenyl, for example, have remarkably similar OKE spectra despite marked differences in their shapes, sizes, and moments of inertia--and even more chemically distinct aromatics tend to have noticeable similarities in their spectra. We explore this universality by using a molecular dynamics simulation to investigate the librational dynamics of molten biphenyl and to predict its OKE spectrum, comparing the results with our previous calculations for liquid benzene. We suggest that the impressive level of quantitative agreement between these two liquids is largely a reflection of the fact that librations in these and other aromatic liquids act as torsional oscillations with oscillator frequencies selected from the liquid's librational bands. Since these bands are centered about the librational Einstein frequencies, the quantitative similarities between the liquids are essentially reflections of the near identities of their Einstein frequencies. Why then are the Einstein frequencies themselves so insensitive to molecular details? We show that, for nearly planar molecules, mean-square torques and moments of inertia tend to scale with molecular dimensions in much the same way. We demonstrate that this near cancellation provides both a quantitative explanation of the close relationship between benzene and biphenyl and a more general perspective on the similarities seen in the ultrafast dynamics of aromatic liquids.

Effects of Molecular Association on Polarizability Relaxation in Liquid Mixtures of Benzene and Hexafluorobenzene

In this work we have studied the relaxation dynamics of the many-body polarizability anisotropy in liquid mixtures of benzene (Bz) and hexafluorobenzene (Hf) at room temperature by femtosecond optical heterodyne-detected Raman-induced Kerr effect spectroscopy (OHD-RIKES) experiments and molecular dynamics (MD) simulations. The computed polarizability response arising from intermolecular interactions was included using the first-order dipole-induced-dipole model with the molecular polarizability distributed over the carbon sites of each molecule. We found good qualitative agreement between experiments and simulations in the features exhibited by the nuclear response function R(t) for pure liquids and mixtures. The long-time diffusive decay of R(t) was observed to vary substantially with composition, slowing down noticeably with dilution of each of the species as compared with that in the corresponding pure liquids. MD simulation shows that the effect on R(t) is due to the formation of strong and localized intermolecular association between Bz and Hf species that hinder the rotational diffusive dynamics. The formation of these Bz-Hf complexes in the liquid mixtures also modifies the rotational diffusive dynamics of the component species in such a way that cannot be explained solely in terms of a viscosity effect. Even though the computed orientational diffusive relaxation times associated with Bz and Hf are larger by a factor of approximately 2 than those from experiments, we found similar trends in experiments and simulations for these characteristic times as a function of composition. Namely, the collective and single-molecule orientational correlation times associated with Bz are observed to grow monotonically with the dilution of Bz, while those corresponding to Hf species exhibit a maximum at the equimolar composition. We attribute the quantitative discrepancy between experiments and simulations to the use of the Williams potential, which seems to overestimate the intermolecular interactions and thus predicts not only a slower translational dynamics but also a slower rotational diffusion dynamics than in real fluids.

Physical Properties and Intermolecular Dynamics of an Ionic Liquid Compared with Its Isoelectronic Neutral Binary Solution

In this study, we address the following question about room-temperature ionic liquids (RTILs). Are the properties of a RTIL more dependent on the charges of the molecular ions or on the fact that the liquid is a complex mixture of two species, one or both of which are asymmetric? To address this question and to better understand the interactions and dynamics in RTILs, we have prepared the organic ionic liquid 1-methoxyethylpyridinium dicyanoamide (MOEPy(+)/DCA(-)) and compared this RTIL with the analogous isoelectronic binary solution, comprised of equal parts of 1-methoxyethylbenzene (MOEBz) and dicyanomethane (DCM). In essence, we have created a RTIL and a nearly identical neutral pair in which we have effectively turned off the charges. To understand the intermolecular interactions in both of these liquids, we have characterized the bulk density and shear viscosity. Using femtosecond optical Kerr effect spectroscopy, we have also characterized the intermolecular vibrational dynamics and diffusive reorientation. To verify that the shape, polarizability, and electronic structure of the RTIL ions and the components of the neutral pair are truly quite similar, we have carried out density functional theory calculations on the individual molecular ion and neutral species.

