Proton interactions, solvent-solute

The solvent triangle classification method of Snyder Is the most cosDBon approach to solvent characterization used by chromatographers (510,517). The solvent polarity index, P, and solvent selectivity factors, X), which characterize the relative importemce of orientation and proton donor/acceptor interactions to the total polarity, were based on Rohrscbneider s compilation of experimental gas-liquid distribution constants for a number of test solutes in 75 common, volatile solvents. Snyder chose the solutes nitromethane, ethanol and dloxane as probes for a solvent s capacity for orientation, proton acceptor and proton donor capacity, respectively. The influence of solute molecular size, solute/solvent dispersion interactions, and solute/solvent induction interactions as a result of solvent polarizability were subtracted from the experimental distribution constants first multiplying the experimental distribution constant by the solvent molar volume and thm referencing this quantity to the value calculated for a hypothetical n-alkane with a molar volume identical to the test solute. Each value was then corrected empirically to give a value of zero for the polar distribution constant of the test solutes for saturated hydrocarbon solvents. These residual, values were supposed to arise from inductive and... 

SOLVOLYSIS. A generalized conception of the relation between a solvent and a solute (i.e., a relation between two components of a single-phase homogeneous system) whereby new compounds arc produced, In most instances, the solvent molecule donates a proton to. or accepts a proton from a molecule of solute, or both, forming one or more different molecules. A particular case of special interest occurs when water is used as solvent, m which case die interaction between solute and solvent is called hydrolysis. 

It follows from these similarities in solvent properties that equilibrium or rate constants of reactions in which the solvent molecules do not directly participate generally show comparatively small changes when the deuterium content of the medium is altered. This is true even for rates of proton transfer between neutral substrates and acetate ions, which as a rule are reduced by 20-40% on going from H20 to D20 (Bell, 1965). Because of the anionic nature of one of the reactants and of the transition state these reactions are of a type in which solvent-solute interactions through hydrogen bonds are probably particularly large, and yet the solvent isotope effect is fairly small. Reactions in... 

Intermolecular solvent-solute interactions influence the charge distribution on a carbohydrate molecule. Subtle electronic changes that occur as a result of these interactions are responsible for the solvent dependence of carbon -proton coupling constants. The general aspects of solvent effects on NMR parameters have been reviewed,78-79 and consequently, only a very brief outline of the theoretical model within FPT INDO SCF MO formalism is considered here. 

Most studies of magnetic relaxation in polymers have dealt with solid or melted polymers (73)] however, Odajima (20) has studied proton relaxation in solutions of polystyrene and polyisobutylene. It is desirable to extend and refine such measurements. In concentrated solution, some insight into the motional effects of polymer-solvent interactions should be obtainable and if, despite low sensitivity, reliable 7 values can be obtained for polymers in dilute solutions, valuable information concerning the detailed motional behavior of isolated polymer molecules may be provided. 

Acidity constants always compare the acidity of a proton donator with the basicity of the solvent. Therefore, only acidity constants relating to the same solvent can be compared [iii, iv]. The acidity constant strongly depends on the dielectric constant of the solvent and solvent-solute interaction parameters. About relations between the acidity constants of one acid in different solvents see reference [v]. 

A selection of AN values has already been given in Table 2-5 of Section 2.2.6 cf. also Table 6-6 in Section 6.5.1. The observed solvent-dependent P chemical shifts result mainly from the polarization of the dipolar P=0 group, induced by the interaction with electrophilic solvents A, particularly HBD solvents. The decrease in electron density at the phosphorus atom results in a deshielding proportional to the strength of the probe/solvent interaction. In solutions of protic acids, the P chemical shift of the 0-protonated triethyl hydroxyphosphonium salt is observed. Since Et3PO is very hygroscopic and therefore not very suitable from an experimental point of view, the use of (n-Bu)3PO instead of Et3PO as probe molecule has been recommended [250]. 

The starting point to elucidate the way the proton moves in solution is to consider its movement through the solvent at a steady state—constant velocity—and at a concentration so low that there is no interionic interaction (zeroth approximation). This occurs when the electric driving force ze X balances the Stokes viscous force, 6nrr v. Thus, the Stokes mobility is... 

Levy and coworkers97 have measured 13C spin-lattice relaxation times, 7), for 3- and 4-aminobiphenyls in a number of solvent systems, and of the corresponding ammonium ions in acidic and nonacidic media. The observed 7) values indicated that the molecular tumbling is anisotropic for these species. In addition, the known biphenyl geometry allowed indentification and semiquantitative evaluation of internal rotation-libration motion. The protonated amine function is motionally more restricted by solvent-solute and ion-pair interactions than the corresponding neutral amine. Thus, in the 3-biphenylammonium ion, the principal axis for molecular reorientation is aligned close to the C3—NHj-bond, whereas in the amine the principal axis lies closer to the biphenyl C2-symmetry axis. In both 3- and 4-aminobiphenyls, the unsubstituted phenyl rings are less restricted due to rapid phenyl rotation or libration. Table 14 presents 13C Tj-data for 4-aminobiphenyl 37 (NH2 on C4) and 4-biphenylammonium acetate 38 and trifluoroacetate 39. 

Substituting in equation 11 the known experimental parameters for phenol dissociation (AG, = 13.8 kcalmol" calculated from the ground-state equilibrium constant, pX, = 10.0), AGt((PhO ) — (PhOH)) of the phenolate/phenol system is about —76 kcalmoH, which is about 10% less than the accepted value for the electrostatic solvation energy of the chloride anion in water, AGe(Cr) = —85 kcalmol". These simple considerations imply that the AGt((PhO ) — (PhOH)) contribution to the overall free energy of solvation is largely electrostatic, and that relatively small differences in the gas-phase proton affinity of the base and in specific solvent-solute interactions of the photoacid and the base determine the relatively narrow (in free-energy units) acidity scale in aqueous solution. It... 

Xanthone, flavone and similar compounds. A difference in chloroform specificity from 2-methoxyethanol is again demonstrated In Figure 11. Xanthydrol is eluted before xanthone and flavone with chloroform as the modifier. Perhaps this shows the useful coupling of a proton donor solvent and proton acceptor solute (a large secondary solvent effect in an adsorption system (43)), the Interaction that was hoped for with the selection of chloroform as one of the selectivity triangle modifiers and apparent here because of a less strong adsorption of the xanthydrol on the fully active silica than some of the other basic solutes used in the preliminary studies. 

These approaches date back to the classical papers by Onsager(9) and Kirkwood(lO). A self-consistent reaction field (SCRF) was developed and applied to several solvent effects with reasonable success(8, 11). However, it seems to become clear that these models where the solute and the solvent are represented by separated non-overlapping wave functions are too crude in some cases(12). Studies of solvent effects involving the interaction of the solute with a proton donor solvent (the so-called protic solvents) for instance usually leads to hydrogen bonds and therefore neglecting the overlap of the electron densities between the two subsystem is very difficult to justify. A similar difficulty is obtained for those absorptions where electron is trans-... 

That the shifts reported 101) for this compound in perdeuteriotoluene are quite different (in particular the inner protons are shifted upheld to r 14.22) implies strong solvent/solute interactions in this system. 

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