HF dissociation in water clusters by computer simulations

We perform Restrained hybrid Monte Carlo simulations to compute the equilibrium constant of the dissociation reaction of HF in HF(H₂O)₇. We find that, like in the bulk, hydrofluoric acid, is a weak acid also in the cubic HF(H₂O)₇ cluster, and that its acidity is higher at lower T. This latter phenomenon has a (vibrational) entropic origin, namely it is due to the reduction of the (negative) T∆S contribution to the variation of free energy between the reactant and product. We found also a temperature dependence of the reactions mechanism. At low T (≤225 K) the dissociation reaction follows a concerted path, with the H atoms belonging to the relevant hydrogen bond chain moving synchronously. At higher T (300 K), first two hydrogen atoms move together, forming an intermediate metastable state having the structure of an Eigen ion H₉O₄<super>+</super>, then the third hydrogen migrates completing the reaction. We also compute the dissociation rate constant, k_rp. We find that at very low T (≤75 K), k_rp depends strongly on the temperature, while it is almost constant at higher Ts. With respect to the bulk, the HF dissociation in HF(H₂O)₇ is about one order of magnitude faster. This is due to a lower free energy barrier for dissociation in the cluster.