Search

Quasiclassical trajectories is computed describing the H + O2 OH + O with the O2 reagent vibrationally excited to different levels at different temperatures. The vibrational energy levels are selected by using a semiclassical Einstein-Brillouin-Keller quantization procedure while the relative translational energy is sampled from a Boltzmann weighted distribution.

The Meyer-Clarke synthetic method was used in the German process for large scale production of mustard gas during World War I, which clearly shows the historical connection of synthesis of mustard gas.

Several ab initio studies have focused on the minimum energy path region of the hydroperoxyl potential energy surface (PES) [J. Chem. Phys. 88, 6273 (1988)] and the saddle point region for H-atom exchange via a T-shaped HO2 complex [J. Chem. Phys. 91, 2373 (1989)]. Further, the results of additional calculations [J. Chem. Phys. 94, 7068 (1991)] have been reported, which, when combined with the earlier studies, provide a global description (but not an analytic representation) of the PES for this reaction. In this work, information at the stationary points of the ab initio PES is used within the framework of conventional Transition State Theory (TST)/RRKM. Theory to compute estimates of the thermal termolecular rate coefficients for the reaction between the H(D) atom and O2 in the presence of two different bath gases, argon and nitrogen, as a function of pressure and temperature. These calculations span a pressure range from 1.0 Torr to the high-pressure limit and a temperature range from 298.15 to 6000.0 K. Conventional TST/RRKM Theory was utilized within the framework of two models: an equilibrium model employing the strong collision assumption (model I), [R. G. Gilbert and S. C. Smith, Theory of Unimolecular and Recombination Reactions (Blackwell, Oxford, 1990), as implemented in the UNIMOL program suite]; and a steady-state model that includes chemical activation (model II), using the collisional energy transfer approximation proposed by J. Troe [J. Chem. Phys. 66, 4745, 4758 (1977); 97, 288 (1992)]. In this work we first summarize the pressure-dependent fall-off curves (calculated with model I) and the high-pressure limit rate coefficients (calculated with models I and II) over the entire temperature range, and then focus on the fall-off behavior for temperatures between 298.15 and 2000.0 K. Direct comparisons are made between the experimentally determined termolecular rate coefficients (either from direct measurements or based on recommended... [ABSTRACT FROM AUTHOR]

Recent ab initio calculations have focused on the minimum energy path region of this surface [J. Chem. Phys. 88, 6273 (1988), Paper I] and on the saddle point region for H atom exchange via a T-shaped HO2 complex (J. Chem. Phys. 91, 2373 (1989), Paper II). In this paper, the results of additional calculations are discussed that, combined with the previously reported results, provide a global representation of the potential energy surface for this reaction. Complete active space SCF/externally contracted configuration interaction calculations (CASSCF/CCI) were carried out using the same wave function employed in II. The new calculations presented here characterize the potential energy surface for a variety of H atom approach angles, ranging from perpendicular to collinear (measured with respect to the O2 bond) and for a variety of H atom to O2 center-of-mass distances. Additionally, a new collinear exchange saddle point is reported. Using the ab initio results from these new calculations, optimal geometries, and harmonic frequencies based on local polynomial representations of the potential energy surface are reported along the constrained energy minimum path, while anharmonic frequencies are calculated at both the O2 and OH asymptotes and at the HO2 intermediate. [ABSTRACT FROM AUTHOR]

A new model is developed for the analysis of muon spin relaxation (μSR) measurements of muonium (Mu) reaction rates with other chemical species. The kinetics component of the model is specifically developed for addition reactions but can be extended to direct reactions. The polarization component of the model assumes that the adduct formed possesses a single, effective spin precession frequency. The complete model is solved to obtain an analytic expression for the μSR signal as a function of time. Both a time-ordered sequence method and a Boltzmann spin equation method give equivalent solutions. These solutions are analyzed under various kinetic conditions. It is concluded that the coefficient controlling the decay of the μSR signal is closely related to the adduct formation rate constant, i.e., the high pressure limit of the apparent bimolecular addition rate constant. In the most favorable case, the decay constant gives the adduct formation rate constant directly at all pressures of buffer gas. [ABSTRACT FROM AUTHOR]

The minimum energy path (MEP) for the addition of a hydrogen atom to N2 is characterized in CASSCF/externally contracted CI calculations using a [4s3p2d1f/3s2p1d] ANO basis set, with additional single point calculations at the stationary points of the potential energy surface using a [5s4p3d2f/4s3p2s] ANO basis set. These calculations represent the most extensive set of ab initio calculations on HN2 completed to date, yielding a zero-point corrected barrier for HN2 dissociation of ≊8.5 kcal mol-1. The lifetime of the HN2 species is estimated from the calculated geometries and energetics using both conventional transition state theory and a method which utilizes an Eckart barrier to compute one-dimensional quantum mechanical tunneling effects. This study concludes that the lifetime of the HN2 species is very short, greatly limiting its role in combustion processes. [ABSTRACT FROM AUTHOR]

