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 reagent was modeled using density functional theory with a Gaussian basis set; two different force fields for the classical bath were investigated: the TIP4P-FQ fluctuating charge and the TIP4P mean field potentials. Basis sets, functionals, and force field parameters have been validated by performing calculations on [HO-](H2O), [Br-](H2O), [HBr](H2O), and [H2O](H2O) isolated dimers at 0 K. Molecular dynamics simulations of the system [HOHBr](-)(H2O)(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 halide and the generation of a water molecule within a few femtoseconds. In addition, our experiments show that the process involves a fast damping of the potential energy concomitant with a sudden increase of the vibrational kinetic 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 increase in the cluster sizes, suggesting the onset of cluster fragmentation; both phenomena evolve faster in the smallest clusters. The role of polarization effects in the classical subsystem on the reaction dynamics was also investigated by performing simulation experiments with the TIP4P potential. In these cases, the proton transfer is more exothermic, leading to fragmentation of the aggregates at earlier stages.