Proton transfer from water to aromatic N-heterocyclic anions from DFT-MD simulations.

The phenomenon of proton transfer from water to six N-heterocyclic anions and free energy landscapes of this process are studied using both electronic structure calculations and first principles molecular metadynamics simulations. Our investigation involves microhydrated and aqueous phase interaction of water with six aromatic heterocyclic anions relevant to chemistry and biology: imidazolide, pyrrolide, benzimidazolide, 2-cyanopyrrolide, indolide, and indazolide. The basic structures of all these heterocyclic anions differ by substituted functional groups as well as fused rings. We study the proton transfer reaction and the minimum number of required water molecules for the reaction in hydrated microclusters. We find out that at least four water molecules are necessary for hydrated clusters to facilitate the intracluster proton transfer reaction from water to anions except for pyrrolide, for which this magic number is 3. To obtain the reaction free energy and activation barrier of the proton transfer process in an aqueous solution, the metadynamics method based first principles molecular dynamics simulations were performed. The complete proton transfer was observed in aqueous solutions for all the anions. The water molecule directly involved in proton transfer becomes acidic due to the cooperative effect of neighboring water molecules. From the metadynamics simulation, we obtain the values of activation barrier for the proton transfer processes from neutral water to anions, and the highest activation barrier is obtained for benzimidazolide, whereas the lowest activation barrier is obtained for pyrrolide. The structures and free energy profiles of the process for all the anions are discussed, and a comparative outlook of the study is presented here.