Solute-solvent Complexes Research Articles

Temperature effects on the fluorescence of indolic molecules in polar solvents. Anomalous stokes shift

The fluorescence Stokes shift of polar solutions of indole is explained on the basis of the interaction of solute-solvent hydrogen-bonded complexes with their polar cages. Temperature dependence of N-methylindole versus indole fluorescence is used in this study.

PREDICTION OF DIFFUSIVITIES FOR ASSOCIATION COMPLEX FORMING ALCOHOL SOLVENT SYSTEMS

Solute's self association and solute-solvent complex formation in alcohol-solvent liquid mixtures can result in large errors in the prediction of diffusivities for such systems by the hitherto proposed generalized correlations. This work reports an attempt to develop a model for predicting diffusivities for alcohol-solvent systems taking into account both the self association and interassociation phenomena and assuming that the former takes place as dimer formation and the latter involves one molecule of solute that is the alcohol and one of the solvent, the non-alcohol component such as unsaturated hydrocarbons, esters ketones etc. It has been observed that an appreciable improvement in the prediction results even if these complex formation phenomena are accounted for in this simplified way and this has been shown for a few selected systems.

Thermochemical investigations of associated solutions. II. Calculation of iodine-benzene equilibrium constants from solute solubility in binary solvent mixtures

Solubilities have been determined at 25 °C for iodine in binary mixtures of benzene with n-hexane, n-heptane and isooctane. The results of these measurements are compared to solution models previously developed for solubility in systems containing only non-specific interactions and to models assuming solute-solvent complexation. The Nearly Ideal Binary Solvent Model (NIBS) based entirely on non-specific interactions provides rather poor predictions of the iodine solubilities, with some of deviations greater than 25%. In comparison, a 1:1 solute-solvent complexation model describes the experimental solubilities to within a maximum deviation of 6%, An example is presented to illustrate the calculation of association constants from solubility measurements.

Charge transfer to a solvent state. VIII. Localization of charge during the transition state of the solute: The solute cation localization energy

A good linear correlation between the polarization energy P + of the solute cation and the corresponding gas-phase ionization energy I g suggested recently, is now obtained for various hydrocarbons in solution. The radius r of the solute cation is calculated from a solute-solvent cage charge-transfer model and P + is derived from the Born formula: P + = −( e 2/2 rX1 - D op −1). In contrast to the non-linear plot of − P +′ versus I g (where − P −′ is the polarization energy with r calculated from the molar volume), the linear plot of − P + versus I g gives a slope which deviates from the slope of unity obtained for the well-known linear correlation of E 1/2 ox versus I g( E 1/2 ox is the half-wave oxidation potential). During the transition state, there is localization of charge. The electron transfer from the excited solute to the electrophilic solvent cage is through the rth C atom where the cation omega localization energy L r w is lowest, r corresponds to the position of the most active C atom in the electrophilic aromatic substitution reaction. When applying the solute-solvent cage CT complex model to aqueous micelle systems and taking into account the large difference in photoionization energies between the gas and condensed phase (Δ I = 2.5 eV), the oxidation potential of perylene and ferrocene seems to vary very little with the charge surfactant of the micelles. The energy V 0 of the conduction band of the aqueous solvent and the hydrophilic character of the solute cation seem to be the main factors which cause the lowering in photoionization energy of the solute. The present electrochemical results do not permit us to conclude that the charge of the aqueous micellle surfactant affects the yield in ionic species, an observation which has been well noted in the photoexcitation case.

Stepwise solvation of molecules as studies by picosecond-jet spectroscopy: dynamics and spectra

We report on the stepwise solvated complexes of isoquinoline with water, methanol, and acetone under supersonic jet conditions. Lifetimes, dispersed fluorescence and excitation spectra are presented for different solute-solvent complexes. These studies reveal manifestations of solute-solvent hydrogen-bonding and of excited state hydrogen bond dissociation.

Charge transfer to a solvent state. VI. Inter-relation between electrochemical and photoionization data of a solute in condensed phase

The electrochemical data obtained from aqueous solutions of tryptophan, of indole, and from tryptophan in ethanol agree well with those derived from the photoionization mechanism which is based on a charge transfer solute–solvent complex model (Ref. 1), particularly the conduction band depth V0, the effective cavity radius r, hence the solvation energy of the solute cation. A relationship is now established between the electrochemical data and the photoionization data. This relationship permits one to deduce V0 and the oxidation energy eEox1/2 of a solute in a solvent when its photoionization data are known or vice versa.

