Michael R. Salazar

Does ozone have a barrier to dissociation and recombination

Abstract The barrier associated with the dissociation and recombination of ozone has been calculated using highly correlated ab initio methods. Our calculations show that, for fixed equilibrium values of the bending angle and one bond distance, there is a very small barrier, 100 cm −1 , as opposed to much larger values previously reported. When the saddle point geometry is optimized, the reaction path still contains a barrier but the top of the barrier lies below the dissociation limit.

The potential energy surface for spin-aligned Li3(1 4A′) and the potential energy curve for spin-aligned Li2(a 3Σu+)

A global potential energy surface (PES) for the 1 4A′ spin-aligned state of Li3 is presented. The surface is constructed as a many body expansion of the potential which is the sum of pairwise additive two-body potentials plus a three-body term. The two-body potential is that for the a 3Σu+ state of the lithium dimer. It combines the most recent Rydberg–Klein–Rees potential available [A. Ross (private communication)] with well-known short and long range expansions and accurately reproduces all known experimental data. To obtain the three-body contributions, an ab initio PES was computed at 1122 points using full configuration interaction for the three valence electrons with an augmented Gaussian basis and the effective core potentials of Stevens, Basch, and Krauss [W. J. Stevens et al., J. Chem. Phys. 81, 6026 (1984)] for the other electrons. The two-body interactions are also calculated using the same basis and then subtracted from the full interaction to give the three-body term. To construct the three-b...

Molecular and atomic oxygen on unpromoted and cesium promoted Ag(111) surfaces

Using density functional theory, the energetics of chemisorption and dissociation are computed for molecular oxygen on an unpromoted and a cesium promoted cluster model of the reduced [111] surface of silver. The computed barriers and thermochemistry for our model clusters suggest that dissociation on unpromoted Ag(111) surfaces is activated, but thermochemically favored. Our computed barrier heights and product energies for the unpromoted silver cluster are compared with experimental data. The activation energy for dissociation on the cesium promoted surface shows a cesium coverage dependence, but, again the dissociated product is thermochemically favored. Atomic charges on all atoms were computed for both the unpromoted and promoted cluster models. Analysis of the atomic charges indicates that cesium acts as an electron donor to the silver cluster and that oxygen extracts a portion of the donated electron density from the cluster.

Properties of a Method for Performing Adaptive, Multilevel QM Simulations of Complex Chemical Reactions in the Gas-Phase.

The properties of a new method of performing molecular dynamic simulations of complex chemical processes are presented. The method is formulated to give a time-dependent, multilevel representation of the total potential that is derived from spatially resolved quantum mechanical regions. An illustrative simulation is performed on a 110 atom system to demonstrate the continuity and energy conserving properties of the method. The effect of a discontinuous total potential upon the kinetic energy of the system is examined. The discontinuities in the magnitude of atomic force vectors due to changing the electronic structure during the simulation are examined as well as the effect that these discontinuities have upon the atomic kinetic energies. The method, while not conserving total energy, does yield canonical (NVT) simulations. The time reversibility property of the simulation with an extremely discontinuous total potential is discussed. The computational scaling associated with the formation of the spatially resolved, time-dependent groups is also investigated.

Potential energy surfaces and reactive dynamics of Zn(3P) with H2

The ab initio potential energy surfaces pertinent to Zn(3P)+H2→ZnH(2Σ)+H have been calculated and are described. For thermal collisions, the dominant reactive surfaces is identified and a mechanism for the reaction on this surface, which may explain the rotational state populations in the ZnH products, is proposed. A novel dynamical technique of running classical simulations on ab initio surfaces, using a piecewise tessellation rather than a global functional fit, is briefly introduced with sample trajectories shown.

General methodology in two dimensions for classical simulation of reactive and nonreactive events on ab initio potential energy surfaces

A completely general two‐dimensional (2D) methodology for the classical simulation of reactive and nonreactive events on ab initio potential energy surfaces is introduced and tested. The methodology requires the minimum amount of information given a priori—geometries and energies at these geometries. From a list of ab initio geometries and energies, simulations may be executed and a distribution of outcomes obtained. The method introduced attempts a local approach at simulating the dynamics of the system, rather than a global analytic fit to the potential energy surface. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 1431–1444, 1998

Dissociation of molecular oxygen on unpromoted and cesium-promoted Ag(111) surfaces

Density functional calculations for the dissociation of molecular oxygen on a Ag10 cluster model (seven surface and three subsurface atoms) of the [111] surface of silver are presented. The calculations show that the reaction pathway for the dissociation of molecular oxygen depends on the amount of cesium present on the surface. For the unpromoted surface, dissociation should have a low rate as the process is activated. As a single Cs atom is added to the cluster, O2 should readily dissociate since the process is kinetically and thermochemically favored. However, as a second Cs atom is added, the dissociation of O2 is quenched as the process is highly activated.

Reactive dynamics for Zn(3P)+H2/D2/HD→ZnH/ZnD+H/D: Rotational populations in ZnH/ZnD products

Using ab initio multiconfigurational potential-energy surfaces pertinent to the reaction Zn(3P)+H2→ZnH(2∑)+H and local surface tessellation and interpolation methods developed earlier, we carried out classical trajectory simulations of the title reactions, running large ensembles of trajectories with initial conditions representative of full-collision experiments. Then, using binning techniques, the ZnH/ZnD vibrational and rotational populations have been extracted. Our simulation results duplicate certain unexpected findings noted in experiments of Breckenridge and Wang [Chem. Phys. Lett. 123, 17 (1986); J. Chem. Phys. 87, 2630 (1987)]. Specifically, it is observed that the rotational profiles found for ZnH produced from H2 or from HD are nearly identical, as are the ZnD populations obtained in reactions with D2 or HD. By interrogating the progress of reactive trajectories, we have been able to identify the origin of (much of) the rotational angular momentum in the ZnH/ZnD products, which then allows us ...

ReaxFF molecular dynamics simulations of intermediate species in dicyanamide anion and nitric acid hypergolic combustion

Ionic liquids based on the dicyanamide anion (DCA) are of interest as replacements for current hypergolic fuels, which are highly toxic. To better understand the reaction dynamics of these ionic liquid fuels, this study reports the results of molecular dynamics simulations performed for two predicted intermediate compounds in DCA-based ionic liquids/nitric acid (HNO3) combustion, i.e. protonated DCA (DCAH) and nitro-dicyanamide-carbonyl (NDC). Calculations were performed using a ReaxFF reactive force field. Single component simulations show that neat NDC undergo exothermic decomposition and ignition. Simulations with HNO3 were performed at both a low (0.25 g ml−1) and high (1.00 g ml−1) densities, to investigate the reaction in a dense vapor and liquid phase, respectively. Both DCAH and NDC react hypergolically with HNO3, and increased density led to shorter times for the onset of thermal runaway. Contrary to a proposed mechanism for DCA combustion, neither DCAH nor NDC are converted to 1,5-Dinitrobiuret (DNB) before thermal runaway. Details of reaction pathways for these processes are discussed.

A completely general methodology for fitting three-dimensional ab initio potential energy surfaces

A completely general three-dimensional (3D) method of generating energies and gradients from a prior list of ab initio energies is developed. The method assumes the minimum amount of input data given a priori, i.e., only the potential energy surface (PES). The method introduced may be cast into either a local or global regime. In the local regime, the domain of the PES is sorted, broken into small regions, and interpolation of the energy and gradients takes place in these regions. In the global regime, the energies of the PES are used to formulate a single global interpolant.