Interaction-induced contributions to Rayleigh and allowed Raman bands

Optical Kerr effect spectroscopy of CS2 in monocationic and dicationic ionic liquids: insights into the intermolecular interactions in ionic liquids

A comparative study of the intermolecular dynamics of CS₂ in monocationic and dicationic ionic liquids (ILs) was performed using optical heterodyne-detected Raman-induced Kerr effect spectroscopy (OHD-RIKES). The reduced spectral densities (RSDs) of mixtures of CS₂ in 1-alkyl-3-methylimidazolium bis[(trifluoromethane)sulfonyl]amide ([C_nC₁im][NTf₂] for n = 3-5) and 1,2n-bis(3-methylimidazolium-1-yl) alkane bis[(trifluoromethane)sulfonyl]amide ([(C₁im)₂C_2n][NTf₂]₂ for n = 3-5) were investigated as a function of concentration at 295 K. An additivity model was used to obtain the CS₂ contribution to the RSD of a mixture in the 0-200 cm^-1 region. One of the aims of this study is to show how CS₂ can be used as a probe of intermolecular/interionic interactions in ILs. The concentrations were chosen such that the CS₂-to-imidazolium ring mole fraction of a mixture with [(C₁im)₂C_2n][NTf₂]₂ (DIL(2n)) is the same as that of a mixture with [C_nC₁im][NTf₂] (MIL(n)). As found previously for CS₂ in monocationic ILs, the intermolecular spectrum of CS₂ in dicationic ILs is lower in frequency and narrower than that of neat CS₂. The new result is that the intermolecular spectrum of CS₂ is higher in frequency in DIL(2n) than in the corresponding MIL(n), indicating that CS₂ molecules experience a stiffer potential in dicationic ILs than in monocationic ILs. The intermolecular dynamics of CS₂ being higher in frequency in DIL(2n) than in MIL(n) is consistent with recent molecular dynamics simulations (Lynden-Bell and Quitevis, J. Chem. Phys., 2018, 148, 193844) that show the stiffer potential is the result of greater confinement of CS₂ in DIL(2n) than in MIL(n). We also show in this study how effects due to dilution and the intermolecular potential seen by a solute molecule in solution are unraveled.

Vibrational Spectroscopy of Ionic Liquids

Vibrational spectroscopy has continued use as a powerful tool to characterize ionic liquids since the literature on room temperature molten salts experienced the rapid increase in number of publications in the 1990's. In the past years, infrared (IR) and Raman spectroscopies have provided insights on ionic interactions and the resulting liquid structure in ionic liquids. A large body of information is now available concerning vibrational spectra of ionic liquids made of many different combinations of anions and cations, but reviews on this literature are scarce. This review is an attempt at filling this gap. Some basic care needed while recording IR or Raman spectra of ionic liquids is explained. We have reviewed the conceptual basis of theoretical frameworks which have been used to interpret vibrational spectra of ionic liquids, helping the reader to distinguish the scope of application of different methods of calculation. Vibrational frequencies observed in IR and Raman spectra of ionic liquids based on different anions and cations are discussed and eventual disagreements between different sources are critically reviewed. The aim is that the reader can use this information while assigning vibrational spectra of an ionic liquid containing another particular combination of anions and cations. Different applications of IR and Raman spectroscopies are given for both pure ionic liquids and solutions. Further issues addressed in this review are the intermolecular vibrations that are more directly probed by the low-frequency range of IR and Raman spectra and the applications of vibrational spectroscopy in studying phase transitions of ionic liquids.

Structure and dynamics of liquid CS2: Going from ambient to elevated pressure conditions

Molecular dynamics simulation studies were performed to investigate the structural and dynamic properties of liquid carbon disulfide (CS₂) from ambient to elevated pressure conditions. The results obtained have revealed structural changes at high pressures, which are related to the more dense packing of the molecules inside the first solvation shell. The calculated neutron and X-ray structure factors have been compared with available experimental diffraction data, also revealing the pressure effects on the short-range structure of the liquid. The pressure effects on the translational, reorientational, and residence dynamics are very strong, revealing a significant slowing down when going from ambient pressure to 1.2 GPa. The translational dynamics of the linear CS₂ molecules have been found to be more anisotropic at elevated pressures, where cage effects and librational motions are reflected on the shape of the calculated time correlation functions and their corresponding spectral densities.

The importance of polarizability: comparison of models of carbon disulphide in the ionic liquids [C1C1im][NTf2] and [C4C1im][NTf2]

Three dimensional distribution of CS₂ around a [C₁C₁im]⁺ ion showing the difference in behaviour of polarizable (red) and unpolarizable (blue) models of CS₂.

Assessing the non-ideality of the CO2-CS2 system at molecular level: a Raman scattering study

The dense phase of CO2-CS2 mixtures has been analysed by Raman spectroscopy as a function of the CO2 concentration (0.02-0.95 mole fractions) by varying the pressure (0.5 MPa up to 7.7 MPa) at constant temperature (313 K). The polarised and depolarised spectra of the induced (ν2, ν3) modes of CS2 and of the ν1-2ν2 Fermi resonance dyad of both CO2 and CS2 have been measured. Upon dilution with CO2, the evolution of the spectroscopic observables of all these modes displays a "plateau-like" region in the CO2 mole fraction 0.3-0.7 never previously observed in CO2-organic liquids mixtures. The bandshape and intensity of the induced modes of CS2 are similar to those of pure CS2 up to equimolar concentration, after which variations occur. The preservation of the local ordering from pure CS2 to equimolar concentration together with the non-linear evolution of the spectroscopic observables allows inferring that two solvation regimes exist with a transition occurring in the plateau domain. In the first regime, corresponding to CS2 concentrated mixtures, the liquid phase is segregated with dominant CS2 clusters, whereas, in the second one, CO2 monomers and dimers and CO2-CS2 hetero-dimers coexist dynamically on a picosecond time-scale. It is demonstrated that the subtle interplay between attractive and repulsive interactions which provides a molecular interpretation of the non-ideality of the CO2-CS2 mixture allows rationalizing the volume expansion and the existence of the plateau-like region observed in the pressure-composition diagram previously ascribed to the proximity of an upper critical solution temperature at lower temperatures.

