The role of nonelectrostatic solvation to chemical reactions in liquid phase

Abstract

A importância de cada termo de solvatação não eletrostática, mais especificamente, formação de cavidade e dispersão-repulsão, para reações químicas em fase líquida foi analisada tomando como exemplo a reação de uma e duas moléculas de H 2 O com CCl 2 . O cálculo do efeito do solvente para estas reações em solução aquosa, via perturbação de energia livre, está disponível na literatura e foi usado para comparação. Neste trabalho, utilizamos a fórmula de Pierotti e o método de ClaveriePierotti para computar o valor da contribuição de formação de cavidade, enquanto que o termo de dispersão-repulsão foi calculado pelo método de Floris e Tomasi. Nossos resultados mostram que o termo de cavidade é o mais importante, sendo que o termo de dispersão-repulsão corresponde de 25% a 35% do termo de cavidade. Além disso, observamos que a aplicação direta da fórmula de Pierotti para a energia livre de cavidade é mais acurada do que o método de Claverie-Pierotti para este sistema.The relative importance of cavity formation and dispersion-repulsion contributions to the activation free energy of chemical reactions in liquid phase is discussed, taken as example the reaction of one and two H 2 O molecules with CCl 2 in aqueous solution. The solvent effect on these systems was investigated by free energy perturbation in previous publications. In the present report, were used the Pierotti scaled particle theory and the Claverie-Pierotti method to compute the free energy of cavity formation, while the dispersion-repulsion contribution was determined by the Floris and Tomasi method. We have found that the cavity term is the most important nonelectrostatic contribution of the solvent to the activation free energy, while the dispersion-repulsion contribution accounts for 25% to 35% of that term. In addition, we have observed that the direct Pierotti formula is more accurate than the Claverie-Pierotti method for the present system. Keywords: solvent effect, continuum model, reaction kinetics IntroductionThe important role of the solvent on the outcome and kinetics of chemical reactions is well established. 1-14 Usually, the electrostatic solute-solvent interaction is the most important contribution of the solvent to the free energy of solvation ( G el ), mainly in polar media and in situations where ionic species are involved. The nonelectrostatic contribution ( G nel ) is regarded as playing a minor role, but it is important for a fine turning of the solvation process modeling. This contribution can be separated in two terms: free energy of cavity formation ( G cav ) and dispersionrepulsion contribution ( G dr ). Thus, the full solvation free energy of a molecule can be written as:An important question to be analyzed is the level of importance of each nonelectrostatic term. It is known that both nonelectrostatic contributions can be relatively large and have opposite signals, leading to a small G nel . However, for a chemical process in solution, the variation of the solvation free energy ( G * solv , from the reactants to the transition ...

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