Solvent effect on proton transfer

Figure 1. Schematic representation of solvent effect on proton transfer from naphthols.

Sections 3.3.1 and 4.2.1 dealt with Bronsted acid/base equilibria in which the solvent itself is involved in the chemical reaction as either an acid or a base. This Section describes some examples of solvent effects on proton-transfer (PT) reactions in which the solvent does not intervene directly as a reaction partner. New interest in the investigation of such acid/base equilibria in non-aqueous solvents has been generated by the pioneering work of Barrow et al. [164]. He studied the acid/base reactions between carboxylic acids and amines in tetra- and trichloromethane. A more recent compilation of Bronsted acid/base equilibrium constants, determined in up to twelve dipolar aprotic solvents, demonstrates the appreciable solvent influence on acid ionization constants [264]. For example, the p.Ka value of benzoic acid varies from 4.2 in water, 11.0 in dimethyl sulfoxide, 12.3 in A,A-dimethylformamide, up to 20.7 in acetonitrile, that is by about 16 powers of ten [264]. 

The solvent effect on proton-transfer reactions is determined by two effects In cases of slow proton transfers, which are not diffusion-controlled, e.g. reactions between carbon acids and weak bases the solvent effect is determined by hydrogen-bonding and formation of ion pairs whereas tte viscosity of the solvent prevails in very fast diffusion-controll l reactions. 

The relationship of solvent effects on proton-transfer equilibria to the number and the acidity of acid protons in cationic conjugate acids (BH ) is considered in detail in Chapter 3. A similar analysis of solvent effects on the acidities of hydroxylic compounds has not been achieved but can be expected for the near future. 

It was recognized by La Mer and Noonan (1939 Noonan and La Mer, 1939) that the comparison of chemical processes in isotopically different solvents involves a general medium or transfer effect, in addition to any effect due to the occurrence of exchange equilibria. The point was further developed by Kingerley and La Mer (1941) in relation to acid-base equilibria, but these authors also showed that in many cases the transfer contribution is of minor importance. Because of the relative size of the two contributions, most workers have paid little or no attention to the transfer contribution. Precisely the opposite view, namely that the entire or, at least, a large part of the solvent isotope effect on proton transfer processes was due to the transfer effect, was propounded by Long and his group (Halevi et al., 1961) but has been abandoned in their more recent work. 

H. R (1996) Evidence by NMR of temperature-dependent solvent electric field effects on proton transfer and hydrogen bond geometries. Z. Phys. Chem., 196, 73-84. 

Wei, D., Salahub, D. R., 1994, Hydrated Proton Clusters and Solvent Effects on the Proton Transfer Barrier ... 

Wei, D. and D. R. Salahub. 1994. Hydrated proton clusters and solvent effects on the proton transfer barrier A density functional study. J. Chem. Phys. 101, 7633. 

The several theoretical and/or simulation methods developed for modelling the solvation phenomena can be applied to the treatment of solvent effects on chemical reactivity. A variety of systems - ranging from small molecules to very large ones, such as biomolecules [236-238], biological membranes [239] and polymers [240] -and problems - mechanism of organic reactions [25, 79, 223, 241-247], chemical reactions in supercritical fluids [216, 248-250], ultrafast spectroscopy [251-255], electrochemical processes [256, 257], proton transfer [74, 75, 231], electron transfer [76, 77, 104, 258-261], charge transfer reactions and complexes [262-264], molecular and ionic spectra and excited states [24, 265-268], solvent-induced polarizability [221, 269], reaction dynamics [28, 78, 270-276], isomerization [110, 277-279], tautomeric equilibrium [280-282], conformational changes [283], dissociation reactions [199, 200, 227], stability [284] - have been treated by these techniques. Some of these... 

Tortonda, F. R., Pascual-Ahuir, J. L., Silla, E. and Tunon, I. Solvent effects on the thermodynamics and kinetics of the proton transfer between hydronium ions and ammonia. A Theoretical study using the continuum and the discrete models, J. Phys. Chem., 99 (1995), 12525-12531... 

Transient absorption experiments have shown that all of the major DNA and RNA nucleosides have fluorescence lifetimes of less than one picosecond [2—4], and that covalently modified bases [5], and even individual tautomers [6], differ dramatically in their excited-state dynamics. Femtosecond fluorescence up-conversion studies have also shown that the lowest singlet excited states of monomeric bases, nucleosides, and nucleotides decay by ultrafast internal conversion [7-9]. As discussed elsewhere [2], solvent effects on the fluorescence lifetimes are quite modest, and no evidence has been found to date to support excited-state proton transfer as a decay mechanism. These observations have focused attention on the possibility of internal conversion via one or more conical intersections. Recently, computational studies have succeeded in locating conical intersections on the excited state potential energy surfaces of several isolated nucleobases [10-12]. 

