Time-Domain Theoretical Analysis of the Noncoincidence Effect, Diagonal Frequency Shift, and the Extent of Delocalization of the CO Stretching Mode of Acetone/Dimethyl Sulfoxide Binary Liquid Mixtures

The intermolecular polar bond interaction and coupling induced spectral splitting phenomenon for a binary mixture

To explain the polarization Raman noncoincidence effect of specific polar bonds and the noncoincidence phenomenon between FT-Raman and FT-IR spectra, aggregation-induced spectral splitting theory was proposed. In this paper, the vibration splitting theory was demonstrated using two strategies: improving the spectral resolution with cryogenic matrix isolation techniques and identifying cases where the coupling splitting is large enough to be distinguishable. The monomer and dimer splitting bands of acetone were detected when cryogenically isolated by the Ar matrix. Additionally, the polarization Raman and two-dimensional infrared spectra of a β-propiolactone (PIL)/CCl₄ binary mixture were collected at room temperature, and the spectral splitting phenomenon was clearly observed. The dynamic transformation between the monomer and dimer could be achieved and detected by adjusting the PIL concentration. The observed splitting phenomenon was further confirmed by theoretical DFT calculations based on the monomer and dimer of PIL, as well as the FT-IR and FT-Raman spectra of PIL. Concentration-triggered 2D-COS synchronous and asynchronous spectra also confirmed the splitting phenomenon and the dilution kinetics of PIL/CCl₄.

The polarized Raman spectra of N-methylpyrrolidone-an effective method for determination of intermolecular interaction and H-bond formation

The isotropic and anisotropic component Raman spectra and H NMR of N-methylpyrrolidone (NMP)/carbon tetrachloride and NMP/methanol binary mixture at different volume fractions have been collected. The polarization Raman frequencies and frequency differences of CO stretching vibration for NMP/methanol mixture show unique concentration-dependence and abrupt jump feature. It is found that the H-bond between solute and solvent does not destroy the noncoincidence (NCE) phenomenon, but has a significant synergistic effect on the NCE. Two distinctive clusters constrained by H-bond and intermolecular interactions were easily determined by means of linear extension method from abrupt jump curve. The experimental phenomena can be well explained by aggregation-induced splitting theory with the proposed dimer structure and H-bond cluster model. Applying the same methodology the conformation of NMP in water has been determined successfully. The establishment of this method will play an important role in the determination of biomolecule aggregation behavior and supramolecular self-assembly structure.

Noncoincidence Effects of Dimethyl Carbonate in Binary Mixtures Probed by Raman Spectroscopy: Experimental and DFT Calculations

The components of isotropic Raman and anisotropic Raman for dimethyl carbonate (DMC) dispersed in cyclohexane and acetone at different volume fractions were recorded separately. The noncoincidence effects (NCE) of the ν₇(C=O) stretching mode were calculated accordingly. The NCE values (Δν_NCE) of the ν₇(C=O) versus DMC volume fractions in the DMC/C₆H₁₂ mixtures exhibits a convex (upward) curvature pattern, while the Δν_NCE vs concentration in the DMC/CH₃COCH₃ mixtures exhibits a concave (downward) curvature. These different NCE behaviors in the different binary mixtures may arise from the solvent-induced aggregation character. Thus, monomer and dimer structures of DMC were optimized and the vibration spectra were obtained using density functional theory (DFT) calculations. An aggregation model was suggested to expound the DMC's characteristic NCE behavior and concentration effect. We found that the theoretical spectra from DFT/polarizable continuum model calculation based on the aggregation model is in accordance with our experimental data. Solvent-dependent experiments show the Δν_NCE values increase with the decrease of the solvent dielectric constant under the identical volume fractions.

