Solute rotation in polar liquids: microscopic basis for the Stokes-Einstein-Debye model

Here, we develop a framework for a molecular level understanding of the celebrated Stokes-Einstein-Debye (SED) formula. In particular, we explore reasons behind the surprising success of the SED model in describing dipolar solute rotation in complex polar media. Relative importance of solvent viscosity and solute-solvent dipolar interaction is quantified via a self-consistent treatment for the total friction on a rotating solute where the hydrodynamic contribution is modified by the friction arising from the longer ranged solute-solvent dipolar interaction. Although the solute-solvent dipolar coupling is obtained via the Mori-Zwanzig formalism, the inclusion of solvent structure via the wave vector dependent viscosity in the hydrodynamic contribution incorporates solvent molecularity in the present theory. This approach satisfactorily describes the experimental rotation times measured using a dipolar solute, coumarin 153 (C153), in protic and aprotic polar liquids, and more importantly, provides microscopic explanation for insignificant contribution of electrical interactions on solute rotation, in contrast to the substantial role played by the translational dielectric friction in the context of ionic mobility. It is also discussed on how the present theory can be suitably extended to study the rotation of a realistic solute in media other than dipolar solvents.     

Vibrational dephasing and hydrodynamic effects on vibrational relaxation rates in acetophenone: Raman bandshape analysis

The isotropic component of Raman band for C=O stretching mode of acetophenone in solution was analyzed by estimating the correlation coefficient with reference to Lorentzian lineshape. In the intermediate region of solute/solvent concentration there is a sharp decrease in the correlation coefficient and there appears to be a transition from non-Lorentzian to Lorentzian lineshape. The vibrational relaxation rates have been estimated from the isotropic component of Raman band in different solvents. The rate is shown to be dependent on several macroscopic as well as microscopic properties of the solute-solvent system and intermolecular interactions. The hydrodynamic and dispersion forces appear to play a major role in determining the vibrational relaxation rate and the broadening of the bands.     

Vibrational relaxation studies in methyl acetate: role of microenvironment and hydrodynamic forces

The Raman spectra were recorded for the C=O stretching vibration of methyl acetate as solutions in various polar and non-polar solvents. The isotropic component was obtained and the vibrational relaxation rates were calculated. On the basis of dependence of isotropic Raman bandwidth on the hydrodynamic properties of the solute-solvent systems, the information was obtained on the microenvironment prevailing in the neighborhood of the C=O stretching mode. Significant correlation is observed between vibrational relaxation rate and solvent parameters namely viscosity, density, refractive index and molecular radius. Microviscosity, involving the size of solute and solvent molecules, is found to be crucial in determining the bandwidth, hence the relaxation rate. The microenvironment appears to play an important role in the vibrational relaxation process.     

Amide I vibrational dynamics of N-methylacetamide in polar solvents: the role of electrostatic interactions

The vibrational frequency of the amide I transition of peptides is known to be sensitive to the strength of its hydrogen bonding interactions. In an effort to account for interactions with hydrogen bonding solvents in terms of electrostatics, we study the vibrational dynamics of the amide I coordinate of N-methylacetamide in prototypical polar solvents: D2O, CDCl3, and DMSO-d6. These three solvents have varying hydrogen bonding strengths, and provide three distinct solvent environments for the amide group. The frequency-frequency correlation function, the orientational correlation function, and the vibrational relaxation rate of the amide I vibration in each solvent are retrieved by using three-pulse vibrational photon echoes, two-dimensional infrared spectroscopy, and pump-probe spectroscopy. Direct comparisons are made to molecular dynamics simulations. We find good quantitative agreement between the experimentally retrieved and simulated correlation functions over all time scales when the solute-solvent interactions are determined from the electrostatic potential between the solvent and the atomic sites of the amide group.     

Rotational diffusion of a nonpolar and a dipolar solute in 1-butyl-3-methylimidazolium hexafluorophosphate and glycerol: interplay of size effects and specific interactions

Temperature dependent rotational diffusion of a nonpolar solute, 9-phenylanthracene (9-PA), and a dipolar solute, rhodamine 110 (R110), has been examined in an ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim(+)][PF(6) (-)]) and in a conventional solvent, glycerol. This study has been undertaken to explore how parameters such as solvent size and free volume influence solute rotation in the case of a nonpolar solute, 9-PA. To understand the role of specific solute-solvent interactions, similar measurements have been performed with a dipolar analogue, R110. It has been observed that the viscosity normalized reorientation times of 9-PA are longer by a factor of 1.4-1.6 in glycerol compared to those in [bmim(+)][PF(6) (-)]. While the most commonly used Stokes-Einstein-Debye hydrodynamic theory is not successful in explaining this experimental observation, Gierer-Wirtz and Dote-Kivelson-Schwartz quasihydrodynamic theories could rationalize this trend, albeit in a qualitative manner. Rotational diffusion of R110, on the other hand, follows an exactly opposite trend compared to 9-PA. The normalized reorientation times of R110 are longer by a factor of 1.3-1.4 in [bmim(+)][PF(6) (-)] compared to glycerol, which is due to the formation of stronger solute-solvent hydrogen bonds between the positively charged R110 and the ionic liquid.     

