Optical Heterodyne-detected Raman-induced Kerr Effect Spectroscopy Research Articles

Orientational and low-frequency (0–450 cm−1) dynamics of methyl methacrylate: OHD-RIKES measurements and DFT calculations

This paper presents results of a study in which optical heterodyne-detected Raman-induced Kerr effect spectroscopy (OHD-RIKES) and density functional theory (DFT) were used to glean information about the low-frequency (0–450 cm−1) intramolecular and intermolecular dynamics and picosecond reorientational dynamics of methyl methacrylate (MMA) as a function of temperature. Based on DFT calculations, the two intramolecular bands at 220 and 380 cm−1 in this spectral region are assigned, respectively, to the (O)CH3 torsional and C=C out-of-plane modes of the cis-conformer of MMA. In the 2–40 ps time range, the OHD-RIKES response of MMA can be well fit by a biexponential decay function with a fast component (relaxation time τ1) associated with intermolecular dynamics, and a slow component (relaxation time τ2) associated with collective reorientation. The temperature dependence of τ2 can be understood in terms of a modified Debye-Stoke-Einstein (DSE) equation with the rotation of MMA molecules being governed by slip boundary conditions, which is consistent with MMA being a non‑hydrogen-bonded liquid. τ1 and τ2 are found to be linearly correlated with each other, which is attributed to τ1 being dominated by structural relaxation and therefore to motional narrowing. The low-frequency broad band in the 0–200 cm−1 region of the Kerr spectrum of MMA was analyzed in terms of a multicomponent line-shape model, with the sum of the first three components being primarily associated with the intermolecular modes of MMA. With increasing temperature, the intermolecular band shifts to lower frequency and narrows with increasing temperature. The shift to lower frequency is attributed to the softening of the effective intermolecular potential due to decreasing density with increasing temperature and the narrowing of the intermolecular band due to motional narrowing. Finally, it is shown that the Kerr spectrum of MMA provides support for the interpretation of the main band of the low-frequency reduced-Raman spectrum of poly(methyl methacrylate) (PMMA) being due to the librational motion of the monomer units in the polymer. This explanation is physically reasonably based on fits of the multicomponent line-shape model to the spectra of PMMA and MMA. It is proposed that this librational motion is associated with the torsional motion of the methoxycarbonyl pendant group on the polymer chain.

A simulation study of CS2 solutions in two related ionic liquids with dications and monocations.

Atomistic simulations of solutions of CS2 in an ionic liquid, , with a divalent cation and in the corresponding ionic liquid with a monovalent cation, [C4C1im][NTf2], were carried out. The low-frequency librational density of states of the CS2 was of particular interest in view of recent optical heterodyne-detected Raman-induced Kerr effect spectroscopy (OHD-RIKES). Compared to the monocation ionic liquid, the maximum shifts to higher frequencies in the dication ionic liquid under ambient conditions, but was found to be significantly pressure-dependent. CS2 molecules lie above and below the plane of the imidazolium rings and found to be close to the butyl tails of the monocation. The diffusion rates and embedding energies of solvent ions and CS2 in the two ionic liquids were measured.

Comparative OHD-RIKES Study of the Low-Frequency (0–250 cm–1) Vibrational Dynamics of Dibenzyl- and Monobenzyl-Substituted Imidazolium Ionic Liquids and Benzene/Dimethylimidazolium Mixtures

The low-frequency (0–450 cm–1) Kerr spectra of 1-benzyl-3-methylimidazolium bis[(trifluoromethane)sulfonyl]amide ([BnzC1im][NTf2]) and 1,3-dibenzylimidazolium bis[(trifluoromethane)sulfonyl]amide ([(Bnz)2im][NTf2]) and 2:1 and 1:1 mixtures of benzene in 1,3-dimethylimidazolium bis[(trifluoromethane)sulfonyl]amide ([C1C1im][NTf2]) were measured by the use of optical heterodyne-detected Raman-induced Kerr effect spectroscopy (OHD-RIKES). Surprisingly, the line-shape of the intermolecular part of the height-normalized Kerr spectrum is the same for both the 1:1 and 2:1 C6H6/[C1C1im][NTf2] mixtures, which may be a signature of the local liquid structure being characterized by π-stacked benzene–cation complexes. The intermolecular part of the Kerr spectrum of [BnzC1im][NTf2] is lower in intensity, higher in frequency, and broader than that of the 1:1 C6H6/[C1C1im][NTf2] mixture. Similarly, the intermolecular part of the Kerr spectrum of [(Bnz)2im][NTf2] is lower in intensity, higher in frequency, and broader th...

