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.

Pressure control in interfacial systems: Atomistic simulations of vapor nucleation

“…To study vapor nucleation, we employ the hybrid Restrained Monte Carlo (hRMC) approach, 17,18,26 which is well suited for non-analytical collective variables (CV), such as the size of the largest vapor bubble used here. hRMC allows to sample the conditional probability density function at the current value of the volume of the vapor bubble, and to compute conditional averages.…”

“…Atomistic simulations are performed for a Lennard-Jones (LJ) liquid in the same nominal thermodynamic conditions as those available in the literature 7,8 (both references use global barostats). A hybrid restrained Monte Carlo (hRMC) scheme 17,18 is adopted in order to cope with the problem of rare events 19 typical of nucleation and in order to compute the related free energy profile; the volume of the largest bubble is used as the order parameter. 20 The good agreement between macro-and microscopic results suggests that the intuitive argument of domains at different pressures is, indeed, at the origin of the artifacts associated with global barostats.…”

Isolation of the simplest hydrated acid

“…Hydrogen fluoride (HF) is the smallest acid and has been studied well, mostly in the gas phase, both theoretically and experimentally ( 6 ). One extensively discussed issue on HF is the minimum number of H 2 O molecules that is necessary to solvate an HF molecule resulting in the formation of the solvent-shared ion pair [H 3 O + (H 2 O) n F – ] ( 3 , 7 , 8 ). However, the central obstacle to resolution of this subject includes the difficulty of preparation of any of the possible HF·(H 2 O) n complexes in a pure form with definite components.…”

C70 Fullerene Cage as a Novel Catalyst for Efficient Proton Transfer Reactions between Small Molecules: A Theoretical study

“…Proton transfer is at the heart of acid-base chemistry, ubiquitous in many chemical systems and relevant to a wide range of disciplines 1–3 . Partial (or facile) proton transfer from acids occurs when the acidic species is solvated by several H 2 O molecules; this is increasingly recognized for the acids HX (X = Cl, Br, I) that experience dissociation near 0 K in (H 2 O) n with n ≥ 4.…”

“…How individual Brønsted-Lowry acids HX (X = F, Cl, Br, I) interact with Brønsted-Lowry bases such as NH 3 in an aqueous environment has been studied for decades 1–3 . More recently, interest has developed in exploring how they behave in different environments, such as in the inside of the C 60 and C 70 fullerene cages, for example 4–13 .…”