Time-resolved optical Kerr-effect investigation on CS2/polystyrene mixtures

The relaxation dynamics of carbon disulfide are investigated in mixtures with polystyrene (PS) using the time-resolved optical heterodyne-detected optical Kerr effect (OHD-OKE). The data are analyzed using both the model-dependent approach, which assumes four distinct temporal responses, and the model-independent Fourier transform approach, which generates a spectral response that can be compared with results obtained by depolarized Rayleigh scattering. A slow dynamics is observed for the OHD-OKE transient decaying exponentially with a time constant that varies from 1.68 ps for neat CS2 to 3.76 ps for the most concentrated CS2PS mixture. The increase of this time constant accompanies an increase in the viscosity of the mixture, so we can associate this component with the diffusive reorientation process of the induced polarizability anisotropy of the carbon disulfide in the mixture. The short-time nuclear response is characterized in the frequency domain by a broad band that peaks around 30 cm(-1) for neat carbon disulfide, and is associated with a complex relaxation pattern. The vibrational distribution shifts to higher frequencies when the PS concentration is increased in the mixture. This result is discussed in terms of an increase in the interaction strength between the PS phenyl rings and the carbon disulfide molecules.

Probing Excited-State Dynamics and Intramolecular Proton Transfer in 1-Acylaminoanthraquinones via the Intermolecular Solvent Response

The dynamics of a series of 1-acylaminoanthraquinones with varying degrees of excited-state intramolecular proton transfer are studied in acetonitrile and dichloromethane. Events are followed via changes in the third-order intermolecular Raman response as a function of time after resonant excitation of the chromophore. Compared to electronically resonant probes of the solute, measuring the ultrafast dynamics using the nonresonant solvent response offers a new and complementary perspective on the events that accompany excitation and proton transfer. Experimentally observed changes in the nuclear polarizability of the solvent follow dynamic changes in the solvent-solute interactions. Reorganization of the solvent in response to the significant changes in the intermolecular interactions upon proton transfer is found to play an important role in the reaction dynamics. With transfer of the proton taking place rapidly, the solvent controls the dynamics via the time-dependent evolution of the free energy surface, even on subpicosecond time scales. In addition, the solvent response probes the effects of intermolecular energy transfer as energy released during the reactive event is rapidly transferred to the local solvent environment and then dissipates to the bulk solvent on about a 10 ps time scale. A brief initial account of a portion of this work has appeared previously, J. Am. Chem. Soc. 2004, 126, 8620-8621.

Measuring the Change in the Intermolecular Raman Spectrum during Dipolar Solvation

We demonstrate a method to directly measure the change in the spectrum of intermolecular solvent fluctuations as a function of time after electronic excitation of a solute, and this method is applied to the dye Coumarin 102 (C102) in acetonitrile. The complete intermolecular response is captured following resonant excitation with time domain third-order Raman spectroscopy. In a previous report, we introduced this method and used it to probe one point in the intermolecular response as a function of time after solute excitation (Underwood, D. F., Blank, D. A. J. Phys. Chem. A 2003, 107 (7), 956). Here we extend this approach to recover the change in the entire intermolecular response as a function of time. To our knowledge the results provide the first direct measurement of the difference in the equilibrated intermolecular response after excitation of a solute and its evolution during a dipolar solvation event. Excitation of C102 results in a significant increase in the solvent-solute interaction due to a large increase in the dipole moment. The observed change in the intermolecular response is consistent with a rapid change in local solvent density, with intermolecular kinetic energy transfer changing the response on longer time scales. Evolution of the response exhibits a strong frequency dependence and suggests changes over longer distances at longer delay times. The measured change in the spectrum of solvent fluctuations represents a direct experimental confirmation of the breakdown of linear response and confirms predictions from molecular dynamics simulations.