Toward a First-Principles Framework for Predicting Collective Properties of Electrolytes.

Given the universal importance of electrolyte solutions, it is natural to expect that we have a nearly complete understanding of the fundamental properties of these solutions (e.g., the chemical potential) and that we can therefore explain, predict, and control the phenomena occurring in them. In fact, reality falls short of these expectations. But, recent advances in the simulation and modeling of electrolyte solutions indicate that it should soon be possible to make progress toward these goals. In this Account, we will discuss the use of first-principles interaction potentials based in quantum mechanics (QM) to enhance our understanding of electrolyte solutions. Specifically, we will focus on the use of quantum density functional theory (DFT) combined with molecular dynamics simulation (DFT-MD) as the foundation for our approach. The overarching concept is to understand and accurately reproduce the balance between local or short-ranged (SR) structural details and long-range (LR) correlations, allowing the prediction of the thermodynamics of both single ions in solution as well as the collective interactions characterized by activity/osmotic coefficients. In doing so, relevant collective motions and driving forces characterized by chemical potentials can be determined.In this Account, we will make the case that understanding electrolyte solutions requires a faithful QM representation of the SR nature of the ion-ion, ion-water, and water-water interactions. However, the number of molecules that is required for collective behavior makes the direct application of high-level QM methods that contain the best SR physics untenable, making methods that balance accuracy and efficiency a practical goal. Alternatives such as continuum solvent models (CSMs) and empirically based classical molecular dynamics have been extensively employed to resolve this problem but without yet overcoming the fundamental issue of SR accuracy. We will demonstrate that accurately describing the SR interaction is imperative for predicting both intrinsic properties, namely, at infinite dilution, and collective properties of electrolyte solutions.DFT has played an important role in our understanding of condensed phase systems, e.g., bulk liquid water, the air-water interface, ions in bulk, and at the air-water interface. This approach holds huge promise to provide benchmark calculations of electrolyte solution properties that will allow for the development and improvement of more efficient methods, as well as an enhanced understanding of fundamental phenomena. However, the standard protocol using the generalized gradient approximation with van der Waals (vdW) correction requires improvement in order to achieve a high level of quantitative accuracy. Simply simulating with higher level DFT functionals may not be the best route considering the significant computational cost. Alternative methods of incorporating information from higher levels of QM should be explored; e.g., using force matching techniques on small clusters, where high level benchmark calculations are possible, to develop ideal correction terms to the DFT functional is a promising possibility. We argue that DFT with statistical mechanics is becoming an increasingly useful framework enabling the prediction of collective electrolyte properties.

Molecular-level origin of the carboxylate head group response to divalent metal ion complexation at the air-water interface.

We exploit gas-phase cluster ion techniques to provide insight into the local interactions underlying divalent metal ion-driven changes in the spectra of carboxylic acids at the air-water interface. This information clarifies the experimental findings that the CO stretching bands of long-chain acids appear at very similar energies when the head group is deprotonated by high subphase pH or exposed to relatively high concentrations of Ca2+ metal ions. To this end, we report the evolution of the vibrational spectra of size-selected [Ca2+·RCO2-]+·(H2O) n=0to12 and RCO2-·(H2O) n=0to14 cluster ions toward the features observed at the air-water interface. Surprisingly, not only does stepwise hydration of the RCO2- anion and the [Ca2+·RCO2-]+ contact ion pair yield solvatochromic responses in opposite directions, but in both cases, the responses of the 2 (symmetric and asymmetric stretching) CO bands to hydration are opposite to each other. The result is that both CO bands evolve toward their interfacial asymptotes from opposite directions. Simulations of the [Ca2+·RCO2-]+·(H2O) n clusters indicate that the metal ion remains directly bound to the head group in a contact ion pair motif as the asymmetric CO stretch converges at the interfacial value by n = 12. This establishes that direct metal complexation or deprotonation can account for the interfacial behavior. We discuss these effects in the context of a model that invokes the water network-dependent local electric field along the C-C bond that connects the head group to the hydrocarbon tail as the key microscopic parameter that is correlated with the observed trends.

Electric fields and potentials in condensed phases.