Molecular rotation as a tool for exploring specific solute-solvent interactions.

Solute-solvent interactions play an important role in determining the physicochemical properties of liquids and solutions. As a consequence, understanding these interactions has been one of the long-standing problems in physical chemistry. This Minireview describes our approach towards attaining this goal, which is to investigate rotational relaxation of a pair of closely related, medium-sized nondipolar solutes in a set of appropriately chosen solvents. Our studies indicate that solute-solvent hydrogen bonding significantly hinders solute rotation. We have also examined the role of solvent size both in the absence and presence of specific interactions and it has been observed that the size of the solvent has a bearing on solute rotation especially in the absence of specific interactions. Our results point to the fact that only strong solute-solvent hydrogen bonds have the ability to impede the rotation of the solute molecule because, in such a scenario, hydrogen-bonding dynamics and rotational dynamics transpire on comparable time scales. This aspect has been substantiated by measuring the reorientation times of the chosen solutes in solvents such as ethanol and trifluoroethanol, which have distinct hydrogen-bond donating and accepting abilities, and correlating them with solute-solvent interaction strengths. As an alternative treatment, it has been shown that specific interactions between the solute and the solvent can be modeled as dielectric friction with the extended charge distribution model. This approach is not unrealistic considering the fact that specific as well as non-specific interactions are electrostatic by nature and the differences between them are subtle.     

Ultrafast two-dimensional infrared vibrational echo chemical exchange experiments and theory

Ultrafast two-dimensional (2D) infrared vibrational echo experiments and theory are used to examine chemical exchange between solute-solvent complexes and the free solute for the solute phenol and three solvent complex partners, p-xylene, benzene, and bromobenzene, in mixed solvents of the partner and CCl4. The experiments measure the time evolution of the 2D spectra of the hydroxyl (OD) stretching mode of the phenol. The time-dependent 2D spectra are analyzed using time-dependent diagrammatic perturbation theory with a model that includes the chemical exchange (formation and dissociation of the complexes), spectral diffusion of both the complex and the free phenol, orientational relaxation of the complexes and free phenol, and the vibrational lifetimes. The detailed calculations are able to reproduce the experimental results and demonstrate that a method employed previously that used a kinetic model for the volumes of the peaks is adequate to extract the exchange kinetics. The current analysis also yields the spectral diffusion (time evolution of the dynamic line widths) and shows that the spectral diffusion is significantly different for phenol complexes and free phenol.     

Vibrational energy relaxation of a diatomic molecule in a room-temperature ionic liquid

Vibrational energy relaxation (VER) dynamics of a diatomic solute in ionic liquid 1-ethyl-3-methylimidazolium hexafluorophosphate (EMI(+)PF(6) (-)) are studied via equilibrium and nonequilibrium molecular dynamics simulations. The time scale for VER is found to decrease markedly with the increasing solute dipole moment, consonant with many previous studies in polar solvents. A detailed analysis of nonequilibrium results shows that for a dipolar solute, dissipation of an excess solute vibrational energy occurs almost exclusively via the Lennard-Jones interactions between the solute and solvent, while an oscillatory energy exchange between the two is mainly controlled by their electrostatic interactions. Regardless of the anharmonicity of the solute vibrational potential, VER becomes accelerated as the initial vibrational energy increases. This is attributed primarily to the enhancement in variations of the solvent force on the solute bond, induced by large-amplitude solute vibrations. One interesting finding is that if a time variable scaled with the initial excitation energy is employed, dissipation dynamics of the excess vibrational energy of the dipolar solute tend to show a universal behavior irrespective of its initial vibrational state. Comparison with water and acetonitrile shows that overall characteristics of VER in EMI(+)PF(6) (-) are similar to those in acetonitrile, while relaxation in water is much faster than the two. It is also found that the Landau-Teller theory predictions for VER time scale obtained via equilibrium simulations of the solvent force autocorrelation function are in reasonable agreement with the nonequilibrium results.     