Study on the noncoincidence effect phenomenon using matrix isolated Raman spectra and the proposed structural organization model of acetone in condense phase

The isotropic and anisotropic Raman spectra of acetone and deuterated acetone isolated in an argon matrix have been recorded for the understanding of noncoincidence effect (NCE) phenomenon. According to the matrix isolated Raman spectra and DFT calculations, we proposed aggregated model for the explanations of the acetone C=O vibration NCE phenomenon and its concentration effect. The experimental data were in consistence with the DFT calculations performed at the B3LYP-D3/6-311 G (d,p) levels based on the proposed model. The experimental identification of the monomer, dimer and trimer are reported here, and the dynamic of the transformation from monomer to aggregated structure can be easily controlled by tuning annealing temperature.

Heterogeneity in binary mixtures of dimethyl sulfoxide and glycerol: fluorescence correlation spectroscopy

Diffusion of four coumarin dyes in a binary mixture of dimethyl sulfoxide (DMSO) and glycerol is studied using fluorescence correlation spectroscopy (FCS). The coumarin dyes are C151, C152, C480, and C481. In pure DMSO, all the four dyes exhibit a very narrow (almost uni-modal) distribution of diffusion coefficient (Dt). In contrast, in the binary mixtures all of them display a bimodal distribution of Dt with broadly two components. One of the components of D(t) corresponds to the bulk viscosity. The other one is similar to that in pure DMSO. This clearly indicates the presence of two distinctly different nano-domains inside the binary mixture. In the first, the micro-environment of the solute consists of both DMSO and glycerol approximately at the bulk composition. The other corresponds to a situation where the first layer of the solute consists of DMSO only. The burst integrated fluorescence lifetime (BIFL) analysis also indicates presence of two micro-environments one of which resembles DMSO. The relative contribution of the DMSO-like environment obtained from the BIFL analysis is much larger than that obtained from FCS measurements. It is proposed that BIFL corresponds to an instantaneous environment in a small region (a few nm) around the probe. FCS, on the contrary, describes the long time trajectory of the probes in a region of dimension ~200 nm. The results are explained in terms of the theory of binary mixtures and recent simulations of binary mixtures containing DMSO.

Solvent dependent frequency shift and Raman noncoincidence effect of S=O stretching mode of Dimethyl sulfoxide in liquid binary mixtures

The isotropic and anisotropic Raman peak frequencies of S=O stretching mode of Dimethyl sulfoxide (DMSO) have been discussed in different chemical and isotopic solvent molecules using different mechanisms. The shifting of peak frequency in further dilution of DMSO with solvent molecule is observed for all solvents. Transition dipole - transition dipole interaction and hydrogen bonding may play a major role in shifting of peak frequencies. The non-coincidence effect (NCE) of DMSO was determined for all the solvents and compared with four theoretical models such as McHale's model, Mirone's modification of McHale's model, Logan's model and Onsager-Fröhlich dielectric continuum model respectively. Most of the theoretical models are largely consistent with our experimental data.

Intra- and intermolecular charge fluxes induced by the OH stretching mode of water and their effects on the infrared intensities and intermolecular vibrational coupling

Intra- and intermolecular charge fluxes induced by the OH stretching mode of water and their effects on the infrared (IR) intensities and intermolecular vibrational coupling are analyzed theoretically. Density functional theoretical (DFT) calculations are carried out for the clusters consisting of 28 and 30 water molecules, and the change in the electron density induced by the OH stretching mode, i.e., the electron density derivative ∂ρ((el))(r)/∂Q(OH), is calculated and its effect is analyzed. It is shown that the electron density derivative extends to the spatial region of the water molecule that accepts the hydrogen bond donated by the vibrating OH bond, indicating that intermolecular charge flux is induced. Both the amount of vibration-induced change in the electric charge (the charge derivative) and the distance between charge centers are large for the intermolecular charge flux, giving rise to a significant enhancement of the dipole derivative, and hence the IR intensity. After showing quantitatively that it is possible to understand the intermolecular vibrational coupling between the OH stretching oscillators with the picture of the "electric field modulation by one vibration" and the "response by the other vibration to the electric field modulation", the effect of the intermolecular charge flux on the intermolecular vibrational coupling is analyzed. It is shown that the intermolecular charge flux plays an important role also in this relation, but the two factors determining the dipole derivative (the charge derivative and the distance between charge centers) should be carefully taken into account.