The ESPT of naphthylammonium [190] and phenanthrylammonium [191] ions in their 18-crown-6 ether complexes in MeOH-H20 (9 1) solvent shows that the excited-state proton-transfer rate decreases markedly on complexing. The back-protonation rate in the excited state is negligibly small compared with those of the other decay processes, which essentially means that there is no excited-state protropic equilibrium in the crown complexes. The one-way proton-transfer reaction is elucidated by the presence of the excited neutral amine-crown complex (RNH2-crown) produced by deprotonation of (RN+H3-crown). There is a large steric effect on protonation to the amino group of the excited neutral complex. 

In contrast to the subsystem representation, the adiabatic basis depends on the environmental coordinates. As such, one obtains a physically intuitive description in terms of classical trajectories along Born-Oppenheimer surfaces. A variety of systems have been studied using QCL dynamics in this basis. These include the reaction rate and the kinetic isotope effect of proton transfer in a polar condensed phase solvent and a cluster [29-33], vibrational energy relaxation of a hydrogen bonded complex in a polar liquid [34], photodissociation of F2 [35], dynamical analysis of vibrational frequency shifts in a Xe fluid [36], and the spin-boson model [37,38], which is of particular importance as exact quantum results are available for comparison. 

Many reactions become possible only in such superbasic solutions, while others can be carried out under much milder conditions. Only some examples of preparative interest (which depend on the ionization of a C—H or N—H bond) will be mentioned here. The subsequent reaction of the resulting carbanion may involve electrophilic substitution, isomerization, elimination, or condensation [321, 322]. Systematic studies of solvent effects on intrinsic rate constants of proton-transfer reactions between carbon acids and carboxylate ions as well as amines as bases in various dimethyl sulfoxide/ water mixtures have been carried out by Bernasconi et al. [769]. 

All those spectral changes which arise from alteration of the chemical nature of the chromophore-containing molecules by the medium, such as proton or electron transfer between solvent and solute, solvent-dependent aggregation, ionization, complexation, or isomerization equilibria lie outside the scope of this chapter. Theories of solvent effects on absorption spectra assume principally that the chemical states of the isolated and solvated chromophore-containing molecules are the same and treat these effects only as a physical perturbation of the relevant molecular states of the chromophores [435-437]. 

A number of other proton transfer reactions from carbon which have been studied using this approach are shown in Table 8. The results should be treated with reserve as it has not yet been established fully that the derived Bronsted exponents correspond exactly with those determined in the conventional way. One problem concerns the assumption that the activity coefficient ratios cancel, but doubts have also been raised by one of the originators of the method that, unless solvent effects on the transition state are intermediate between those on the reactants and products, anomalous Bronsted exponents will be obtained [172(c)]. The Bronsted exponents determined for menthone and the other ketones in Table 8 are roughly those expected by comparison with the values obtained for ketones using the conventional procedure (Table 2). For nitroethane the two values j3 = 0.72 and 0.65 which are shown in Table 8 result from the use of different H functions determined with amine and carbon acid indicators, respectively. Both values are roughly similar to the values (0.50 [103], 0.65 [104]), obtained by varying the base catalyst in aqueous solution. The result for 2-methyl-3-phenylpropionitrile fits in well with the exponents determined for malononitriles by general base catalysis but differs from the value j3 0.71 shown for l,4-dicyano-2-butene in Table 8. This latter result is also different from the values j3 = 0.94 and 0.98 determined for l,4-dicyano-2-butene in aqueous solution with phenolate ions and amines, respectively. However, the different results for l,4-dicyano-2-butene are to be expected, since hydroxide ion is the base catalyst used in the acidity function procedure and this does not fit the Bronsted plot observed for phenolate ions and amines. The primary kinetic isotope effects [114] also show that there are differences between the hydroxide ion catalysed reaction (feH/feD = 3.5) and the reaction catalysed by phenolate ions (kH /kP = 1.4). The result for chloroform, (3 = 0.98 shown in Table 8, fits in satisfactorily with the most recent results for amine catalysed detritiation [171(a)] from which a value 3 = 1.15 0.07 was obtained. 

Detailed analyses of intramolecular structures are possible. Comparison of NMR and fluorescence data shows meso- and racemic diastereoisomers are found from 2,4-di(2-pyrenyl)pentane 24 jhe polarization of monomer and excimer of 4,9, disubstituted pyrenes have been measured in nematic liquid crystals 25 Quenching of pyrene fluorescence by alcohols in cyclodextrin inclusion complexes has also been studied in detail 26 Solvent effects on the photophysical properties of pyrene-3-carboxylic acid has been used to measure the pJJ, in different solvents 27 Geminate recombination in excited state proton transfer reactions has been studied with... 

---