Studies on solute solvent interaction. Charge transfer band maxima of N-alkyl pyridinium iodides in solution — A correlation with a dielectric parameter

The longest wavelength band of n-alkyl pyridinium iodides (NAPI) in solution, which is due to charge transfer processes within a contact ion pair species, serves as an empirical measure of solute-solvent interaction for a polar solute in polar solvents. An attempt has been made to correlate the energy of the transition with the chemical potential of the dipolar ion pair in a solvent. The latter quantity has been calculated using a dielectrically saturable Block-Walker reaction field model. It has been found that, for protic solvents there is a good linear correlation between the two parameters enabling the calculation of the transition energy in water. An alternative correlation involving the individual molecule dipole reaction field is also discussed. For dipolar aprotic solvents both correlations yield poorer results indicating a single parameter correlation is not sufficient. In alcoholic binary systems the solute-solvent interaction is a linear function of the bulk reaction fields of the component solvents. But in the case of aprotic-alcoholic solvents, where specificities of interaction differ, the solute sees an environment the composition of which differs from that of the bulk.     

一种不含胶凝剂的凝胶电解质在准固态染料敏化太阳能电池中的应用

报道了一种不含胶凝剂的凝胶电解质的制备及在准固态染料敏化太阳能电池中的应用.这种新型凝胶电解质仅含有机溶剂和碘盐,即3-甲氧基丙腈、苯胺、三碘化铝和碘.上述混合物通过路易斯酸性三碘化铝离子导体和路易斯碱性苯胺有机溶剂间的路易斯酸-碱相互作用形成凝胶,无需额外添加传统凝胶电解质的关键组分—胶凝剂.形成的三碘化铝-苯胺复合物在凝胶电解质中能同时发挥离子导体和胶凝剂的作用.红外光谱图中苯胺的氨基和苯环特征峰的变化证实了三碘化铝-苯胺复合物的形成.含这种新型凝胶电解质的准固态染料敏化太阳能电池光电性能和稳定性与含三碘化铝-3-甲氧基丙腈液体电解质的染料敏化太阳能电池相比有很大提高.     

Monte Carlo simulation of the influence of solvent on nucleic acid base associations

The results of Monte Carlo calculations of the association between nucleic acid bases in a nonpolar solvent (CCl4) are described. The influence of the solvent on planar and stacked associations of bases was examined by analyzing the total energy of the system, including solute-solute, solute-solvent, and solvent-solvent contributions. Good quantitative agreement with the available experimental data was obtained. Solute-solvent interactions are primarily determined by dispersion forces; consequently, solute-solvent interactions vertical to the solute plane that maximize dispersion interactions are most favored, and a rough proportionally between solute-solvent energy and the surface of the solute was observed. Analysis of solvent-solvent energy is not necessarily reduced when surface area decreases, contrary to the simple cavity concept. "Single molecule probe" calculations were performed to explain the differences in base associations in H2O and CCl4. In CCl4 dispersion forces dominate and planar complexes are stabilized by maximum exposure of molecular planes to the solvent. In H2O electrostatic forces dominate so that the most stable structures are stacked association that allow the maximum number of hydrophilic centers to be exposed to the solvent.     

Empirical parameters of lewis acidity and basicity for aqueous binary solvent mixtures

Empirical parameters of Lewis acidity, E^N_T, introduced by Reichardt et al., and Lewis basicity, B_KT , introduced by Kamlet and Taft, have been determined for mixtures of water with ten organic solvents. In the case of water/alcohol mixtures a distinct dependence between these acidity and basicity parameters have been found. For the other solvent mixtures the E^N_T on B_KT dependence is more complex even if these parameters are purified from non-specific solute/solvent interactions.     

Electrostatic interactions of a neutral dipolar solute with a fused salt: a new model for solvation in ionic liquids

Ionic liquids represent a novel and poorly understood class of solvents, and one challenge in understanding these systems is how one should view the electrostatic character of solute-solvent interactions. The highly structured nature of a fused salt makes a dielectric continuum approximation difficult to implement, and there is no obvious connection between the structure of an individual ion and the polarization character of the medium. We address this problem by making the ansatz that rather than polarizing the medium, the solute may be viewed as intercalating in the charge distribution of the neat liquid such that the solvent screens the electric field of the solute. This approach allows derivation of an analytical expression for the distribution of solvent charge about the solute, and this distribution is found to be a close match to simulation data. The theory also predicts that the electrostatic character of solute-solvent interactions should be determined primarily by the number density of solvent ions, a prediction proven correct by analysis of existing experimental data. The approach represents a new model for the interpretation of solvation phenomena in ionic liquids.     

