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.     

H NMR spectrum simplification of phenyl compounds containing electronegative groups by intermolecular interactions

Resolutions of ¹HNMR spectra of aromatic protons have been greatly improved by using simple alcohols, aliphatic amines and aliphatic acids as solvent-induced shift reagents. The hydrogen-bonding between the solvent and the solute molecules, and the self-association characteristics of the solvents are responsible for the spectrum resolution enhancement.     

Rotational diffusion of dyes in solvents of low viscosity from transient-dichroism experiments

Rotational diffusion data from pulsed laser experiments are presented for dye molecules dissolved in alcohols and non-alcohols. Cresyl violet and fluorescein both exhibit strong dependence of the rotational motion upon solvent molecular structure. In complete contrast the rotational diffusion of the oblong dyes pyronine G and acridine orange do not reveal any specific solute-solvent interaction.     

Rotational diffusion of dyes in micellar media from transient absorption

Solubilizer effects in dye solutions have been investigated by subnanosecond laser-pulse photometry. Different models for the transient dichroism of dye-micelle solutions are proposed. Computer fits of the experimental curves indicate that the model where dye molecules rotate within the liquid-like interior of the micelles is the most realistic one. Since the measured rotational diffusion times differ for different dyes in almost the same way as for pure highly-viscous solvents, the dye molecules do not act merely as labels for the rotation of the micelles, but rotate themselves considerably. It is shown that the rotational diffusion of dye molecules and micelles can be separated giving reasonable results for the size of the micelles. The results are compared with previous steady-state experiments.     

Solvent effects on electronic absorption spectra of nitrochlorobenzenes,nitrophenols and nitroanilines—II. Studies in polar solvents

A general method for estimating the specific solute—solvent interaction energies (E_s) from analysis of solvent effects on electronic absorption spectra has been outlined. The E_s values for a number of mono- and disubstituted benzenes in a variety of solvents such as water, alcohols, chloroalkanes, ether and acetonitrile have been estimated and the results are discussed in relation to the interacting groups in the solute and solvent molecules. These interactions, which can be classified as H bonding or EDA type, are mainly electrostatic in nature. These studies indicate that, in the case of disubstituted benzenes, the intramolecular electronic interactions are stronger than the solute—solvent interactions.     

Rotational diffusion in aprotic and protic solvents

We present the orientational relaxation times in protic and aprotic solvents for rose bengal in its lowest excited singlet state. The method uses a mode locked dye laser for polarized excitation, and time correlated single photon counting for determination of the time resolved polarized fluorescence. The observed orientational decay for the dipolar aprotic solvents and the alcohols are in agreement with the values predicted by the Stokes-Einstein diffusion equation. In the latter solvents, volume and shape corrections must be made for attachment of the alcohol to the two anion sites of the dye molecule. The solvent N-methylformamide, however, shows rose bengal reorienting much faster than the alcohols. Our interpretation of this data suggests that agreement with the Stokes-Einstein equation (stick boundary conditions) is coincidental. We propose a solvent torque model in which the solvent interaction at each anion site of rose bengal controls the deviations from an expected slip boundary condition. This qualitative model is used to correlate our data as well as relevant data in the literature. The values in picoseconds for the observed orientational relaxation times are given in parenthesis; acetone (70), DMF (160), DMSO (420), MeOH (190), EtOH (450), isopropanol (840), NMF (500).     

Relative predominance of azo and hydrazone tautomers of 4-carboxyl-2,6-dinitrophenylazohydroxynaphthalenes in binary solvent mixtures

Azo-hydrazone tautomerism is a phenomenon that occurs in azo dyes possessing substituents conjugated to the azo linkage which has labile proton that can be exchanged intramolecularly. Thus the predominance of one tautomer over another is a function of many factors among which are solvent polarity, solvent type, solute-solvent interactions and the structure of the dye molecule itself. The 4-carboxyl-2,6-dinitrophenylazohydroxynaphthalenes, previously shown to exhibit azo-hydrazone tautomerism, were studied for the relative predominance of one form over another based on interaction at the microenvironment of binary solvent mixtures containing DMF and non-hydrogen bonding (CCl(4)), hydrogen bond donor (toluene, chloroform), hydrogen bond acceptor (acetonitrile, acetone) and the alcohols; ethanol and methanol as solvent pairs. The three dyes gave two main bands in the 50:50 mixture of DMF with these solvents consisting of a high energy band at 250-382 nm while the low energy bands for the dyes occurred at 415-485 nm. Spectral shifts in the binary solvent mixtures were related to the solvent dipolarity, basicity of the less polar component relative to DMF, substituent type, molar transition energy, formation constant for the hydrogen-bonding solvated complexes and the standard free energy change for hydrogen bonding with DMF. The relative predominance of the hydrazone tautomer bears a direct relationship to the basicity of the solvent, presence of hydrogen bond donor substituent and was associated with high molar transition energies and low formation constant. The microenvironment surrounding the dye molecules played a major role in the stability of one tautomer relative to the other.     

