title: Equilibrium and rate constants, and reaction mechanism of the HF dissociation in the HF(H2O)7 cluster by ab initio rare event simulations.
abstract: We perform restrained hybrid Monte Carlo (MC) simulations to compute the equilibrium constant of the dissociation reaction of HF in HF(H2O)7. We find that the HF is a stronger acid in the cluster than in the bulk, and its acidity is higher at lower T. The latter phenomenon has a vibrational entropic origin, resulting from a counterintuitive balance of intra- and intermolecular terms. We find 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), the first two hydrogen atoms move together, forming an intermediate metastable state having the structure of an eigen ion (H9O4(+)), and then the third hydrogen migrates completing the reaction. We also compute the dissociation rate constant, kRP. At very low T (≤75 K) kRP depends strongly on the temperature, whereas it gets almost constant at higher T’s. With respect to the bulk, the HF dissociation in the HF(H2O)7 is about 1 order of magnitude faster. This is due to a lower free energy barrier for the dissociation in the cluster.
authors.affiliation: School of Physics, Room 302 UCD-EMSC, University College Dublin , Belfield, Dublin 4, Dublin, Ireland.
authors.forename: Alin Marin
authors.initials: AM
authors.lastname: Elena
authors.forename: Simone
authors.initials: S
authors.lastname: Meloni
authors.forename: Giovanni
authors.initials: G
authors.lastname: Ciccotti
mesh_terms: D006858:Hydrofluoric Acid / Q000737:chemistry*; D007700:Kinetics; D009010:Monte Carlo Method; D011789:Quantum Theory; D013816:Thermodynamics; D014867:Water / Q000737:chemistry*
chemical_list: D014867:Water; D006858:Hydrofluoric Acid