The electric fields and potentials inside and at the interface of matter are relevant to many branches of physics, chemistry, and biology. Accurate quantification of these fields and/or potentials is essential to control and exploit chemical and physical transformations. Before we understand the response of matter to external fields, it is first important to understand the intrinsic interior and interfacial fields and potentials, both classically and quantum mechanically, as well as how they are probed experimentally. Here we compare and contrast, beginning with the hydrogen atom in vacuum and ending with concentrated aqueous NaCl electrolyte, both classical and quantum mechanical electric potentials and fields. We make contact with experimental vibrational Stark, electrochemical, X-ray, and electron spectroscopic probes of these potentials and fields, outline relevant conceptual difficulties, and underscore the advantage of electron holography as a basis to better understand electrostatics in matter.

Mean Inner Potential of Liquid Water.

Improving our experimental and theoretical knowledge of electric potentials at liquid-solid boundaries is essential to achieve a deeper understanding of the driving forces behind interfacial processes. Electron holography has proved successful in probing solid-solid interfaces but requires knowledge of the materials' mean inner potential (MIP, V_{0}), which is a fundamental bulk material property. Combining off-axis electron holography with liquid phase transmission electron microscopy (LPTEM), we provide the first quantitative MIP determination of liquid water V_{0}=+4.48±0.19 V. This value is larger than most theoretical predictions, and to explain the disagreement we assess the dominant factors needed in quantum simulations of liquid water. A precise MIP lays the foundations for nanoscale holographic potential measurements in liquids, and provides a benchmark to improve quantum mechanical descriptions of aqueous systems and their interfaces in, e.g., electrochemistry, solvation processes, and spectroscopy.

Isotopomer-selective spectra of a single intact H2O molecule in the Cs(+)(D2O)5H2O isotopologue: Going beyond pattern recognition to harvest the structural information encoded in vibrational spectra.

We report the vibrational signatures of a single H2O molecule occupying distinct sites of the hydration network in the Cs(+)(H2O)6 cluster. This is accomplished using isotopomer-selective IR-IR hole-burning on the Cs(+)(D2O)5(H2O) clusters formed by gas-phase exchange of a single, intact H2O molecule for D2O in the Cs(+)(D2O)6 ion. The OH stretching pattern of the Cs(+)(H2O)6 isotopologue is accurately recovered by superposition of the isotopomer spectra, thus establishing that the H2O incorporation is random and that the OH stretching manifold is largely due to contributions from decoupled water molecules. This behavior enables a powerful new way to extract structural information from vibrational spectra of size-selected clusters by explicitly identifying the local environments responsible for specific infrared features. The Cs(+)(H2O)6 structure was unambiguously assigned to the 4.1.1 isomer (a homodromic water tetramer with two additional flanking water molecules) from the fact that its computed IR spectrum matches the observed overall pattern and recovers the embedded correlations in the two OH stretching bands of the water molecule in the Cs(+)(D2O)5(H2O) isotopomers. The 4.1.1 isomer is the lowest in energy among other candidate networks at advanced (e.g., CCSD(T)) levels of theoretical treatment after corrections for (anharmonic) zero-point energy. With the structure in hand, we then explore the mechanical origin of the various band locations using a local electric field formalism. This approach promises to provide a transferrable scheme for the prediction of the OH stretching fundamentals displayed by water networks in close proximity to solute ions.

Molecular characterization of brown carbon (BrC) chromophores in secondary organic aerosol generated from photo-oxidation of toluene.

Atmospheric brown carbon (BrC) is a significant contributor to light absorption and climate forcing. However, little is known about a fundamental relationship between the chemical composition of BrC and its optical properties. In this work, light-absorbing secondary organic aerosol (SOA) was generated in the PNNL chamber from toluene photo-oxidation in the presence of NOx (Tol-SOA). Molecular structures of BrC components were examined using nanospray desorption electrospray ionization (nano-DESI) and liquid chromatography (LC) combined with UV/Vis spectroscopy and electrospray ionization (ESI) high-resolution mass spectrometry (HRMS). The chemical composition of BrC chromophores and the light absorption properties of toluene SOA (Tol-SOA) depend strongly on the initial NOx concentration. Specifically, Tol-SOA generated under high-NOx conditions (defined here as initial NOx/toluene of 5/1) appears yellow and mass absorption coefficient of the bulk sample (MACbulk@365 nm = 0.78 m(2) g(-1)) is nearly 80 fold higher than that measured for the Tol-SOA sample generated under low-NOx conditions (NOx/toluene < 1/300). Fifteen compounds, most of which are nitrophenols, are identified as major BrC chromophores responsible for the enhanced light absorption of Tol-SOA material produced in the presence of NOx. The integrated absorbance of these fifteen chromophores accounts for 40-60% of the total light absorbance by Tol-SOA at wavelengths between 300 nm and 500 nm. The combination of tandem LC-UV/Vis-ESI/HRMS measurements provides an analytical platform for predictive understanding of light absorption properties by BrC and their relationship to the structure of individual chromophores. General trends in the UV/Vis absorption by plausible isomers of the BrC chromophores were evaluated using theoretical chemistry calculations. The molecular-level understanding of BrC chemistry is helpful for better understanding the evolution and behavior of light absorbing aerosols in the atmosphere.