Molecular reorientation dynamics and microscopic friction in liquids

Molecular rotation reorientation times are investigated using time resolved fluorescence depolarization studies of three solutes of similar size and shape (nile red, neutral nile blue and cationic nile blue) dissolved in alcohol and alkane solvents as well as an extensive compilation of previous results for neautral and charged solutes dissolved in non-polar, polar and associated solvents. A universal correlation is foung between reorientation time, solvent viscosity, and solute volume for solutes dissolved in alkanes, while strongly interacting solutes experience relatively enhanced friction, and non-polar solutes dissolved in alcohols experience reduced friction. The results are compared and contrasted with slip and stick hydrodynamic predictions, and used to develop empirical correlations, which can be used to predict molecular reorientation times with an uncetainty on the order of a factor of two in virtually any solute-solvent system.     

Investigation of the influence of solute-solvent interactions on the vibrational energy relaxation dynamics of large molecules in liquids

The influence of solute-solvent interactions on the vibrational energy relaxation dynamics of perylene and substituted perylenes in the first singlet excited-state upon excitation with moderate (<0.4 eV) excess energy has been investigated by monitoring the early narrowing of their fluorescence spectrum. This narrowing was found to occur on timescales ranging from a few hundreds of femtoseconds to a few picoseconds. Other processes, such as a partial decay of the fluorescence anisotropy and the damping of a low-frequency oscillation due to the propagation of a vibrational wavepacket, were found to take place on a very similar time scale. No significant relationship between the strength of nonspecific solute-solvent interactions and the vibrational energy relaxation dynamics of the solutes could be evidenced. On the other hand, in alcohols the spectral narrowing is faster with a solute having H-bonding sites, indicating that this specific interaction tends to favor vibrational energy relaxation. No relationship between the dynamics of spectral narrowing and macroscopic solvent properties, such as the thermal diffusivity, could be found. On the other hand, a correlation between this narrowing dynamics and the number of low-frequency modes of the solvent molecules was evidenced. All these observations cannot be discussed with a model where vibrational energy relaxation occurs via two consecutive and dynamically well-separated steps, namely ultrafast intramolecular vibrational redistribution followed by slower vibrational cooling. On the contrary, the results indicate that both intra- and intermolecular vibrational energy redistribution processes are closely entangled and occur, at least partially, on similar timescales.     

Towards a modellisation of the solvation energy in multi-component solvents. The interesting case of a charged solute imbedded in a polymer-containing electrolyte solution

In this paper we propose a mean-field theory to calculate the solvation free energy of a charged solute imbedded in a complex multi-component solvent. We considered a solvent made up of a mixture of small (electrolyte solution) and large (polymer) components. The presence of macromolecules ensures reduced mixing entropy among the different solvent components, an effect due to polymer connectivity. The reduced entropy favours strong preferential distribution of a particular solvent even in the presence of weak preferential solute–solvent interactions. In addition, two energy terms must be considered: (a) the interaction between the solute electrostatic potential and the electrolyte solution and (b) the formation of a polymer–solute interface. Because of the different dielectric permittivity of the solvent components, the electrolyte and polymer distribution functions are strongly coupled: ions, indeed, are more solvated in regions of higher local dielectric permittivity arising from the inhomogeneous mixing of solvent and polymer. We combined together the different energy terms in the framework of the de Gennes free energy functional for polymer solutions along with a generalised Poisson–Boltzmann equation developed for inhomogeneous dielectric media. Moreover, the preferential electrolyte solvation in regions of greater polarity was considered by an extension of the Born equation. Setting the polymer dielectric permittivity smaller than the solvent one and making null the specific polymer–solute interactions, we calculated enhanced electrolyte concentration and reduced polymer concentration near the solute surface on raising the solute surface charge density. The theory shows also the breakdown of the widely used separation between electrostatic and surface tension-dependent contributions to solvation energy when non-ideal mixed solvents are considered. In fact, according to the model, the surface tension of such mixed solvents strongly depends on the solute surface charge density: at high potentials the interfacial tension may increase rather than decrease on raising the polymer volume fraction. The theoretical results have been compared with experimental data on polymer+electrolyte solution surface tension and with solubility data of colloidal particles. The comparison evidences the complex behaviour of multi-component solvents going well beyond the trivial weighted average of the dielectric permittivity and surface tension of the isolated chemical components. Deviations from the simple behaviour predicted by an average picture of multi-component solvents could be understood by developing more sophisticated, but still simple, approaches like that proposed in this paper.Contribution to the Jacopo Tomasi Honorary Issue. This paper is dedicated to Jacopo Tomasi. I learned much of the difficult art of transforming complex problems into simple models after reading his early works on solvation energy.     

