Theoretical analysis of pyridine protonation in water clusters of increasing size

The protonation of pyridine in water clusters as a function of the number of water molecules was theoretically analyzed as a prototypical case for the protonation of organic bases. We determined the variation of structural, bonding, and energetic properties on protonation, as well as the stabilization of the ionic species formed. Thus, we used supermolecular models in which pyridine interacts with clusters of up to five water molecules. For each complex, we determined the most stable unprotonated and protonated structures from a simulated annealing at the semi ab initio level. The structures were optimized at the B3LYP/cc-pVDZ level. We found that the hydroxyl group formed on protonation of pyridine abstracts a proton from the ortho-carbon atom of the pyridine ring. The "atoms in molecules" theory showed that this C-H group loses its covalent character. However, starting with clusters of four water molecules, the C-H bond recovers its covalent nature. This effect is associated with the presence of more than one ring between the water molecules and pyridine. These rings stabilize, by delocalization, the negative charge on the hydroxyl oxygen atom. Considering the protonation energy, we find that the protonated forms are increasingly stabilized with increasing size of the water cluster. When zero-point energy is included, the variation follows closely an exponential decrease with increasing number of water molecules. Analysis of the vibrational modes for the strongest bands in the IR spectra of the complexes suggests that the protonation of pyridine occurs by concerted proton transfers among the different water rings in the structure. Symmetric water stretching was found to be responsible for hydrogen transfer from the water molecule to the pyridine nitrogen atom.