A Model of Liquid Adsorption Chromatography Involving Solute-Solvent Interaction in the Mobile Phase, Energetic Heterogeneity of the Adsorbent, and Differences in Molecular Sizes of Solute and Solvents

Abstract A simple model for liquid-solid chromatography (LSC) process with mixed mobile phases has been proposed. According to this model the LSC process is represented by suitable reversible phase-exchange reactions between molecules of solute and solvents and reversible solute-solvent reactions in the mobile phase. These reactions describe the competitive adsorption of solute molecules and formation of solute-solvent complexes in the mobile phase. Analytical equations for the capacity ratio, derived in terms of the model, involve solute-solvent interaction in the mobile phase, differences in molecular sizes of solute and solvents, energetic heterogeneity of the adsorbent and ideality of the ourface phase. Linear forms of these equations are very convenient for analysing the experimental chromatographic data.

Solubility of polar organic solutes in nonaqueous systems: Role of specific interactions

The changes in solubility of several polar organic solutes when polar organic solvents are added to a relatively inert solvent such as isooctane were determined. The relative changes in solubility predicted from regular solution theory using solubility parameters often did not agree with the observed results. However, the solubilities could be rationalized mathematically by assuming the formation of specific solute–solvent complexes. Agreement of the thermodynamic data reported here with such models provides further evidence that specific interactions, when they occur, are more important than the bulk properties of the pure components in determining drug solubilities in nonaqueous systems. Specific examples demonstrate the relationship between the solubility and molecular structure of the solute and solvent. For example, solubility can be related to the hydrogen-donating and hydrogen-accepting abilities of the solute and solvent. Steric factors also appear to play a role in solubility, while structural modifications in a solute or solvent molecule far removed from the interactive functional group have little influence on molar solubility changes with the added polar cosolvent.

Solvent effects on carbonyl and ethylenic stretching frequencies in 3-keto unsaturated steroids

Solvent effects relative to n—hexane, on the carbonyl stretching frequencies of androstanolone (I) testosterone (III) and 1,2-dehydrotestosterone (VI) and related 3-keto steroids have been investigated. Solvent shifts increase in moving from saturated to α-unsaturated keto, but introduction of a second α-double bond reduces the shift, contrary to the anticipated increase with increasing unsaturation. Evidence is presented, suggesting solute—solvent complexation between proton-donating solvents and ethylenic π-electrons in the dienes. The apparently anomalous pattern of frequency shifts is explained in terms of this interaction.

Biphotonic process in ionic photodissociation of weak charge transfer complexes

Dependences of the photoinduced radical-anion formation of solute-solvent charge-transfer complexes upon the exciting light intensity have been studied spectroscopically in ether-isopentane glass as well as in 2-methyltetrahydrofuran (MTHF) glass at 77 K. Biophotonic ionic dissociations have been found for the following complexes: TCNB(tetracyanobenzene)-ether, PMDA(pyromellitic dianhydride)-ether, TCPA(tetrachlorophthalic anhydride)-ether and TCPA-MTHF.

Ionic photodissociation of charge transfer complexes : Solute-solvent charge-transfer complexes

Ionic photodissociation of charge transfer complexes : Solute-solvent charge-transfer complexes

Studies of the electronic absorption and emission spectra of 2,1,3-benzothiadiazole