Spectroscopic investigation on intermolecular and intramolecular motions of concentrated aqueous KSCN solution

Both intermolecular and intramolecular motions of concentrated aqueous KSCN solution were investigated by means of femtosecond optical heterodyne-detected Raman induced Kerr effect spectroscopy (OHD-RIKES) and Raman spectroscopy. We found that all concentrated aqueous KSCN solutions present significantly large anisotropic hyperpolarizabilities in their femtosecond OHD-RIKES measurements, which is attributed to a 45cm−1 librational motion of SCN−. We also found that the weakening of the 170cm−1 stretching motion of water intermolecular hydrogen bond (HB) by 0.10m (molality) KSCN is equivalent to that by 4.00m KBr in aqueous solution, which implies SCN− strongly distort the HB network of water. Besides, we found that the rotational relaxation time constant of SCN− increases with the increase of KSCN concentration in aqueous solution, which is correlated with the corresponding viscosity of respective aqueous KSCN solution.

Additivity in the Optical Kerr Effect Spectra of Binary Ionic Liquid Mixtures: Implications for Nanostructural Organization

Low-frequency spectra of binary room-temperature ionic liquid (RTIL) mixtures of 1-pentyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide and 1-pentyl-3-methylimidazolium bromide in the 0-250 cm(-1) region were studied as a function of mole fraction at 295 K. The spectra were obtained by use of optical heterodyne-detected Raman-induced Kerr effect spectroscopy (OHD-RIKES). The spectra of these binary mixtures are well described by the weighted sums of the spectra for the neat RTILs. This surprising result implies that the intermolecular modes giving rise to the spectra of the neat liquids must also produce the spectra of the mixtures. Additivity of the OKE spectra can be explained by a model in which locally ordered domains are assumed to exist in the neat liquid with the structures of these locally ordered domains preserved upon mixing. Recently published molecular dynamics simulations show that RTILs are nanostructurally organized with ionic networks and nonpolar regions. If ionic networks also exist in the mixture, the additivity of the OKE spectra implies that there are "blocks" along the network of the mixture that are ordered in the same way as in the neat liquids. These "block co-networks" would have a nanostructural organization resembling that of a block copolymer.

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.

Why Are Viscosities Lower for Ionic Liquids with −CH2Si(CH3)3 vs −CH2C(CH3)3 Substitutions on the Imidazolium Cations?

We have prepared novel room temperature ionic liquids (RTILs) with trimethylsilylmethyl (TMSiM)-substituted imidazolium cations and compared the properties of these liquids with those for which the TMSiM group is replaced by the analogous neopentyl group. The ionic liquids are prepared with both tetrafluoroborate (BF(4)(-)) and bis(trifluoromethylsulfonyl)imide (NTf(2)(-)) anions paired with the imidazolium cations. At 22 degrees C, the TMSiM-substituted imidazolium ILs have shear viscosities that are reduced by a factor of 1.6 and 7.4 relative to the alkylimidazolium ILs for the NTf(2)(-) and BF(4)(-) anions, respectively. To understand the effect of silicon substitution on the viscosity, the charge densities have been calculated by using density functional theory electronic structure calculations. The ultrafast intermolecular, vibrational, and orientational dynamics of these RTILs have been measured by using femtosecond optical heterodyne-detected Raman-induced Kerr effect spectroscopy (OHD-RIKES). The intermolecular dynamical spectrum provides an estimate of the strength of interactions between the ions in the RTILs, and provides a qualitative explanation for the observed reduction in viscosity for the silicon-substituted RTILs.