A Fourier transform method for calculating action variables and semiclassical eigenvalues in molecules starting from Cartesian coordinates and momenta is developed and applied to the determination of rotational semiclassical eigenvalues for rigid asymmetric top molecules. The method involves calculating rotational actions by using Fourier representations of the symmetric top action-angle variables to express the integrals. The question of what is the optimum quantization axis is considered, and it is found that the same primitive semiclassical eigenvalues are obtained independent of which principal axis is used for quantization provided that the replacement J → J+ 1/2 is made. The replacement J → [J(J+1)]1/2 leads to eigenvalues that are slightly dependent on quantization axis, but more accurate. For either choice of J replacement, the primitive eigenvalues are found to be double valued when trajectory motion is librational relative to both the prolate and oblate top quantization axes, and also when motion is rotational relative to both axes. This problem can be resolved by implementing uniform semiclassical theory, but this is hard to generalize to nonrigid molecule eigenvalue calculations. Here we propose several simple modifications to the primitive semiclassical method which determine uniquely defined eigenvalues for both the J(J+1) or (J+ 1/2 )2 replacements. These modified primitive semiclassical methods are also applicable to nonrigid molecule calculations, and one of these (the linear interpolation method) is found to give eigenvalues that are quite accurate without requiring information beyond that which is needed in a primitive semiclassical calculation. [ABSTRACT FROM AUTHOR]

The variational transition state theory rate constant is calculated for the H+CH3 → CH4 bimolecular reaction and compared with the quasiclassical trajectory value reported previously. The calculations are performed on two different potential energy surfaces, MAPS/CH4-I and -II. To calculate the variational transition state theory rate constant, the reaction path and frequencies orthogonal to the reaction path are required. The general classical trajectory computer program mercury was modified to perform these calculations. Good agreement is found between the variational transition state theory and quasiclassical trajectory rate constants for both the MAPS/CH4-I and -II surfaces. The rate constants for the two surfaces differ by an order of magnitude. These calculated values are compared with a recent experimental measurement. [ABSTRACT FROM AUTHOR]

The Monte Carlo quasiclassical trajectory method is used to study the molecular dynamics of the H+CH3→CH4 association reaction. The bimolecular rate constant is calculated for a temperature of 300 K and compared with experimental measurements. Both the nature of the reactive trajectories and the bimolecular rate constant are very sensitive to the shape of the C–H stretching potential energy curve. [ABSTRACT FROM AUTHOR]

Quasiclassical trajectories have been computed on the Melius−Blint (MB) Potential Energy Surface (PES) and on the Double Many-Body Expansion (DMBE) IV PES of Pastrana et al. describing the H + O2 ⇔ OH + O reaction with the nonrotating (J = 0) O2 reagent vibrationally excited to levels v = 6, 7, 8, 9, and 10 at four temperatures: 1000, 2000, 3000, and 4000 K. The vibrational energy levels were selected by using a semiclassical Einstein−Brillouin−Keller (EBK) quantization procedure while the relative translational energy was sampled from a Boltzmann weighted distribution. The rate coefficient for the formation of the OH + O products is seen to increase monotonically with quantum number and nearly monotonically with temperature. On the MB PES, at T = 1000 K, the total rate coefficient increases by a factor of 5.2 as the initial vibrational quantum number of the O2 diatom increases from v = 6 to v = 10. For T = 2000 K, this factor drops to 3.3, to 2.9 for T = 3000 K, and to 2.5 for T = 4000 K. On the DMBE IV PES, at T = 1000 K the total rate coefficient increases by a factor of 4.1 as the initial vibrational quantum number of the O2 diatom increases from v = 6 to v = 10. For T = 2000 K, this factor drops to 3.5, to 2.1 for T = 3000 K, and to 2.0 for T = 4000 K. The less-direct group (defined below) of trajectories is sensitive to the initial O2 vibrational excitation in several different temperature ranges, apparently retaining the effect of reagent vibrational excitation. The more-direct group (defined below) of trajectories does not exhibit this behavior. Reagent vibrational excitation does not increase the total rate coefficients for the title reaction more than the increase due to a simple temperature increase. The less-direct and more-direct groups of trajectories differ in their contribution to the rate coefficient for the title reaction. In particular, at T = 4000 K, the two PESs used in this work differ dramatically in the roles of the less-direct and more-direct trajectories. The behavior of the more-direct and less-direct groups of trajectories can be understood in terms of the efficiency of intramolecular vibrational energy transfer. This work utilizes the recently introduced PES Library, POTLIB 2001, which made the comparisons between the two PESs discussed in this work possible in a very straightforward way. [ABSTRACT FROM AUTHOR]