A study has been made of the spectral properties of 2,1,3-benzothiadiazole (BTD) and of BTD-d4. Absorption spectra have been obtained in polar and nonpolar solvents at room temperature and in an EPA rigid glass at 77 K. The complete loss of resolution of the first singlet absorption in ethanol at room temperature is explained in terms of the formation of weak solute solvent complexes. This solvent effect is taken as evidence that the heterocyclic ring portion of the molecule is involved in the electronic transition. Fluorescence and phosphorescence emission spectra have been obtained in polar, nonpolar, and heavy atom rigid glasses at 77 K. Triplet decay lifetimes have been obtained both by directly measuring the phosphorescence decay and by monitoring the decay of triplet-triplet (T-T) absorption. The T-T absorption spectrum of BTD-d4 has been obtained in EPA rigid glass at 77 K in the regions from 20 000 cm−1 to 28 000 cm−1. The deuterium effect on the triplet state lifetime and the external heavy atom effect on phosphorescence intensity are interpreted as providing evidence for ππ∗ character in the lowest triplet state. On the other hand, the phosphorescence energy and the triplet lifetime are interpreted as providing evidence for nπ∗ character in the same state. Similary, the T-T spectrum is suggestive of nπ∗-ππ∗ interaction in the triplet manifold. A difference in the triplet state lifetime obtained by the two measurement techniques is discussed.

Effects of Solute–Solvent Complexation Reactions on Dissolution Kinetics: Testing of a Model by Using a Concentration Jump Technique

The rates of dissolution of 2-naphthol in cyclohexane and in cyclohexane containing different concentrations of additives which complex with dissolved solute such as n-propanol, undecanol, N,N-dimethylpropionamide, and N,N-dimethyldodecamide were measured. To minimize the effects on the rates of: (a) differences in area of the solid-liquid interface from one experiment to the next, (b) changes in the area of the solid-liquid interface that occur during an experiment, and (c) small variations in hydrodynamics, a “concentration jump” technique was employed. The rates of dissolution could be accounted for by using a modified form of Olander's film theory model, which included the effects of complexation equilibria on mass transfer. The model and results clearly demonstrated the importance of the magnitudes of both the diffusion coefficient of the complexing component in the solvent and of the stability constant of the complex on dissolution kinetics.

Excited state prototropism and solvent dependence of the fluorescence of 3-aminoquinoline

The effects of solvent polarity and hydrogen-bonding capability on the fluorescence of 3-aminoquinoline have been studied. The failure of the solvent dependence of the absorption spectrum to parallel that of the fluorescence spectrum is attributed to differences in hydrogen bonding in ground and lowest excited singlet states. The large Stokes shifts produced by addition of small amounts of hydrogen-bonding solvents to solutions of 3-aminoquinoline in non-activating solvents suggest the formation of a well defined solute-solvent complex in the excited state. 3-Amino-quinoline is a stronger acid in the excited state than in the ground state with respect to dissociation from the amino group. The same is true for the dication. Fo¨rster cycle calculations, however, indicate that dissociation from the heterocyclic nitrogen is more difficult in the excited state. It is shown that when hydrogen-bonding interactions are present, solvent relaxation effects after excitation are best studied in non-activating solvents rather than in rigid hydrogen-bonding solvents.

High-resolution nuclear magnetic resonance spectra of phenanthrenes. Part III. Benzo[c]phenanthrene

Complete analyses of the 1H n.m.r. spectra of benzo[c]phenanthrene (I) have been carried out for solutions in carbon disulphide, acetone, [2H6]benzene, and mixtures of carbon disulphide and [2H6]benzene. Spin-decoupling and variable temperature experiments have shown that broadening of the H(1,12) resonance must be attributed to unresolved long-range coupling to H(5) and H(8) and not to intramolecular relaxation. Interaction of (I) with [2H6]benzene has been investigated by dilution and variable temperature experiments. It is concluded that there is, at most, a structuring of an indefinite number of solvent molecules around each solute molecule, rather than the formation of a specific solute–solvent complex. The distinct effect of [2H6] benzene on the chemical shift of the sterically-hindered H(1), relative to the shifts of other protons, is best explained by restrictions on the close approach of solvent molecules. Intramolecular chemical shifts correspond well with those calculated on the basis of ring currents in a planar molecular model; for H(1), steric deshielding is apparently compensated by a reduced ring-current deshielding.

Solvent effect on NMR spectra of poly(methyl methacrylate)

AbstractThe NMR spectra of stereoblock poly(methyl methacrylate) in several solvents were measured. It is concluded from the following experimental results that the solute–solvent complexes are formed in benzene solution: the chemical shifts measured in C6H6 go to a lower field than do those in CDCl3, except those of the ester methyl group, which splits into three resonances, and the shifts in the aromatic solvents are so different from those in the aliphatic solvents that Buckingham's theory cannot be applied to the results. The analysis of the temperature dependence of the chemical shifts of PMMA in benzene solution gave the heat of formation of the complex: ΔH = 2.8 ± 0.5 kcal./mole.

