David J. Henry

G3-RAD and G3X-RAD: Modified Gaussian-3 (G3) and Gaussian-3X (G3X) procedures for radical thermochemistry

The authors gratefully acknowledge generous allocations of computing time on the Compaq Alphaserver of the National Facility of the Australian Partnership for Advanced Computing, Australian National University Supercomputer Facility, and the support of the Australian Research Council.

Performance of Numerical Basis Set DFT for Aluminum Clusters

We have investigated and compared the ability of numerical and Gaussian-type basis sets combined with density functional theory (DFT) to accurately describe the geometries, binding energies, and electronic properties of aluminum clusters, Al12XHn (X = Al, Si; n = 0, 1, 2). DFT results are compared against high-level benchmark calculations and experimental data where available. Properties compared include geometries, binding energies, ionization potentials, electron affinities, and HOMO-LUMO gaps. Generally, the PBE functional with the double numerical basis set with polarization (DNP) performs very well against experiment and the analytical basis sets for considerably less computational expense.

Dissociative Adsorption of Hydrogen Molecule on Aluminum Clusters : Effect of Charge and Doping

The dissociative chemisorption of molecular hydrogen on charged and neutral aluminum clusters Al12X (X = Mg, Al, Si) was investigated using DFT and a modified G3(MP2)-RAD procedure. Reaction barriers and enthalpies were determined for both neutral and singly charged clusters. The lowest barrier for dissociative adsorption of H2 on a neutral cluster was found for the Al12Mg cluster, whereas the highest barrier was found to be on the closed-shell Al12Si. The interaction of H2 with Al13(+) is found to proceed via an association complex that is 0.07 eV lower in energy than the isolated species and from which the barrier to H2 dissociative adsorption is only 0.16 eV. The most exothermic reaction of H2 with Al12X occurs for the Al13(+)/H2 system. In comparison, reactions with the closed-shell Al13(-) and Al12Si clusters are found to be endothermic. The barriers for H2 desorption from the dihydrogenated clusters are generally quite substantial.

Monolayer Structure and Evaporation Resistance: A Molecular Dynamics Study of Octadecanol on Water

This study examines intermolecular interactions of a monolayer of octadecanol (CH(3)(CH(2))(17)OH) on water as a function of surface density and temperature, using classical molecular dynamics simulations. We observe increased interaction between the alkyl chains (van der Waals) and hydroxyl groups (H-bonding) with increased surface density, which leads to increased order and packing within the monolayer. We also identified clear trends in the intermolecular interactions, ordering and packing of the monolayer molecules as a function of temperature. The observed trends can be closely related to features of the current empirical theories of evaporation resistance.

First Principles Investigation of H Addition and Abstraction Reactions on Doped Aluminum Clusters

We have investigated axial interactions of H(2) with Al(12)X (X = Mg, Al, and Si) clusters and found that homolytic dissociation leading to Al(12)XH and H atom proceeds without a barrier but is an extremely endothermic process. The calculated difference in energy of the addition and abstraction reactions indicates that any Al(12)X-based hydrogen storage technology that involves predissociation of H(2) will be limited by the competing processes. We have also discovered that while there is a modest barrier for dissociation of H(2) on a single Al(12)Mg cluster to give the dihydride, the process occurs spontaneously between two closely spaced Al(12)Mg clusters, resulting in the formation of two Al(12)MgH species. Doping of the cluster with an electropositive atom (Mg) enables the transfer of electron density to the Al cage, which enhances H(2) dissociation. The information gained can contribute to the design of novel solid-state materials made of doped Al clusters, which may ultimately be suitable for catalytic processes.

DFT Study of H Adsorption on Magnesium-Doped Aluminum Clusters

In this study we use density functional theory (DFT) to investigate the properties and H adsorption characteristics of structural isomers of the magnesium-doped aluminum cluster, Al(12)Mg. Our results show that the exohedral structure (exo-MgAl(12)) is significantly lower in energy (1.59 eV) than the endohedral structure (endo-Al(12)Mg); however, the exohedral structure shows significant structural distortion. Our calculations demonstrate that H binds favorably to both exohedral and endohedral structures. Generally, binding energies for H to both clusters range from approximately 2.3 to 2.5 eV with atop positions slightly favored, except for addition directly to the exohedral Mg atom, where the binding energy drops to 1.92 eV. We include a DFT molecular dynamics study of the endo-Al(12)Mg and endo-Al(12)MgH clusters which revealed the isomerization to the respective exostructures at finite temperatures (100-600 K). Interestingly, hydrogen adsorption appears to enhance the isomerization.

Theoretical study of adhesion between graphite, polyester and silica surfaces

This study examines the interaction between graphite and polyester-based polymers and silica in order to compare the adhesive properties of these surfaces. Surface interaction energies were calculated at different interfacial separations, and the resultant adhesion energy curves were used to determine the Work of Separation (W sep) and equilibrium interfacial separation (d 0). Adhesion between graphite and polyester was calculated to be significantly greater than between graphite and silica. Our calculations indicate that Van der Waals Forces lead to significant adhesion between graphite and polyester. However, the Van der Waals attraction is approximately 30% less between graphite and silica.

Comparative study of commonly used molecular dynamics force fields for modeling organic monolayers on water

This study compares the performance of the all-atom molecular dynamics force fields OPLS-AA and COMPASS, and the united-atom GROMOS96 ff53a6 force field, for organic monolayers at aqueous interfaces, as a function of surface density, temperature, and system size. Where possible, comparison with experimental data was undertaken and used to scrutinize the performance of each force field. We find close agreement between the all-atom force fields (OPLS and COMPASS) and experiment for the description of organic monolayers on water. However, the united-atom force field 53a6 tends to exhibit poorer agreement than the all-atom force fields.

Steric trends and kinetic parameters for radical reductions involving alkyldiphenyltin hydrides

Absolute rate constants and Arrhenius parameters for hydrogen atom abstraction by primary alkyl radicals from methyldiphenyl-, ethyldiphenyl-, butyldiphenyl-, isopropyldiphenyl-, cyclohexyldiphenyl- and (trimethylsilyl)methyldiphenyltin hydride were determined in tert-butylbenzene through utilization of the ‘5-hexenyl radical clock’ reaction. At 80 °C, rate constants (kH) for all hydrides were found to lie in the range (8.2–11.5) × 106 lmol−1 s−1, with similar Arrhenius expressions for all reactions studied [viz. log kH = (8.92–8.97)−(3.03–3.24)/2.3RT]. The nature of the alkyl substituent appears to have a subtle effect on the function of the hydride such that the order or reactivity of stannanes (RPh2SnH) is Me > Et > Bu > i-Pr > c-Hex ≥ Me3SiCH2; this trend can be directly traced to steric effects operating in the transition states for hydrogen transfer from tin to carbon. The implications of these observations are discussed.

Comparison of embedded atom method potentials for small aluminium cluster simulations

In this paper, we present a comparison of the performance of a series of embedded atom method potentials for the evaluation of bulk and small aluminium cluster geometries and relative energies, against benchmark density functional theory calculations. In general, the non-pairwise potential-B (NP-B), which was parametrized against Al cluster data, performs the best.