Solvent-dependent transition states for decarboxylations

The rate constants and kinetic isotope effects for decarboxylation of 4-pyr idylacetic acid depend strongly on whether the solvent is water or dioxane, and the present paper interprets this finding. We calculate the solvent de pendence of the free energy barrier and of the C-13 and O-18 kinetic isotop e effects using a quantum mechanical solvation model based on class IV char ges and semiempirical atomic surface tensions. The calculations provide a c onsistent interpretation of the experimental results, which provides a stri king confirmation of the soundness of the solvation modeling. Even more sig nificantly, the agreement of theory and experiment gives us confidence in t he physical picture of the reaction provided by the model. This indicates t hat the location of the transition state, as measured by the length of the breaking C-C bond, is 0.24 Angstrom later than the gas phase in dioxane and 0.37 Angstrom later than the gas phase in water. Charge development at the transition state also depends strongly on the solvent; in particular the C O2 moiety is 0.07 electronic charge units more negative at the transition s tate in dioxane than in water.