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

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 (NH4X)n, 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 (NH4F)n particles (n = 2–5) exhibit
three distinct types of hydrogen bonds as a function of size (traditional,
ion-pair and proton-shared). However, (NH4Br)n and (NH4Cl)n 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.