Energetics, Thermodynamics, and Hydrogen Bonding Diversity in Ammonium Halide Clusters

Abstract

Contributions from different intermolecular and interionic forces, as well as variations in bond energies, produce size-dependent variations in the structures of acid-base molecular clusters. In this work the structures and interaction energetics of cluster particles with the nominal formulas (NHX), X = (F, Cl, Br) are predicted using either "mag-walking" sawtooth simulated annealing Monte Carlo calculations or model building, followed by M06-2X or RI-MP2 geometry optimization and single-point energy calculations. Whereas the n = 1 clusters all exhibit a single hydrogen bond, small (NHF) particles (n = 2-5) exhibit three distinct types of hydrogen bonds as a function of size (traditional, ion-pair and proton-shared). However, (NHBr) and (NHCl) particles (n = 2-13) all solely exhibit ion-pair hydrogen bonding, with even values of n exhibiting pronounced relative stability. The computed differential interaction energy of the bromide and chloride systems is generally near the bulk limit of the difference in their accepted lattice energies, despite the fact that their structures do not resemble the bulk crystal structures. Nanoparticle growth reactions are predicted to be thermodynamically spontaneous under standard conditions, with significant size and system dependencies. This work is designed to further our understanding of the nature of hydrogen bonding and other intermolecular forces, particularly within ionic nanocrystallites, as well as the thermodynamics of cluster formation.

Nonuniform Proton Transfer and Strong Hydrogen Bonding within Cation, Anion, and Neutral Clusters of Ammonia and Hydrogen Fluoride

“…Earlier work on these systems revealed that M06-2X optimized structures agreed well with experiments as well as MP2, CCSD­(T), and QCISD calculations for ammonium chloride and bromide clusters . In a subsequent assessment of the reliability and accuracy of 200 density functionals, Mardirossian and Head-Gordon found the most promising overall functional that they tested to be ωB97M-V, a range-separated hybrid meta-GGA with nonlocal correlation, designed using a combinatorial approach.…”

“…The correct choice of a functional is, as always, important, and several functionals are considered in the present study. For example, in previous work, we found that the M06-2X method performed well for geometry optimization and reaction energetics studies of fairly large ammonium chloride and ammonium bromide clusters. , In the present work, we primarily use the ωB97M-V method developed by Mardirossian and Head-Gordon but also show evidence that both this functional and the MN15 functional developed by Truhlar and co-workers are good choices for DFT modeling of ammonium fluoride clusters.…”

“…The calculations presented here were completed using Gaussian 16, Spartan 18/Q-Chem 5.1, , and Psi4. − For the purpose of geometry optimization, initial structures for the clusters were obtained using structures obtained by simulated annealing Monte Carlo methods and model building which were obtained in previous work on ammonium fluoride, chloride, and bromide clusters as starting points . No evidence was found for the existence of low-lying cluster isomers for the systems considered in the present work.…”

“…These clusters were found to exhibit three distinct types of hydrogen bonds (traditional, ion-pair, and proton-shared) as a function of cluster size. In particular, the n = 3 system exhibited an unusual proton-shared hydrogen bond, the n = 4 system consisted completely of ion pairs connected by ion-pair hydrogen bonds, and the n = 5 system showed a combination of traditional and ion-pair hydrogen bonds . The relative complexity of (NH 4 F) n clusters can be considered to be connected to the weak acidity of HF combined with its tendency to form a rather strong hydrogen bond with NH 3 .…”

“…In the case of the chloride and bromide systems, the (NH 4 X) n particles ( n = 2–13) all exhibited ion-pair hydrogen bonding, and clusters as large as n = 13 did not converge to the bulk (CsCl) lattice structure, instead favoring even-numbered cubic structures reminiscent of solid NaCl. Free energies of nucleation under anhydrous conditions were predicted to be highly exergonic, whether by a plume condensation reaction (at high concentrations) or by addition of ammonia and hydrogen halide molecules (at low concentrations) support the feasibility of the latter growth mechanism at low concentrations, within which considerable size dependence of the reaction thermodynamics was noted.…”