Snapshots of Proton Accommodation at a Microscopic Water Surface: Understanding the Vibrational Spectral Signatures of the Charge Defect in Cryogenically Cooled H(+)(H2O)(n=2-28) Clusters.

We review the role that gas-phase, size-selected protonated water clusters, H(+)(H2O)n, have played in unraveling the microscopic mechanics responsible for the spectroscopic behavior of the excess proton in bulk water. Because the larger (n ≥ 10) assemblies are formed with three-dimensional cage morphologies that more closely mimic the bulk environment, we report the spectra of cryogenically cooled (10 K) clusters over the size range 2 ≤ n ≤ 28, over which the structures evolve from two-dimensional arrangements to cages at around n = 10. The clusters that feature a complete second solvation shell around a surface-embedded hydronium ion yield spectral signatures of the proton defect similar to those observed in dilute acids. The origins of the large observed shifts in the proton vibrational signature upon cluster growth were explored with two types of theoretical analyses. First, we calculate the cubic and semidiagonal quartic force constants and use these in vibrational perturbation theory calculations to establish the couplings responsible for the large anharmonic red shifts. We then investigate how the extended electronic wave functions that are responsible for the shapes of the potential surfaces depend on the nature of the H-bonded networks surrounding the charge defect. These considerations indicate that, in addition to the sizable anharmonic couplings, the position of the OH stretch most associated with the excess proton can be traced to large increases in the electric fields exerted on the embedded hydronium ion upon formation of the first and second solvation shells. The correlation between the underlying local structure and the observed spectral features is quantified using a model based on Badger's rule as well as via the examination of the electric fields obtained from electronic structure calculations.

Molecular simulations of heterogeneous ice nucleation. I. Controlling ice nucleation through surface hydrophilicity.

Ice formation is one of the most common and important processes on earth and almost always occurs at the surface of a material. A basic understanding of how the physicochemical properties of a material's surface affect its ability to form ice has remained elusive. Here, we use molecular dynamics simulations to directly probe heterogeneous ice nucleation at a hexagonal surface of a nanoparticle of varying hydrophilicity. Surprisingly, we find that structurally identical surfaces can both inhibit and promote ice formation and analogous to a chemical catalyst, it is found that an optimal interaction between the surface and the water exists for promoting ice nucleation. We use our microscopic understanding of the mechanism to design a modified surface in silico with enhanced ice nucleating ability.

Molecular simulations of heterogeneous ice nucleation. II. Peeling back the layers.

Coarse grained molecular dynamics simulations are presented in which the sensitivity of the ice nucleation rate to the hydrophilicity of a graphene nanoflake is investigated. We find that an optimal interaction strength for promoting ice nucleation exists, which coincides with that found previously for a face centered cubic (111) surface. We further investigate the role that the layering of interfacial water plays in heterogeneous ice nucleation and demonstrate that the extent of layering is not a good indicator of ice nucleating ability for all surfaces. Our results suggest that to be an efficient ice nucleating agent, a surface should not bind water too strongly if it is able to accommodate high coverages of water.

A matter of quantum voltages.

Voltages inside matter are relevant to crystallization, materials science, biology, catalysis, and aqueous chemistry. The variation of voltages in matter can be measured by experiment, however, modern supercomputers allow the calculation of accurate quantum voltages with spatial resolutions of bulk systems well beyond what can currently be measured provided a sufficient level of theory is employed. Of particular interest is the Mean Inner Potential (V(o))--the spatial average of these quantum voltages referenced to the vacuum. Here we establish a protocol to reliably evaluate V(o) from quantum calculations. Voltages are very sensitive to the distribution of electrons and provide metrics to understand interactions in condensed phases. In the present study, we find excellent agreement with measurements of V(o) for vitrified water and salt crystals and demonstrate the impact of covalent and ionic bonding as well as intermolecular/atomic interactions. Certain aspects in this regard are highlighted making use of simple model systems/approximations. Furthermore, we predict V(o) as well as the fluctuations of these voltages in aqueous NaCl electrolytes and characterize the changes in their behavior as the resolution increases below the size of atoms.