A new ultrafast technique for measuring the terahertz dynamics of chiral molecules: The theory of optical heterodyne-detected Raman-induced Kerr optical activity

Optical heterodyne-detected Raman-induced Kerr optical activity (OHD-RIKOA) is a nonresonant ultrafast chiroptical technique for measuring the terahertz-frequency Raman spectrum of chirally active modes in liquids, solutions, and glasses of chiral molecules. OHD-RIKOA has the potential to provide much more information on the structure of molecules and the symmetries of librational and vibrational modes than the well-known nonchirally sensitive technique optical heterodyne-detected Raman-induced Kerr-effect spectroscopy (OHD-RIKES). The theory of OHD-RIKOA is presented and possible practical ways of performing the experiments are analyzed.

Low-Frequency Modes of Peptides and Globular Proteins in Solution Observed by Ultrafast OHD-RIKES Spectroscopy

The low-frequency (1–200cm−1) vibrational spectra of peptides and proteins in solution have been investigated with ultrafast optical heterodyne-detected Raman-induced Kerr-effect spectroscopy (OHD-RIKES). Spectra have been obtained for di-L-alanine (ALA(2)) and the α-helical peptide poly-L-alanine (PLA) in dichloroacetic acid solution. The poly-L-alanine spectrum shows extra amplitude compared to the di-L-alanine spectrum, which can be explained by the secondary structure of the former. The globular proteins lysozyme, α-lactalbumin, pepsin, and β-lactoglobulin in aqueous solution have been studied to determine the possible influence of secondary or tertiary structure on the low-frequency spectra. The spectra of the globular proteins have been analyzed in terms of three nondiffusive Brownian oscillators. The lowest frequency oscillator corresponds to the so-called Boson peak observed in inelastic neutron scattering (INS). The remaining two oscillators are not observed in inelastic neutron scattering, do therefore not involve significant motion of hydrogen atoms, and may be associated with delocalized backbone torsions.

The effects of anion and cation substitution on the ultrafast solvent dynamics of ionic liquids: A time-resolved optical Kerr-effect spectroscopic study

Ultrafast solvent dynamics of room-temperature ionic liquids have been investigated by optical heterodyne-detected Raman-induced Kerr-effect spectroscopy (OHD-RIKES) by studying the effects of cation and anion substitution on the low frequency librational modes. The spectra of two series of imidazolium salts are presented. The first series is based on the 1-butyl-3-methylimidazolium salts [bmim]+ containing the anions trifluoromethanesulfate [TfO]−, bis(trifluoromethanesulfonyl)imide [Tf2N]−, and hexafluorophosphate [PF6]−. The second series is based on [Tf2N]− salts containing the three cations 1-butyl-2,3-dimethylimidazolium [bmmim]+, 1-methyl-3-octylimidazolium [omim]+, and [bmim]+. It is found in all five samples that the signal is due to libration of the imidazolium ring at three frequencies around 30, 65, and 100 cm−1 corresponding to three local configurations of the anion with respect to the cation.

Intermolecular spectrum of 4-octyl-4 ′-cyanobiphenyl in n-heptane: OHD-RIKES measurements

The low frequency spectrum of 4-octyl-4 ′-cyanobiphenyl (8CB) in n-heptane was obtained by using optical heterodyne-detected Raman-induced Kerr effect spectroscopy (OHD-RIKES). The spectrum is characterized by a broad band at ∼61 cm −1 and weaker bands at ∼180 and ∼210 cm −1. The broad band becomes narrower and shifts to lower frequency upon dilution. These spectral changes can be explained by a decrease in the curvature of the effective intermolecular potential upon dilution. The intensities of the weak bands also decrease upon dilution. This behavior is consistent with the bands being due to collective modes of a locally ordered structure.

Time-resolved optical Kerr-effect spectroscopy of low-frequency dynamics in Di-L-alanine, poly-L-alanine, and lysozyme in solution.