Rotational dynamics and conformational kinetics in liquid crystals

Two particular aspects of solute dynamics in ordered media are analysed on the basis of the solution of multivariate diffusion equations: the effects of the solvation dynamics on the rotational motions of dipolar probes in liquid crystal solvents, and the alteration of reaction pathways in isomerization kinetics caused by the solvent order. The introduction of a suitable solvent coordinate allows the interpretation of high frequency contributions in the rotational correlation functions observed by spectroscopic techniques, namely dielectric dispersion, IR and Raman spectroscopy, ESR lineshapes and optical Kerr effect. For molecular systems undergoing conformational changes, a method is offered to evaluate the modification of the torsional barriers resulting from the anisotropic torques modulated by the molecular shape changes along the reaction coordinate.     

Influence of solute-solvent coordination on the orientational relaxation of ion assemblies in polar solvents

We have investigated the rotational dynamics of lithium thiocyanate (LiNCS) dissolved in various polar solvents with time and polarization resolved vibrational spectroscopy. LiNCS forms multiple distinct ionic structures in solution that can be distinguished with the CN stretch vibrational frequency of the different ionic assemblies. By varying the solvent and the LiNCS concentration, the number and type of ionic structures present in solution can be controlled. Control of the ionic structure provides control over the volume, shape, and dipole moment of the solute, critical parameters for hydrodynamic and dielectric continuum models of friction. The use of solutes with sizes comparable to or smaller than the solvent molecules also helps amplify the sensitivity of the measurement to the short-ranged solute-solvent interaction. The measured orientational relaxation dynamics show many clear and distinct deviations from simple hydrodynamic behavior. All ionic structures in all solvents exhibit multi-exponential relaxation dynamics that do not scale with the solute volume. For Lewis base solvents such as benzonitrile, dimethyl carbonate, and ethyl acetate, the observed dynamics strongly show the effect of solute-solvent complex formation. For the weak Lewis base solvent nitromethane, we see no evidence for solute-solvent complex formation, but still see strong deviation from the predictions of simple hydrodynamic theory.     

Spectral Investigations of Preferential Solvation and Solute–Solvent Interactions of Free Base and Protonated 5,10,15,20-Tetrakis(4-trimethyl-ammonio-phenyl)-porphine Tetratosylate in Aqueous Organic Mixed Solvents

The solvatochromic properties of the free base and the protonated 5,10,15,20-tetrakis(4-trimethyl-ammonio-phenyl)-porphine tetratosylate (TTMAPP) were studied in pure water, methanol, ethanol, 2-propanol, and their corresponding aqueous mixtures. The correlation of the empirical solvent polarity scale (E _T) values of TTMAPP with composition of the solvents were analyzed by the solvent exchange model of Bosch and Roses to clarify the preferential solvation of the probe dyes in the binary mixed solvents. The solvation shell composition effects in preferential solvation of the solute dyes were investigated in terms of both solvent–solvent and solute–solvent interactions and also the local mole fraction of each solvent composition was calculated in the cybotactic region of the probe. The effective mole fraction variation may provide significant physicochemical insights in the microscopic and molecular level of interactions between TTMAPP species and the solvent components and, therefore, can be used to interpret the solvent effect on kinetics and thermodynamics of TTMAPP.     

Building cavities in a fluid of spherical or rod-like particles: a contribution to the solvation free energy in isotropic and anisotropic polarizable continuum model

A general formalism for the calculation of cavitation energies in the framework of the scaled particle theory has been implemented in the Polarizable Continuum Model (PCM), contributing to the nonelectrostatic part of the molecular free energy in solution. The solute cavity and the solvent molecules are described as hard spherocylinders, whose radius and length are related to the actual molecular shape, while the solvent density is estimated from experimental data, or from the solvent molecular volume, suitably scaled. The present model can describe isotropic solutions of spherical and rod-like molecules in spherical or rod-like solvents, and also anisotropic solutions in which the solvent molecules are oriented in space: in this case, the cavitation energy also depends on the relative orientation of solute and solvent molecules. Test calculations have been performed on simple systems to evaluate the accuracy of the present approach, in comparison with other methods and with the available experimental estimates of the cavitation energy, giving encouraging results.     