Investigating the significance of zero-point motion in small molecular clusters of sulphuric acid and water.

The nucleation of particles from trace gases in the atmosphere is an important source of cloud condensation nuclei, and these are vital for the formation of clouds in view of the high supersaturations required for homogeneous water droplet nucleation. The methods of quantum chemistry have increasingly been employed to model nucleation due to their high accuracy and efficiency in calculating configurational energies; and nucleation rates can be obtained from the associated free energies of particle formation. However, even in such advanced approaches, it is typically assumed that the nuclei have a classical nature, which is questionable for some systems. The importance of zero-point motion (also known as quantum nuclear dynamics) in modelling small clusters of sulphuric acid and water is tested here using the path integral molecular dynamics method at the density functional level of theory. The general effect of zero-point motion is to distort the mean structure slightly, and to promote the extent of proton transfer with respect to classical behaviour. In a particular configuration of one sulphuric acid molecule with three waters, the range of positions explored by a proton between a sulphuric acid and a water molecule at 300 K (a broad range in contrast to the confinement suggested by geometry optimisation at 0 K) is clearly affected by the inclusion of zero point motion, and similar effects are observed for other configurations.

Communication: solute anisotropy effects in hydrated anion and neutral clusters.

Specific ion effects in solvation processes are often rationalized in terms of spherically symmetric models involving an ion's size, charge, and polarizability. The effects of permanent charge anisotropy, related to the polyatomic nature of complex solutes, are expected to play a role in solvation but the extent of their importance remains unexplored. In this work, we provide compelling experimental and theoretical evidence that the anisotropic nature of complex polyoxyanion solutes can have a critical influence on the solvation process. Combined photoelectron spectroscopy and theoretical modeling results show that the electron binding energy of IO(3) (-)(H(2)O)(n) (n = 0-12) clusters is characterized by an anomalous drop at n = 10. Such behavior is unprecedented for rigid solute molecules and is related to the anisotropy of the neutral iodate radical that displays a strong selectivity to solvent configurations generated by the charged anion complex. These results highlight the significance of solute anisotropy and its potential impact on ion specificity and selectivity in aqueous environments.

Non-hexagonal ice at hexagonal surfaces: the role of lattice mismatch.

It has long been known that ice nucleation usually proceeds heterogeneously on the surface of a foreign body. However, little is known at the microscopic level about which properties of a material determine its effectiveness at nucleating ice. This work focuses on the long standing, conceptually simple, view on the role of a good crystallographic match between bulk ice and the underlying substrate. We use grand canonical Monte Carlo to generate the first overlayer of water at the surface and find that the traditional view of heterogeneous nucleation does not adequately account for the array of structures that water may form at the surface. We find that, in order to describe the structures formed, a good match between the substrate and the nearest neighbour oxygen-oxygen distance is a better descriptor than a good match to the bulk ice lattice constant.

Study of ion specific interactions of alkali cations with dicarboxylate dianions.

Alkali metal cations often show pronounced ion-specific interactions and selectivity with macromolecules in biological processes, colloids, and interfacial sciences, but a fundamental understanding about the underlying microscopic mechanism is still very limited. Here we report a direct probe of interactions between alkali metal cations (M(+)) and dicarboxylate dianions, (-)O(2)C(CH(2))(n)CO(2)(-) (D(n)(2-)) in the gas phase by combined photoelectron spectroscopy (PES) and ab initio electronic structure calculations on nine M(+)-D(n)(2-) complexes (M = Li, Na, K; n = 2, 4, 6). PES spectra show that the electron binding energy (EBE) decreases from Li(+) to Na(+) to K(+) for complexes of M(+)-D(2)(2-), whereas the order is Li(+) < Na(+) ≈ K(+) when M(+) interacts with a more flexible D(6)(2-) dianion. Theoretical modeling suggests that M(+) prefers to interact with both ends of the carboxylate -COO(-) groups by bending the flexible aliphatic backbone, and the local binding environments are found to depend upon backbone length n, carboxylate orientation, and the specific cation M(+). The observed variance of EBEs reflects how well each specific dicarboxylate dianion accommodates each M(+). This work demonstrates the delicate interplay among several factors (electrostatic interaction, size matching, and strain energy) that play critical roles in determining the structures and energetics of gaseous clusters as well as ion specificity and selectivity in solutions and biological systems.

