Determination of protonation sites in bases from topological rules

The topological analysis of the electron localization function ELF has been carried out on a series of neutral and protonated bases revealing that the determination of the most favorable protonation site can be done with the help of a simple rule. The protonation occurs in the most populated valence basin of the base which yields the least topological change of the localization gradient field. This can be viewed as resulting of a competition between the Pauli repulsion and the stabilization brought by the proton. Structural comparisons of hydrogen bonded complexes and protonated bases are consistent with a picture in which the formation of the complex is the initial step of the proton transfer reaction B + HX --> BH+ + X-. The angular structure of the complex is such as the proton transfer reaction satisfies the least motion principle of Rice and Teller. This description brings support to the model of Legon and Millen which generalizes the VSEPR concepts to hydrogen bonded complexes B HX in assuming that the hydrogen bond is directed towards the nucleophilic center of the basis, in other words towards the protonation site. (C) 2000 Elsevier Science B.V. All rights reserved.