The low-frequency spectra of peptides and proteins in solution have been investigated with optical heterodyne-detected Raman-induced Kerr-effect spectroscopy. Spectra were obtained for di-l-alanine ALA(2) and poly-l-alanine (PLA) in dichloroacetic acid solution. The conformational dependence of those spectra at low frequency has been analyzed. ALA(2) displays a band centered at 50 cm-1, whereas the alpha-helical PLA shows two shoulders at 60 and 140 cm-1. The similarity of the spectral features observed in PLA to those in water can be explained by analogous acoustic translational modes in the hydrogen network of the PLA alpha-helix. The mostly alpha-helical protein lysozyme in aqueous solution has also been investigated and showed significantly more structure with modes at 10, 35, 73, 106, and 164 cm-1.

Ultrafast dynamics in aqueous polyacrylamide solutions.

We have investigated the ultrafast dynamics of aqueous polyacrylamide ([-CH(2)CH(CONH(2))-](n), or PAAm) solutions using femtosecond optical heterodyne-detected Raman-induced Kerr effect spectroscopy (OHD-RIKES). The observed aqueous PAAm dynamics are nearly identical for both M(w) = 1500 and 10 000. Aqueous propionamide (CH(3)CH(2)CONH(2), or PrAm) solutions were also studied, because PrAm is an exact model for the PAAm constitutional repeat unit (CRU). The longest time scale dynamics observed for both aqueous PAAm and PrAm solutions occur in the 4-10 ps range. Over the range of concentrations from 0 to 40 wt %, the picosecond reorientation time constants for the aqueous PAAm and PrAm solutions scale linearly with the solution concentration, despite the fact that the solution shear viscosities vary exponentially from 1 to 264 cP. For a given value of solution concentration in weight percent, constant ratios of measured reorientation time constants for PAAm to PrAm are obtained. This ratio of PAAm to PrAm reorientation time constants is equal to the ratio of the volume for the PAAm constitutional repeat unit (-CH(2)CHCONH(2)-) to the molecular volume of PrAm. For these reasons, we assign the polymer reorientation dynamics to motions of the entire constitutional repeat unit, not only side group motions. Simple molecular dynamics simulations of H[-CH(2)CH(CONH(2))-](7)H in a periodic box with 180 water molecules support this assignment. Amide-amide and amide-water hydrogen-bonding interactions lead to strongly oscillatory femtosecond dynamics in the Kerr transients, peaking at 80, 410, and 750 fs.

Structure and Intermolecular Dynamics of Liquids: Femtosecond Optical Kerr Effect Measurements in Nonpolar Fluorinated Benzenes

A comparative study of the collective polarizability anisotropy dynamics of benzene, 1,3,5-trifluorobenzene, and hexafluorobenzene was carried out by using optical heterodyne-detected Raman-induced Kerr effect spectroscopy (OHD-RIKES) with 45 fs laser pulses. The OHD-RIKES data were analyzed by using a model-dependent approach and a Fourier transform approach, which yields a spectral density for the liquid. From an analysis of the long-time tails, collective reorientation times of 2.45 ± 0.06, 9.05 ± 0.10, and 13.5 ± 0.1 ps were obtained respectively for C6H6, 1,3,5-C6F3H3, and C6F6. The spectral densities are narrower for 1,3,5-C6F3H3 and C6F6 than for C6H6 by roughly a factor of 2. Information about the intermolecular vibrational modes of the liquid is contained in the reduced spectral density which is obtained by subtracting the tail-matched diffusive reorientational response from the OHD-RIKES data. In the case of C6H6 and C6F6, the intermolecular spectral density can be decomposed into at least two b...

Reorientational and intermolecular dynamics in binary liquid mixtures of hexafluorobenzene and benzene: femtosecond optical Kerr effect measurements

The molecular dynamics of binary liquid mixtures of C 6F 6 and C 6H 6 were studied by using optical heterodyne-detected Raman-induced Kerr effect spectroscopy (OHD-RIKES) with 45 fs laser pulses at room temperature and ambient pressure. The data give evidence for the effect of interspecies interactions in the short-time, nondiffusive part of the OHD-RIKES response.