Rotational dynamics and conformational kinetics in liquid crystals

Abstract

Two particular aspects of solute dynamics in ordered media are analysed on the basis of the solution of multivariate diffusion equations: the effects of the solvation dynamics on the rotational motions of dipolar probes in liquid crystal solvents, and the alteration of reaction pathways in isomerization kinetics caused by the solvent order. The introduction of a suitable solvent coordinate allows the interpretation of high frequency contributions in the rotational correlation functions observed by spectroscopic techniques, namely dielectric dispersion, IR and Raman spectroscopy, ESR lineshapes and optical Kerr effect. For molecular systems undergoing conformational changes, a method is offered to evaluate the modification of the torsional barriers resulting from the anisotropic torques modulated by the molecular shape changes along the reaction coordinate.     

Solvent effects on the thioamide rotational barrier: an experimental and theoretical study.

The solvent effect on the C-N rotational barriers of N,N-dimethylthioformamide (DMTF) and N,N-dimethylthioacetamide (DMTA) has been investigated using ab initio theory and NMR spectroscopy. Selective inversion recovery NMR experiments were used to measure rotational barriers in a series of solvents. These data are compared to ab initio results at the G2(MP2) theoretical level. The latter are corrected for large amplitude vibrational motions to give differences in free energy. The calculated gas phase barriers are in very good agreement with the experimental values. Solvation effects were calculated using reaction field theory. This approach has been found to give barriers that are in good agreement with experiment for many aprotic, nonaromatic solvents that do not engage in specific interactions with the solute molecules. The calculated solution-phase barriers for the thioamides using the above solvents are also in good agreement with the observed barriers. The solvent effect on the thioamide rotational barrier is larger than that for the amides because the thioamides have a larger ground-state dipole moment, and there is a larger change in dipole moment with increasing solvent polarity. The transition-state dipole moments for the amides and thioamides are relatively similar. The origin of the C-N rotational barrier and its relation to the concept of amide "resonance" is examined.     

Diffusion in organic solvents: Interdiffusion coefficient of 1,1,2,2-tetrabromoethane

Interdiffusion coefficient have been measured for 1,1,2,2-tetrabromoethane in 21 organic solvents in order to ascertain the influence of shape, size, viscosity, nature and degree of branching, of the solvent molecules on transport properties. None of the existing correlations for diffusion in binary liquid systems appear to fit all the experimental data. Steric effects due to the solvent molecules are shown to be very important in the interpretation of the diffusion process.     

Is molecular rotation really influenced by subtle changes in molecular shape?

In an attempt to seek out whether the reorientation time of a solute molecule is influenced by marginal changes to its shape, rotational relaxation of four coumarin solutes that are almost identical in size but subtly distinct in shape has been investigated in a viscous nonpolar solvent as a function of temperature. It has been observed that the reorientation times of the four coumarins differ significantly from one another. The four solutes have been treated as asymmetric ellipsoids and Stokes-Einstein-Debye hydrodynamic theory has been employed to calculate the shape factors and boundary condition parameters. The measured reorientation times when normalized by respective shape factors and boundary condition parameters can be scaled on a common curve, which is an indication that ellipsoid based hydrodynamic theory is adequate to model the reorientation times even when the differences in the shapes of the solute molecules are minimal.     

Solvent and concentration effects on the visible spectra of tri-para-dialkylamino-substituted triarylmethane dyes in liquid solutions

