Aggregation-Induced Chemical Reactions: Acid Dissociation in Growing Water Clusters

Abstract

Understanding chemical reactivity at ultracold conditions, thus enabling molecular syntheses via interstellar and atmospheric processes, is a key issue in cryochemistry. In particular, acid dissociation and proton transfer reactions are ubiquitous in aqueous microsolvation environments. Here, the full dissociation of a HCl molecule upon stepwise solvation by a small number of water molecules at low temperatures, as relevant to helium nanodroplet isolation (HENDI) spectroscopy, is analyzed in mechanistic detail. It is found that upon successive aggregation of HCl with H(2)O molecules, a series of cyclic heteromolecular structures, up to and including HCl(H(2)O)(3), are initially obtained before a precursor state for dissociation, HCl(H(2)O)(3)···H(2)O, is observed upon addition of a fourth water molecule. The latter partially aggregated structure can be viewed as an "activated species", which readily leads to dissociation of HCl and to the formation of a solvent-shared ion pair, H(3)O(+)(H(2)O)(3)Cl(-). Overall, the process is mostly downhill in potential energy, and, in addition, small remaining barriers are overcome by using kinetic energy released as a result of forming hydrogen bonds due to aggregation. The associated barrier is not ruled by thermal equilibrium but is generated by athermal non-equilibrium dynamics. These "aggregation-induced chemical reactions" are expected to be of broad relevance to chemistry at ultralow temperature much beyond HENDI spectroscopy.

Communication: A new ab initio potential energy surface for HCl–H2O, diffusion Monte Carlo calculations of D and a delocalized zero-point wavefunction

“…The D 0 of the fully deuterated isotopologue is calculated to be 1476 ± 3 cm −1 , which we hope will stand as a benchmark for future experimental work. [1][2][3][4][5][6] and theoretical [7][8][9][10][11] studies. The high interest in these complexes is due to their impact on atmospheric chemistry, specifically their role in ozone depletion.…”

“…[16][17][18][19][20][21][22] However, recent high level quantum mechanical studies predict the coexistence of non-ionized clusters at the four-monomer limit. 8,10 The complexity of the problem is even more apparent from the lack of experimental consensus on the identification of the ionized cluster. 4,23 A building block to understanding the clusters at the four-water limit and beyond is a detailed knowledge of even smaller HCl-H 2 O clusters, the first of which is the HCl-H 2 O dimer.…”

Extending molecular simulation time scales: Parallel in time integrations for high-level quantum chemistry and complex force representations

“…12,13,60 As shown in Fig. 3 very little energy drift was seen in both simulations up to 1.2 and 2.5 ps; maintaining energy conservation during proton shuttling. Prior AIMD simulations (at the less accurate DFT+GGA level) of this "smallest acid droplet" have shown that adding a fourth water to an HCl(H 2 O) 3 cluster results in a spontaneous dissociation of the proton into the water.…”

“…Prior AIMD simulations (at the less accurate DFT+GGA level) of this "smallest acid droplet" have shown that adding a fourth water to an HCl(H 2 O) 3 cluster results in a spontaneous dissociation of the proton into the water. 12,13 In our calculations it was found that the initial potential energy of the cluster was very sensitive to the basis set size. The cluster dissociates almost immediately when using the smaller basis set.…”

“…V to the simulation of the dynamics of the environmentally important HCl + 4H 2 O clusters 12,13,60 at the MP2/ 3-21G, MP2/6-31G*, and MP2/6-311+G* levels. [61][62][63][64][65] The preconditioner in the PQN algorithm was done at the HF/ 3-21G level.…”