Analysis of the activation and heterolytic dissociation of H2 by frustrated Lewis pairs: NH3/BX3 (X = H, F, and Cl).

We performed a computational study of H(2) activation and heterolytic dissociation promoted by prototype Lewis acid/base pairs NH(3)/BX(3) (X = H, F, and Cl) to understand the mechanism in frustrated Lewis pairs (FLPs). Although the NH(3)/BX(3) pairs form strong dative bonds, electronic structure theories make it possible to explore the potential energy surface away from the dative complex, in regions relevant to H(2) activation in FLPs. A weakly bound precursor complex, H(3)N·H(2)·BX(3), was found in which the H(2) molecule interacts side-on with B and end-on with N. The BX(3) group is pyramidal in the case of X = H, similar to the geometry of BH(5), but planar in the complexes with X = F and Cl. The latter complexes convert to ion pairs, [NH(4)(+)][BHX(3)(-)] with enthalpy changes of 7.3 and -9.4 kcal/mol, respectively. The minimum energy paths between the FLP and the product ion pair of the chloro and fluoro complexes were calculated and analyzed in great detail. At the transition state (TS), the H(2) bond is weakened and the BX(3) moiety has undergone significant pyramidal distortion. As such, the FLP is prepared to accept the incipient proton and hydride ion on the product-side. The interaction energy of the H(2) with the acid/base pair and the different contributions for the precursor and TS complex from an energy decomposition analysis expose the dominant factors affecting the reactivity. We find that structural reorganization of the precursor complex plays a significant role in the activation and that charge-transfer interactions are the dominant stabilizing force in the activated complex. The electric field clearly has a role in polarizing H(2), but its contribution to the overall interaction energy is small compared to that from the overlap of the p(N), σ(H-H), σ*(H-H), and p(B) orbitals at the TS. Our detailed analysis of the interaction of H(2) with the FLP provides insight into the important components that should be taken into account when designing related systems to activate H(2).

Theoretical investigations on the formation and dehydrogenation reaction pathways of H(NH2BH2)(n)H (n = 1-4) oligomers: importance of dihydrogen interactions.

The H(NH(2)BH(2))(n)H oligomers are possible products from dehydrogenation of ammonia borane (NH(3)BH(3)) and ammonium borohydride (NH(4)BH(4)), which belong to a class of boron-nitrogen-hydrogen (BNH(x)) compounds that are promising materials for chemical hydrogen storage. Understanding the kinetics and reaction pathways of formation of these oligomers and their further dehydrogenation is essential for developing BNH(x)-based hydrogen storage materials. We have performed computational modeling using density functional theory (DFT), ab initio wave function theory, and Car-Parrinello molecular dynamics (CPMD) simulations on the energetics and formation pathways for the H(NH(2)BH(2))(n)H (n = 1-4) oligomers, polyaminoborane (PAB), from NH(3)BH(3) monomers and the subsequent dehydrogenation steps to form polyiminoborane (PIB). Through computational transition state searches and evaluation of the intrinsic reaction coordinates, we have investigated the B-N bond cleavage, the reactions of NH(3)BH(3) molecule with intermediates, dihydrogen release through intra- and intermolecular hydrogen transfer, dehydrocoupling/cyclization of the oligomers, and the dimerization of NH(3)BH(3) molecules. We find that the formation of H(NH(2)BH(2))(n+1)H oligomers occurs first through reactions of the H(NH(2)BH(2))(n)H oligomers with BH(3) followed by reactions with NH(3) and the release of H(2), where the BH(3) and NH(3) intermediates are formed through dissociation of NH(3)BH(3). We also find that the dimerization of the NH(3)BH(3) molecules to form cyclic c-(NH(2)BH(2))(2) is slightly exothermic, with an unexpected transition state that leads to the simultaneous release of two H(2) molecules. The dehydrogenations of the oligomers are also exothermic, typically by less than 10 kcal/(mol of H(2)), with the largest exothermicity for n = 3. The transition state search shows that the one-step direct dehydrocoupling cyclization of the oligomers is not a favored pathway because of high activation barriers. The dihydrogen bonding, in which protic (H(N)) hydrogens interact with hydridic (H(B)) hydrogens, plays a vital role in stabilizing different structures of the reactants, transition states, and products. The dihydrogen interaction (DHI) within the R-BH(2)(eta(2)-H(2)) moiety accounts for both the formation mechanisms of the oligomers and for the dehydrogenation of ammonia borane.