We have characterized the spectroscopy properties of crystal violet (CV+) and ethyl violet (EV+) in liquid solutions as a function of the solvent type and dye concentration. The analysis of how solvent properties and dye concentration affects the electronic spectra of these tri-para-dialkylamino substituted tryarylmethane (TAM+) dyes was performed on the basis of two spectroscopic parameters, namely the difference in wavenumber (deltanu) between the maximum and the shoulder that appears in the short-wavelength side of the respective maximum visible band (deltanu = 1/lambda(shoulder)-1/lambda(max) cm(-1)), and the wavelength of the maximum absorption (lambda(max)). The solvent and the concentration effects on lambda(max) and deltanu have indicated that both solute/solute (ion-pairing and dye aggregation) and solute/solvent (H-bonding type) interactions modulate the shape of the visible electronic spectra of these dyes in solution. In solvent with small dieletric constant (epsilon < approximately 10), the formation of ion-pairs represents a major contribution to the shaping of these spectra. Upon increasing dye concentration the formation of ion-pairs was characterized by an increase in deltanu observed concomitantly with a red shift in lambda(max) In chloroform and chlorobenzene the ion-pair association constant of CV+ and EV+ with Cl- ions were found to be in the order of 10(6) and 10(5) M(-1), respectively. In trichloroethylene the association constant for the CV+Cl- pair was 10(8) M(-1). In water, dye aggregation instead of ion-pairing represents a major contribution to the shaping of the visible spectra of CV+ and EV+. Dye aggregation was indicated by an increase in deltanu observed concomitantly with a blue shift in lambda(max) upon increasing dye concentration. The distinct behavior of deltanu for dye aggregation and ion-pairing as a function of dye concentration can therefore assist in the characterization of these two distinct phenomena. The solute/solvent interactions were studied in a series of polar solvents in which solute/solute interactions do not occur in any detectable extent. The dependence found for deltanu as a function of the Kamlet-Tafts solvatochromic parameters (alpha, beta and pi*) is in keeping with previous inferences indicating that the splitting in the overlapped absorption band of CV+ and EV+ in hydroxilated solvents arises from a perturbation in the molecular symmetry induced by hydrogen bonding (donor-acceptor) type interactions with solvent molecules. A distinction between the effects of solute/solute and solute/solvent interactions on the visible spectra of these dyes is provided.     

Monte Carlo simulations of the solution structure of simple alcohols in water-acetonitrile mixtures

Monte Carlo simulations have been performed to explore the solution structure of ethyl, isopropyl, isobutyl, and tertiary butyl alcohols in pure water, pure acetonitrile, and different mixtures of the two solvents. The explicit solvent studies in NpT ensembles at T = 298 K illustrate that the solute "discriminates" the solvent's components and that the composition of the first solvation shell differs from that of the bulk solution. Since the polarizable continuum dielectric method (PCM) does not presently model the solvation of molecules with both polar and apolar sites in mixed protic solvents, we suggest a direction for further program development wherein a continuum dielectric method would accept more than one solvent and the solute sites would be solvated by user-defined solvent components. The prevailing solvation model will be determined upon the lowest free energy calculated for a particular solvation pattern of the solute having a specific conformational/tautomeric state. Characterization of equilibrium hydrogen-bond formation becomes a complicated problem that depends on the chemical properties of the solute and its conformation, as well as upon the varying nature of the first solvation shell. For example, while the number of hydrogen bonds to secondary and tertiary alcohol solutes are nearly constant in pure water and in water-acetonitrile mixtures with at least 50% water content, the number of hydrogen bonds to primary alcohols gradually decreases for most of their conformations when acetonitrile content is increased. Nonetheless, the calculations indicate that O-H...O(water) hydrogen bonds are still possible in a small fraction of the arrangements for the solution models with water content of 30% or less. The isopentene solute does not form any observable hydrogen bonds, despite having an electron-rich, double-bond site.     

Zero electric field gradient mixtures for the elucidation of orientational mechanisms

A novel zero electric field gradient nematic liquid crystal made up from two nematic liquid crystal components is employed as a solvent for a series of molecules ranging from small molecules to mesogens themselves. Nuclear magnetic resonance is used to determine the degree of order of the solute and solvent molecules. Results are compared to those obtained for two completely different zero electric field gradient nematic mixtures. The comparison strongly indicates that for a variety of molecules largely differing in size, shape and flexibility their degree of order can be described by a single orientation mechanism. This mechanism can be adequately modelled by a simple phenomenological mean field model based on the size and shape anisotropy of the dissolved species. The use of zero electric field gradient mixtures in combination with this mean field model allows the prediction of solute order parameters at approximately the 10 per cent level.     

醇类溶剂溶剂化显色极性的理论分析

对一系列醇类溶剂分子进行了理论计算,运用多元线性回归分析方法从分子间相互作用的角度对四种溶剂化显色极性参数(E~T^N,π^*,Py和SPP)进行了理论分析。结果表明,对醇类溶剂而言,参数E~T^N和SPP实质上主要反映的是溶剂的氢键酸性性质;参数π^*中虽然包含了溶剂的极性因素,但同时与溶质-溶剂分子间的电荷转移相互作用有着密切的关系;而参数Py则较好地反映了溶剂的极性性质。