Dynamical effects in line shapes for coupled chromophores: time-averaging approximation

For an isolated resonance of an isolated chromophore in a condensed phase, the absorption line shape is often more sharply peaked than the distribution of transition frequencies as a result of motional narrowing. The latter arises from the time-dependent fluctuations of the transition frequencies. It is well known that one can incorporate these dynamical effects into line shape calculations within a semiclassical approach. For a system of coupled chromophores, both the transition frequencies and the interchromophore couplings fluctuate in time. In principle one can again solve this more complicated problem with a related semiclassical approach, but in practice, for large numbers of chromophores, the computational demands are prohibitive. This has led to the development of a number of approximate theoretical approaches to this problem. In this paper we develop another such approach, using a time-averaging approximation. The idea is that, for a single chromophore, a motionally narrowed line shape can be thought of as a distribution of time-averaged frequencies. This idea is developed and tested on both stochastic and more realistic models of isolated chromophores, and also on realistic models of coupled chromophores, and it is found that in all cases this approximation is quite satisfactory, without undue computational demands. This approach should find application for the vibrational spectroscopy of neat liquids, and also for proteins and other complicated multichromophore systems.

Scheme of detecting microscopic inhomogeneity in binary liquid mixtures utilizing resonantly coupled vibrational modes

Influence of microscopic inhomogeneity in binary liquid mixtures on their vibrational spectra is studied by doing calculations on a model liquid system. The concentration dependence of the noncoincidence effect (NCE), which is a feature of vibrational bands related to the intermolecular resonant coupling of vibrational modes, is analyzed. It is suggested that observation of convex behavior of the NCEs for the vibrational bands of both species, especially that of the less polar species, in a binary liquid mixture is an indication of the occurrence of microscopic inhomogeneity.

Time-Domain Calculations of the Infrared and Polarized Raman Spectra of Tetraalanine in Aqueous Solution

The IR and polarized (isotropic and anisotropic) Raman spectra are calculated for the amide I band of tetraalanine ((Ala)4) in aqueous solution by using a time-domain computational method, which includes the effects of the diagonal frequency modulations (of individual peptide groups), the off-diagonal (interpeptide) vibrational couplings, and structural dynamics. It is shown that the calculated band profiles, especially the existence of a large negative noncoincidence effect (i.e., large frequency separations between the IR, isotropic Raman, and anisotropic Raman bands, with the isotropic Raman being higher in frequency), are in reasonable agreement with the experimental results. This negative noncoincidence effect derives from two conditions: the positive coupling between the amide I vibrations of peptide groups and the angle larger than 90 degrees between the transition dipoles of the coupled vibrations. This result means that the dynamically changing structures mainly in the polyproline II and beta-type conformations containing some repeated interconversions obtained from the molecular dynamics calculation are consistent with the existence of a large negative noncoincidence effect, as far as the structures satisfy the above two conditions. It is also shown that the electric fields from solvent water molecules induce larger frequency shifts than those of intrachain interactions, with rapid underdamped oscillatory modulations ( approximately 43 fs) due to the librational motions of water molecules that give rise to motional narrowing effect on the spectra. The reason for the difference from the behavior seen for the O-H stretching mode of liquid water is discussed. The time-domain analysis of the mode identity shows that the system proceeds halfway to complete mode mixing with a similar time scale ( approximately 60 fs), suggesting the importance of the nonadiabatic effect, which is included in a natural way in the present computational method.

