A computational study of ultrafast acid dissociation and acid-base neutralization reactions. II. The relationship between the coordination state of solvent molecules and concerted versus sequential acid dissociation

作者：Maurer Patrick; Thomas Vibin; Iftimie Radu^*

来源：Journal of Chemical Physics, 2011, 134(9): 094505.

DOI：10.1063/1.3554654

We investigate the role played by the coordination state of pre-existing water wires during the dissociation of moderately strong acids by means of first-principles molecular dynamics calculations. By preparing 2,4,6-tricyanophenol (calc. pKa similar to 0.5) in two different initial states, we are able to observe sequential as well as concerted trajectories of dissociation: On one hand, equilibrium dissociation takes place on a similar to 50 ps timescale; proton conduction occurs through three-coordinated water wires in this case, by means of sequential Grotthus hopping. On the other hand, by preparing 2,4,6-tricyanophenol in a hydration state inherited from that of equilibrated phenol (calc. pKa = 7.6), the moderately strong acid finds itself in a presolvated state from which dissociation can take place on a similar to 1 ps timescale. In this case, concerted dissociation trajectories are observed, which consist of proton translocation through two intervening, four-coordinated, water molecules in 0.1 -1.0 ps. The present results suggest that, in general, the mechanism of proton translocation depends on how the excess proton is injected into a hydrogen bond network. In particular, if the initial conditions favour proton release to a fourfold H-bonded water molecule, proton translocation by as much as 6-8 angstrom can take place on a sub-picosecond timescale.

• 出版日期2011-3-7