Intermolecular spectral densities of liquids: A quantitative comparison of time-domain and frequency-domain techniques

Intermolecular spectral densities measured by two independent experimental techniques, time-domain optical-heterodyne-detected Raman-induced-Kerr-effect spectroscopy (OHD-RIKES) and frequency-domain stimulated gain spectroscopy (SGS), for three molecular liquids (benzene, carbon disulfide, and chloroform) are compared. The agreement in the high-frequency (interaction-induced) portion of the spectrum (20–150 cm−1) is quantitative. The relative merits between time-domain and frequency-domain coherent light-scattering techniques are discussed.

Femtosecond Optical Kerr Effect Studies of Liquid Methyl Iodide

The collective polarizability anisotropy dynamics of liquid methyl iodide at room temperature and ambient pressure was studied by using optical heterodyne-detected Raman-induced Kerr effect spectroscopy (OHD-RIKES) with 45 fs laser pulses. The OHD-RIKES data are analyzed by using both the model-dependent approach, which assumes four distinct temporal responses, and the model-independent Fourier transform approach, which generates a spectral density. Near zero time, the OHD-RIKES transient is dominated by the instantaneous electronic response. The short-time nuclear response is characterized by two components. The first component is interpreted as arising from an inhomogeneously broadened (fwhm ≈ 62 cm-1) underdamped intermolecular vibrational mode with a mean frequency of ∼60 cm-1. The second component is an intermediate quasi-exponential response with a 1/e time constant of ∼200 fs. At longer times, the OHD-RIKES transient decays exponentially with a 1/e time constant of 1.76 ± 0.05 ps, which corresponds to the collective reorientation time of CH3I. The spectral density peaks at ∼24 cm-1 and has a fwhm of ∼80 cm-1. The spectral density can be well fitted by an ohmic distribution function with ωc ≈ 30 cm-1. The spectral density obtained from the OHD-RIKES data is consistent with previously measured depolarized Rayleigh scattering and low-frequency far infrared absorption spectra for liquid CH3I.

Temperature-Dependent Dynamics of Microconfined Cs2

ABSTRACTFemtosecond optical-heterodyne detected Raman-induced Kerr effect spectroscopy (OHD-RIKES) has been used to study the reorientational dynamics of CS2 in microporous glasses over a wide range of temperatures. Microconfinement is shown to affect the behavior of the liquid at all the temperature studied. The results are interpreted in terms of a two-state model of the confined liquid.

Intermolecular Dynamics of Substituted Benzene and Cyclohexane Liquids, Studied by Femtosecond Nonlinear-Optical Polarization Spectroscopy

Femtosecond time-resolved optical-heterodyne detected Raman-induced Kerr effect spectroscopy (OHD-RIKES) is shown to be a powerful and comprehensive tool for studying the intermolecular dynamics occurring in liquids. The observed dynamics include both the underdamped, or coherent inertial motions, and the longer time scale diffusive relaxation. The inertial dynamics include phonon-like intermolecular vibrations, intermolecular collisions, and librational caging motions. Data are presented and analyzed for a series of five liquids: cyclohexane, methylcyclohexane, toluene, benzyl alcohol, and benzonitrile, listed in order of increasing polarity. We explore the effects of aromaticity (e.g., methylcyclohexane vs. toluene), symmetry reduction (cyclohexane vs methylcyclohexane), and substitution effects (e.g., substituted benzene series) on the ultrafast intermolecular dynamics, for a group of molecular liquids of similar size and volume. We analyze the intermolecular dynamics in both the time and frequency do...

Structure and dynamics of molecular liquids investigated by optical-heterodyne detected Raman-induced Kerr effect spectroscopy (OHD-RIKES)

Experimental femtosecond OHD-RIKES results of benzene, 1,4-cyclo-hexadiene, 1,5-cyclo-octadiene, benzonitrile, and o-methylbenzonitrile are reported. The goal of these experiments is to elucidate the structure and dynamics of aromatic liquids and to understand the underlying intermolecular forces. The experimental results are discussed in light of recent advances in theory and experiment that have contributed to our current understanding of the aromatic-aromatic intermolecular forces.