Hybrid quantum classical molecular dynamics simulation of the proton-transfer reaction of HO- with HBr in aqueous clusters

A hybrid quantum classical computational algorithm, which couples a density functional Hamiltonian to a classical bath, is applied to investigate the proton-transfer reaction OH- + HBr --> H2O + Br- in aqueous clusters. The r eagent was modeled using density functional theory with a Gaussian basis se t; two different force fields for the classical bath were investigated: the TIP4P-FQ fluctuating charge and the TIP4P mean field potentials. Basis set s, functionals, and force field parameters have been validated by performin g calculations on [HO-](H2O), [Br-](H2O), [HBr](H2O), and [H2O](H2O) isolat ed dimers at 0 K. Molecular dynamics simulations of the system [HOHBr](-)(H 2O)(n), with n = 2 and 6, show that the reaction is spontaneous and rather exothermic, leading to the full detachment of the bromide ion from the hali de and the generation of a water molecule within a few femtoseconds. In add ition, our experiments show that the process involves a fast damping of the potential energy concomitant with a sudden increase of the vibrational kin etic energy of the newly formed HO bond in the water molecule. The gradual dissipation of the solute energy into the classical region led to an increa se in the cluster sizes, suggesting the onset of cluster fragmentation; bot h phenomena evolve faster in the smallest clusters. The role of polarizatio n effects in the classical subsystem on the reaction dynamics was also inve stigated by performing simulation experiments with the TIP4P potential. In these cases, the proton transfer is more exothermic, leading to fragmentati on of the aggregates at earlier stages.