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Reversible geminate recombination of hydrogen-bonded water molecule pair

Summary: Reversible geminate recombination of hydrogen-bonded water
molecule pair
Omer Markovitcha
and Noam Agmonb
Institute of Chemistry and the Fritz Haber Research Center, The Hebrew University of Jerusalem,
Jerusalem 91904, Israel
Received 15 May 2008; accepted 16 July 2008; published online 27 August 2008
The history independent autocorrelation function for a hydrogen-bonded water molecule pair,
calculated from classical molecular dynamics trajectories of liquid water, exhibits a t-3/2
tail. Its whole time dependence agrees quantitatively with the solution for reversible
diffusion-influenced geminate recombination derived by Agmon and Weiss J. Chem. Phys. 91,
6937 1989 . Agreement with diffusion theory is independent of the precise definition of the bound
state. Given the water self-diffusion constant, this theory enables us to determine the dissociation
and bimolecular recombination rate parameters for a water dimer. The theory is indispensable for
obtaining the bimolecular rate coefficient. Interestingly, the activation energies obtained from the
temperature dependence of these rate coefficients are similar, rather than differing by the
hydrogen-bond HB strength. This suggests that recombination requires displacing another water
molecule, which meanwhile occupied the binding site. Because these activation energies are about
twice the HB strength, cleavage of two HBs may be required to allow pair separation. The


Source: Agmon, Noam - Institute of Chemistry, Hebrew University of Jerusalem


Collections: Chemistry