Nanometer-Scale Correlations in Aqueous Salt Solutions.

The intermediate-range (1-4 nm) correlation of cations, anions, and water in aqueous alkaline earth salt solutions is measured using synchrotron X-ray diffraction. We differentiate from the entire solution structure factor, SX(Q), a separate region at low Q (<1.5 Å-1) containing local diffraction maxima (prepeaks) that indicate nanometer-scale oscillatory behavior. These features are quantitatively reproduced with classical molecular dynamics simulations. At high concentrations, the prepeaks emerge from correlations arising from the existence of small quasi close-packed lattice-like structures comprised of cation hydration spheres. We also analyze the concentration dependence of the prepeak and discuss the overall results in light of the rich literature dealing with intermediate-range correlations underlying universal phenomena in concentrated electrolytes.

Implementation of dynamical nucleation theory with quantum potentials.

A method is implemented within the context of dynamical nucleation theory in order to efficiently determine the ab initio water dimer evaporation rate constant. The drive for increased efficiency in a Monte Carlo methodology is established by the need to use relatively expensive quantum mechanical interaction potentials. A discussion is presented illustrating the theory, algorithm, and implementation of this method to the water dimer. Hartree-Fock and second order Møller-Plesset perturbation theories along with the Dang-Chang polarizable classical potential are utilized to determine the ab initio water dimer evaporation rate constant.

Hydrated structure of Ag(I) ion from symmetry-dependent, K- and L-edge XAFS multiple scattering and molecular dynamics simulations.

Details of the first-shell water structure about Ag(+) are reported from a corefinement of the K- and L(2)-edge multiple scattering signal in the X-ray absorption fine structure (XAFS) spectra. Detailed fits of the Ag K-edge data that include the contributions from multiple scattering processes in the hydrated ion structure cannot distinguish between models containing tetrahedral symmetry versus those containing collinear O-Ag-O bonds. However, we show that the multiple scattering oscillations at the L(2)-edges have distinctly different phase and amplitude functions than at the K-edge. These phase and amplitude functions depend not only on the symmetry of the multiple scattering paths but also on the nature of the final state electronic wave function probed by the dipole-allowed transition. Hence the multiple scattering portions of K- and L(2)-edge spectra provide independent measurements of the local symmetry--not a redundant measurement as is commonly believed. On the basis of the enhanced information content obtained by the simultaneous assessment of both the K- and L(2)-edges, we report that the hydrated Ag(+) structure contains five or six water molecules in the first shell with a significant number of nearly collinear and 90 degrees O-Ag-O bond angles. Finally, the K- and L(2)-edge spectra are used to benchmark the hydration structure that is generated from both DFT-based and classical molecular dynamics simulations. Simulated first-shell structures are compared to the experimental structures.

Neutron powder diffraction and molecular simulation study of the structural evolution of ammonia borane from 15 to 340 K.

The structural behavior of (11)B-, (2)H-enriched ammonia borane, ND(3)(11)BD(3), over the temperature range from 15 to 340 K was investigated using a combination of neutron powder diffraction and ab initio molecular dynamics simulations. In the low temperature orthorhombic phase, the progressive displacement of the borane group under the amine group was observed leading to the alignment of the B-N bond near parallel to the c-axis. The orthorhombic to tetragonal structural phase transition at 225 K is marked by dramatic change in the dynamics of both the amine and borane group. The resulting hydrogen disorder is problematic to extract from the metrics provided by Rietveld refinement but is readily apparent in molecular dynamics simulation and in difference Fourier transform maps. At the phase transition, Rietveld refinement does indicate a disruption of one of two dihydrogen bonds that link adjacent ammonia borane molecules. Metrics determined by Rietveld refinement are in excellent agreement with those determined from molecular simulation. This study highlights the valuable insights added by coupled experimental and computational studies.