A computer simulation study of the formation of liquid crystal nanodroplets from a homogeneous solution

The aggregation of liquid crystal nanodroplets from a homogeneous solution is an important but not well understood step in the preparation of various advanced photonic materials. Here, the authors performed molecular dynamics computer simulations of the formation of liquid crystalline nanodroplets, starting from an isotropic and uniform binary solution of spherical Lennard-Jones (solvent) and elongated ellipsoidal Gay-Berne (solute) rigid particles in low (<10%) concentration. They studied the dynamics of demixing and the mesogen ordering process and characterized the resulting nanodroplets assessing the effect of temperature, composition, and specific solute-solvent interaction on the morphology, structure, and anisotropy. They find that the specific solute-solvent interaction, composition, and temperature can be adjusted to tune the nanodroplet growth and size.     

Early time hydrogen-bonding dynamics of photoexcited coumarin 102 in hydrogen-donating solvents: theoretical study

To study the early time hydrogen-bonding dynamics of chromophore in hydrogen-donating solvents upon photoexcitation, the infrared spectra of the hydrogen-bonded solute-solvent complexes in electronically excited states have been calculated using the time-dependent density functional theory (TDDFT) method. The hydrogen-bonding dynamics in electronically excited states can be widely monitored by the spectral shifts of some characteristic vibrational modes involved in the formation of hydrogen bonds. In this study, we have demonstrated that the intermolecular hydrogen bonds between coumarin 102 (C102) and hydrogen-donating solvents are strengthened in the early time of photoexcitation to the electronically excited state by theoretically monitoring the stretching modes of C=O and H-O groups. This is significantly contrasted with the ultrafast hydrogen bond cleavage taking place within a 200-fs time scale upon electronic excitation, proposed in many femtosecond time-resolved vibrational spectroscopy experiments. The transient hydrogen bond strengthening behaviors in excited states of chromophores in hydrogen-donating solvents, which we have demonstrated here for the first time, may take place widely in many other systems in solution and are very important to explain the fluorescence-quenching phenomena associated with some radiationless deactivation processes, for example, the ultrafast solute-solvent intermolecular electron transfer and the internal conversion process from the fluorescent state to the ground state.     

Molecular motion and interactions in aqueous carbohydrate solutions. I. Dielectric-relaxation studies

Dielectric-relaxation studies in the frequency range 200 kHz to 35 GHz are reported for a range of sugars (from mono- to trisaccharides) in aqueous solution. The complex dielectric spectra were analyzed using a weighted least-squares minimization method to resolve the various component relaxations, and the implications of the analyses in terms of the molecular dynamics of solute and solvent and the interactions between solute and solvent are discussed. For the highest concentration studied (ca. 2M), it was found that the most significant analysis required three discrete relaxation processes, whereas lower concentration samples could usually be satisfactorily fitted with two. Irrespective of any uncertainty in model selection, a number of conclusions regarding the solute-solvent interactions can be made, and it is shown how final quantification of the extents of hydration can be made using the input of information from other techniques.     

乙二醇的分子间氢键结构动力学的飞秒非线性红外光谱

利用稳态线性红外光谱和飞秒泵浦-探测红外光谱技术, 研究了在乙腈(MeCN)、丙酮(AC)、四氢呋喃(THF)和二甲基亚砜(DMSO)溶剂中乙二醇(EG)的结构和羟基(―OH)伸缩振动动力学. 结果表明, 乙二醇的―OH伸缩振动的频率位置、峰宽以及振动弛豫动力学都表现出强烈的溶剂依赖性. 乙二醇溶液中至少存在两种形式的分子间氢键, 一种是溶质-溶剂团簇的分子间氢键, 另一种是溶质-溶质团簇的分子间氢键. 量子化学计算预测的―OH伸缩振动频率的溶剂依赖性与我们的红外光谱实验观测结果一致. 进一步, 我们发现在乙腈中参与形成溶质-溶剂团簇氢键的乙二醇―OH伸缩振动具有最慢的弛豫动力学, 丙酮和四氢呋喃次之, 而最快的弛豫动力学过程发生在二甲基亚砜中. 在每一溶剂条件下, 乙二醇/乙二醇溶质团簇中―OH伸缩振动弛豫都更快一些. 本文结果有助于认识在溶质-溶质、溶质-溶剂分子团簇共存的体系中不同分子间氢键的结构动力学特性.