Hydrogen Bonding and Cation Coordination Effects in Primary and Secondary Amines Dissolved in Carbon Tetrachloride

Raman and infrared spectroscopy were used to investigate hydrogen-bonding interactions and cation coordination effects in solutions of lithium triflate (LiCF3SO3) dissolved in two primary amines, hexylamine (HEXA) and N,N-dimethylethylenediamine (DMEDA), and in a secondary amine, dipropylamine (DPA). Strong intermolecular hydrogen-bonding interactions and weaker intramolecular hydrogen-bonding interactions that occur only in DMEDA were spectroscopically distinguished in a comparison of pure HEXA, pure DMEDA, and the dilute solutions of these amines in CCl4. The spectroscopic shifts in intensity and frequency in the NH stretching region of DPA and DPA diluted in CCl4 were similar to those of HEXA. Dilute electrolyte solutions in carbon tetrachloride were prepared to analyze specifically the cation coordination effect. In these solutions, limited intermolecular hydrogen-bonding interactions are present, and the observed spectral shifts correspond primarily to the cation-induced shifts. The symmetric SO3 stretching region of the triflate anion was investigated to probe further the coordination of the cation. The local structures of the triflate ions and the amine groups in the electrolyte solutions dissolved in CCl4 are similar to the local structures in the corresponding amine-salt crystals previously reported by us.

Time-Domain Calculations of the Polarized Raman Spectra, the Transient Infrared Absorption Anisotropy, and the Extent of Delocalization of the OH Stretching Mode of Liquid Water

The polarized Raman spectrum and the time dependence of the transient infrared (TRIR) absorption anisotropy are calculated for the OH stretching mode of liquid water (neat liquid H2O) by using time-domain formulations, which include the effects of both the diagonal frequency modulations (of individual oscillators) induced by the interactions between the dipole derivatives and the intermolecular electric field, and the off-diagonal (intermolecular) vibrational coupling described by the transition dipole coupling (TDC) mechanism. The IR spectrum of neat liquid H2O and the TRIR anisotropy of a liquid mixture of H2O/HDO/D2O are also calculated. It is shown that the calculated features of these optical signals, including the temperature dependence of the polarized Raman and IR spectra, are in reasonable agreement with the experimental results, indicating that the frequency separation between the isotropic and anisotropic components of the polarized Raman spectrum and the rapid decay (approximately 0.1 ps) of the TRIR anisotropy of the OH stretching mode of neat liquid H2O are mainly controlled by the resonant intermolecular vibrational coupling described by the TDC mechanism. Comparing with the time evolution of vibrational excitations, it is suggested that the TRIR anisotropy decays in the time needed for the initially localized vibrational excitations to delocalize over a few oscillators. It is also shown that the enhancement of the dipole derivatives by the interactions with surrounding molecules is an important factor in generating the spectral profiles of the OH stretching Raman band. The time-domain behavior of the molecular motions that affect the spectroscopic features is discussed.

Effects of Intermolecular Vibrational Coupling and Liquid Dynamics on the Polarized Raman and Two-Dimensional Infrared Spectral Profiles of Liquid N,N-Dimethylformamide Analyzed with a Time-Domain Computational Method

A time-domain method for calculating polarized Raman and two-dimensional infrared (2D-IR) spectra that includes the effects of both the diagonal frequency modulations (of individual molecules in the system) and the off-diagonal (intermolecular) vibrational coupling is presented and applied to the case of the amide I band of liquid N,N-dimethylformamide. It is shown that the effect of the resonant off-diagonal vibrational coupling and the resulting delocalization of vibrational modes is clearly seen as the noncoincidence effect in the polarized Raman spectrum and some spectral features (especially as asymmetric intensity patterns) in the 2D-IR spectra. The type of 2D-IR spectra (concerning the polarization condition) most appropriate for observing this effect is discussed. On the basis of the agreement between the observed and calculated band profiles of the polarized Raman spectrum, the time dependence of the transient IR absorption anisotropy is also calculated. The method of evaluating the extent of delocalization of vibrational modes that is relevant to the features of these optical signals in the time and frequency domains is discussed. The nature of the molecular motions (concerning the liquid dynamics) that are effective on the diagonal frequency modulations is also examined.