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.

Molecular Dynamics Study of Polarizability Anisotropy Relaxation in Aromatic Liquids and Its Connection with Local Structure

The collective polarizability anisotropy dynamics in a set of three aromatic liquids, benzene (Bz), hexafluorobenzene (HFB), and 1,3,5-trifluorobenzene (TFB), has been studied by molecular dynamics simulation. These liquids have very similar shapes, but different electrostatic interactions due to opposite polarities of C-H and C-F bonds, giving rise to different local intermolecular structures in the liquid phase. We have investigated how these structural arrangements affect polarizability anisotropy dynamics observed in optical Kerr-effect (OKE) spectroscopy. We have modeled the interaction-induced polarizability with the first-order dipole-induced dipole approximation, with the molecular polarizability distributed over the carbon sites. Local contributions to the librational OKE spectrum were computed separately for molecules participating in parallel or perpendicular relative orientations within the first coordination shell. We found that the relative locations of parallel and perpendicular librational bands of the OKE spectra are closely related to the corresponding pair energy distributions of the closest four neighbors of a given molecule, corresponding to a model of a harmonic oscillator in a cage of nearest neighbors. This model predicts higher librational frequencies for more attractive intermolecular interactions, which in all three liquids correspond to parallel local arrangements. On the diffusive orientational time scale, all three liquids exhibit slower relaxation of molecules in parallel arrangements, although the difference in relaxation rates is substantial only in TFB, which has the strongest tendency toward parallel stacking. The analysis of the collective polarizability relaxation was performed using two different approaches, the projection scheme (J. Chem. Phys. 1980, 72, 2801) and the theory developed by Steele (Mol. Phys. 1987, 61, 1031) for the second time derivatives applied to collective time correlations. Both approaches allow the decomposition of the OKE response into contributions from orientational relaxation and other dynamical processes. We find that they lead to different predictions on how the response depends on collective reorientation and processes arising from fluctuations in the interaction-induced polarizability. We discuss the reasons for these differences and the advantages and disadvantages of the two analysis schemes.

Two-dimensional Raman spectra of atomic solids and liquids

We calculate third- and fifth-order Raman spectra of simple atoms interacting through a soft-core potential by means of molecular-dynamics (MD) simulations. The total polarizability of molecules is treated by the dipole-induced dipole model. Two- and three-body correlation functions of the polarizability at various temperatures are evaluated from equilibrium MD simulations based on a stability matrix formulation. To analyze the processes involved in the spectroscopic measurements, we divide the fifth-order response functions into symmetric and antisymmetric integrated response functions; the symmetric one is written as a simple three-body correlation function, while the antisymmetric one depends on a stability matrix. This analysis leads to a better understanding of the time scales and molecular motions that govern the two-dimensional (2D) signal. The 2D Raman spectra show novel differences between the solid and liquid phases, which are associated with the decay rates of coherent motions. On the other hand, these differences are not observed in the linear Raman spectra.

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.

Polarizability response in polar solvents: Molecular-dynamics simulations of acetonitrile and chloroform

The relaxation of the many-body polarizability in liquid acetonitrile and chloroform at room temperature was studied by molecular-dynamics simulations. The collective polarizability induced by intermolecular interactions was included using first- and all-orders dipole-induced-dipole models and calculated considering both molecule-centered and distributed site polarizabilities. The anisotropic response was analyzed using a separation scheme that allows a decomposition of the total response in terms of orientational and collision-induced effects. We found the method effective in approximately separating the contributions of these relaxation mechanisms, although the orientational-collision-induced interference makes a non-negligible contribution to the total response. In both liquids the main contribution to the anisotropic response is due to orientational dynamics, but intermolecular collision-induced (or translational) effects are important, especially at short times. We found that higher-order interaction-induced effects were essentially negligible for both liquids. Larger differences were found between the center-center and site-site models, with the latter showing faster polarizability relaxation and better agreement with experiment. Isotropic and anisotropic spectra were computed from the corresponding time correlation functions. The lowest-frequency contributions are largely suppressed in the isotropic spectra and their overall shape is similar to the purely collision-induced contribution to the anisotropic spectra, but with an amplitude which is smaller by a factor of approximately 5 in acetonitrile and approximately 3 in chloroform.

Raman spectra of ionic liquids: Interpretation via computer simulation

Theoretical Raman spectra of the complex-forming ionic liquids LaCl3 and ScCl3, derived from molecular dynamics computer simulations, are presented. These simulations, which use polarizable ion interaction models, have previously been shown to predict structural properties in excellent agreement with diffraction experiments. The dependence of the polarizability of the melt on the ionic positions, which determines the Raman spectrum through the time dependence of the polarizability correlation function, is modeled on the basis of ab initio electronic structure calculations carried out on alkali chlorides. New simulation techniques are introduced in order to allow the spectrum to be calculated with acceptable statistics. The calculated spectra are in semiquantitative agreement with experimental data. The distinctive bands which appear in the spectra of such complex melts are linked to the vibrations of the transient coordination complexes which form in these melts and new interpretations for the origin of several well-known features are proposed. The simulations thus enable a link between the structure of a melt as perceived through Raman spectroscopy and through diffraction experiments to be made.