On the Role of Amides and Imides for Understanding GaN Syntheses from Ammonia Solution: Molecular Mechanics Models of Ammonia, Amide and Imide Interactions with Gallium Nitride.

A key requisite to characterizing GaN precipitation from ammonia solution from molecular simulations is the availability of reliable molecular mechanics models for the interactions of gallium ions with NH3 , NH2- , and NH2- species, respectively. Here, we present a tailor-made force field which is fully compatible to an earlier developed GaN model, thus bridging the analyses of Ga3+ ions in ammonia solution with the aggregation of [Gax (NH)y (NH2 )z ]+3x-2y-z precursors and the modelling of GaN crystals. For this, quantum mechanical characterization of a series of Ga-coordination clusters is used for parameterization and benchmarking the generalized amber force field (GAFF2) and tailor-made refinements needed to achieve good agreement of both structural features and formation energy, respectively. The perspectives of our models for larger scale molecular dynamics simulations are demonstrated by the analyses of amide and imide defects arrangement during the growth of GaN crystal faces.

Molecular dynamics simulation study of NH4+ and NH2- in liquid ammonia: interaction potentials, structural and dynamical properties.

We provide tailor-made GAFF2-type interaction potentials for modeling ammonium and amide ions in ammonia. Based on harmonic approximation of intra-molecular bond stretching and bending, our force fields nicely reproduce the vibrational modes of NH4+ and NH2-, respectively. Moreover, quantum calculations of pair-wise NH4+/NH2--NH3 interactions were used for inter-molecular force field parameterization, while (NH3)n, [(NH4)(NH3)n]+, and [(NH2)(NH3)n]- complexes with n > 2, respectively, were reserved for benchmarking in terms of both structure and formation energy. Despite the limited reliability of molecular mechanics models for describing dimer complexes (n = 1), we find that GAFF2 reasonably reproduces [(NH4)(NH3)n]+ species for n = 2-4. For the assessment of [(NH2)(NH3)n]- complexes with n = 2-4, we however suggest the introduction of specific van der Waals parameters for amide-ammonia interactions. The application of the (extended) GAFF2 models is demonstrated for the study of ammonium and amide solvation in liquid ammonia at 240 K and 1 atm, respectively. On this basis, we suggest the applicability of our model for both gas phase and liquid states of ammonia.

An embedded atom model for Ga-Pd systems: From intermetallic crystals to liquid alloys.

We present an embedded atom model (EAM) potential for modeling Ga-Pd interactions within intermetallic solids and liquid alloys. The molecular mechanics potential was parameterized on the basis of the structure and mechanical properties of GaPd2, whereas a series of other GaxPd1-x phases and liquid alloy systems allowed rigorous benchmarking. For the intermetallic solids, structures and elastic moduli were found in very reasonable agreement with experimental structures and results from DFT calculations. The liquid models were characterized from molecular dynamics simulations that also showed nice agreement with experimental and ab initio reference data. Moreover, the perspectives of the EAM model are illustrated by the elucidation of an alloy nanodroplet model whose characterization includes the kinetics of Pd dopant diffusion from the Ga droplet surface to the bulk liquid and vice versa.

Approaching Dissolved Species in Ammonoacidic GaN Crystal Growth: A Combined Solution NMR and Computational Study.

Solutions of gallium trihalides GaX3 (X=F, Cl, Br, I) and their ammoniates in liquid ammonia were studied at ambient temperature under autogenous pressure by multinuclear (71 Ga, 35 Cl, 81 Br) NMR spectroscopy. To unravel the role of pH, the analyses were done both in absence and in presence of ammonium halides, which are employed as mineralizers during ammonoacidic gallium nitride crystal growth. While gallium trifluoride and its ammoniate were found to be too sparingly soluble to give rise to a NMR signal, the spectra of solutions of the heavier halides reveal the presence of a single gallium-containing species in all cases. DFT calculations and molecular dynamics simulations suggest the identification of this species as consisting of a [Ga(NH3 )6 ]3+ cation and up to six surrounding halide anions, resulting in an overall trend towards negative complex charge. Quantitative 71 Ga NMR studies on saturated solutions of GaCl3 containing various amounts of additional NH4 Cl revealed a near linear increase of GaCl3 solubility with mineralizer concentration of about 0.023 mol GaCl3 per mol NH4 Cl at room temperature. These findings reflect the importance of Coulombic shielding for the inhibition of oligomerization and precipitation processes and help to rationalize both the low solubility of gallium halides in neutral ammonia solution and, in turn, the proliferating effect of the mineralizer during ammonoacidic gallium nitride formation.

Interaction potentials for modelling GaN precipitation and solid state polymorphism.

We outline a molecular mechanics model for the interaction of gallium and nitride ions ranging from small complexes to nanoparticles and bulk crystals. While the current GaN force fields allow the modelling of either bulk crystals or single ions dispersed in solution, our model covers both and hence paves the way to describing aggregate formation and crystal growth processes from molecular simulations. The key to this is the use of formal +3 and -3 charges on the gallium and nitride ions, whilst accounting for the charge transfer in GaN crystals by means of additional potential energy terms. The latter are fitted against experimental data of GaN in the wurtzite structure and benchmarked for the zinc-blende and rock-salt polymorphs. Comparison to quantum chemical references and experiment shows reasonable agreement of structures and formation energy of [GaN] n aggregates, elastic properties of the bulk crystal, the transition pressure of the wurtzite to rock-salt transformation and intrinsic point defects. Furthermore, we demonstrate force field transferability towards the modelling of GaN nanoparticles from simulated annealing runs.

Mechanistic insights into HCO2H dehydrogenation and CO2 hydrogenation catalyzed by Ir(Cp*) containing tetrahydroxy bipyrimidine ligand: the role of sodium and proton shuttle.

The mechanism of HCO2H dehydrogenation catalyzed by [IrCp*(H2O)(bpymO4H4)]2+ (bpymO4H4 = 2,2',6,6'-tetrahydroxy-4,4'-bipyrimidine) was investigated using density functional theory. The relative free energy profiles at various protonation states corrected to pH 3.5 and pH 7.6 suggested that Na+ together with the ortho-oxyanion of bipyrimidine facilitates the Ir-HCO2 formation, subsequent hydride transfer, and H2 formation. HCO2H was found to be a more effective proton shuttle than H2O for H2 formation. Under experimental conditions, the highest catalytic reactivity was found at pH 3.5-4.0, where both HCO2Na and HCO2H were present. At lower pH and low formate concentration, HCO2H dehydrogenation tends to proceed via a Na+ independent pathway, involving a higher energy barrier. At higher pH, although Na+ can mediate hydride transfer and H2 formation, the low amount of HCO2H results in H2O as the proton shuttle, which involves a higher energy barrier than that for HCO2H proton shuttle. In other words, the catalytic activity of HCO2H dehydrogenation by the proton-responsive Ir complexes at different pH values is influenced by the protonation state, involvement of Na+, and the availability of HCO2H as a proton shuttle. For the hydrogenation of CO2 at pH 8.3, the rate determining step is the heterolytic cleavage of H2 mediated by Na+via a HCO3- proton shuttle. Our results demonstrate the importance of alkali metal ions in the design of catalysts for efficient, reversible, CO2 conversion.