Linear free energy relations solvent effects

The Bronsted relation was the first example of a linear free energy relation between rates and equilibrium constants, and would now be regarded as a particular example of this class. Such relations are now widely used for correlating the effects of substituents or change of solvent... 

Joris, L., Mitsky, J. and Taft, R.W. (1972) Effects of polar aprotic solvents on linear free energy relations in hydrogen-bonded complex formation. J. Am. Chem. Soc., 94,3438-3442. 

The second aspect is more fundamental. It is related to the very nature of chemistry (quantum chemistry is physics). Chemistry deals with fuzzy objects, like solvent or substituent effects, that are of paramount importance in tautomerism. These effects can be modeled using LFER (Linear Free Energy Relationships), like the famous Hammett and Taft equations, with considerable success. Quantum calculations apply to individual molecules and perturbations remain relatively difficult to consider (an exception is general solvation using an Onsager-type approach). However, preliminary attempts have been made to treat families of compounds in a variational way [81AQ(C)105]. 

During the last two decades, our group has smdied solvent effects on the process development in different molecular solvents (aprotic and protic, polar and non-polar) or in their binary mixtures, correlating the kinetic data of this reaction with empirical solvent parameters (E, n, a and 3) through Linear Free Energy Relationship s - LFER s - simple and multiparametric equations. The principal S Ar reactions studied comprise l-halo-2,4-dinitrobenzene or l-halo-2,6-dinitrobenzene as substrates and primary and secondary amines as nucleophiles. For the 1-fluoro-dinitrobenzenes derivatives, the reaction can exhibit base catalysis, which is normally solvent dependent. In general, solvent effects were related to reaction rates, mechanisms and catalysis. These studies were extented employing ILs as reaction media. 

The term A is related to the solvent density and molecular weight and to the free volumes of the ions and the ion-pair [59]. Fjoi is the difference between the molar ion-pair solvation energy and the free ion solvation energy. The theory does not predict a simple linearity of In with lie. Actually solvent effects other than that due to the relative permittivity of the solvent are easily predicted, since the macroscopic e is only a rudimentary description of the real attenuation of the ionic interactions due... 

Recendy, validity of the two assumptions regarding the free energy proflie, parabolic and solute independence of the force constants, has been examined by several authors. These assumptions have been predicted from the continuum dielecuic models and commonly adopted in many of the early works. Kakitani et alJ discussed the nonlinearity of solvation related to ET in polar solvents, and Carter and Hynes performed molecular dynamics simulations of the charge separation (CS) and the charge recombination (CR) reactions to observe such non-linear effects. More recently, Ando et alf discussed these problems and they observed no such non-linear effects. Due to the non-linear nature of the hypemetted chain (HNC) closure to solve the RISM equation, our method can shed light on the non-linearity of the free energy profiles. In section III, we apply our method, which is outlined briefly in section II, to the CS reaction which was previously studied by Carter and Hynes, and discuss the problems mentioned above based on the obtained free energy profiles. 

---