Exciplex Studies. II. Indole and Indole Derivatives

An excited-state solute—solvent complex (exciplex) has been shown to be responsible for a large red shift and loss of vibrational structure in the fluorescence spectra of indole and indole derivatives in polar solvents. Solute—solvent stoichiometry of 1:2 and 1:1 is observed with associating and nonassociating solvents, respectively. Hydrogen bonding between the indole >N–H group and solvent is shown not to be responsible for the interaction. It is suggested that the exciplex state is a charge-transfer state and is an intermediate in the process of electron transfer from the solute to the solvent.

Low temperature circular dichroism: conformational effects

Circular dichroism (c. d.) is a very powerful tool for the determination of the geometry of a molecule in solution; the measured values are, however, often strongly dependent upon the experimental conditions used, such as, for example, solvent, concentration and temperature. In this paper only the latter effect and its applica­tion to conformational analysis is discussed. The two main reasons for a change of ∆ ϵ with temperature are solvation and conformational mobility (see, for example, Moscowitz et al . 1963 b ). Very often the investigated substance has some interactions with the solvent, leading to a solute-solvent complex, which might be as strong as a hemiketal, if a ketone is measured Table 1. Effect of temperature variation on C. D. (A) Solute + solvent ⇌[solute — solvent] (B) Conformational equilibria (I) Rigid structure ( R = const.) (II) Compounds with conformational mobility (1) One energy minimum during complete (pseudo) rotation ( I T 2 T 1 measure for steric hindrance) (2) Two energy minima ( R T = ( f ( R a , R b , T , ∆ G )) (3) Three or more energy minima in an alcoholic solution, or as weak as the interaction between, say, the same ketone and a saturated hydrocarbon, due to the induced dipole moment of the latter. In general, such an equilibrium between the components and the complex will be temperature dependent and as the complex may have a different c. d. from the non-solvated molecule, a change of ∆ ϵ may result. Frequently a blue-shift of the maxima is found at low temperatures as well as a more clearly resolved fine structure. These effects are ascribed to such solute-solvent interactions (see table 1).

NUCLEAR MAGNETIC RESONANCE STUDIES: VIII. DETERMINATION OF THE THERMODYNAMIC PARAMETERS GOVERNING ASSOCIATIONS OF SUBSTITUTED BENZALDEHYDES IN TOLUENE SOLUTION

The temperature dependence of the proton chemical shifts of four substituted benzaldehydes in toluene solution has been measured. From these results, estimates have been made of the enthalpies and entropies of formation of the stereospecific solute–solvent complexes which were previously shown to exist in these systems. These calculations indicate that the ΔH and ΔS values are −0.9 ± 0.2 kcal/mole and −4.9 ± 1.3 e.u., respectively. No correlation with substitution is apparent; the results are compared with other available data.

A proton magnetic resonance investigation of some weak interactions in solution

The proton chemical shifts of cyclohexane, methyl iodide and iodoform are measured in a number of solvents. A complete calculation of the contribution to the solute proton chemical shift arising from the magnetic anisotropy of cylindrically symmetric solvents is given. Although the formula predicts the direction of the observed shifts, the observed values for non-polar solutes are always much larger than the calculated values. Some possible reasons for this are given and discussed. The variation of the proton chemical shifts of the polar solutes methyl iodide and iodoform in aliphatic solvents are shown to agree with present theories of these effects. However, in aromatic solvents considerable deviations from the theoretical line are found and these are postulated to arise from solute solvent complexes in which the dipole axis of the solute lies along the hexagonal axis of symmetry of the benzene ring with the protons towards the ring. From the variation of the proton chemical shifts of methyl iodide and iodoform in toluene solution with temperature the following parameters were obtained. For the methyl iodide toluene complex the energy and entropy of formation are 1·3 ± 0·5 kcals/mole and 4·9 ± 0·4 e.u. respectively. For the iodoform toluene complex the corresponding values are 1·6 ± 0·2 kcals/mole and 6·4 ± 0·2 e.u.