RESUMEN
Six-dimensional wave packet calculations on an accurate potential energy surface are used to obtain the quantum mechanical capture (QM C) probabilities for CH + H(2) corresponding to a variety of total angular momenta and internal reactant states. Rate constant calculations are made feasible by employing a Monte Carlo based sampling procedure. The QM C probabilities alone are also used to estimate the high pressure CH + H(2) rate constants corresponding to stabilization or CH(3) formation. The rate constants for CH + H(2) --> CH(2) + H reaction in the low pressure limit are obtained by combining the QM C probabilities with a phase space theory (PST) approximation for product formation from the complex. Our results are compared with the experimental results of Brownsword et al. (J. Chem. Phys. 1997, 106, 7662), as well as with purely classical PST calculations. The QM C probabilities are shown to be highly dependent on the initial rotational states of the reactants corresponding to orientational restrictions on complex formation. Consistent with this, our QM C high pressure rate constants for CH(3) formation are lower than the purely classical PST rate constants. These QM C rate constants also are in reasonable accord with experiment. A similar but somewhat more subtle picture emerges regarding the QM C/PST rate constants for CH(2) + H formation.
RESUMEN
Quantum mechanical energy levels are computed for the hydrogen molecule and its homonuclear isotopes confined within carbon nanotubes of various sizes and structures using three different interaction potentials. Two translational and two rotational degrees of freedom are treated explicitly. We study the dependence on the interaction potential and the size of the nanotube of several features, including zero-pressure quantum sieving selectivities, ortho-para energy splittings, and wave function characteristics. We show that large quantum sieving selectivities, as well as large deviations from gas phase ortho-para splittings, occur only under the condition of extreme two-dimensional confinement, when the characteristic length of the hydrogen-carbon interaction potential is nearly equal to the radius of the nanotube.
RESUMEN
Six-dimensional wave packet calculations are carried out to study the behavior of HOCO subsequent to the photodetachment of an electron from the negative anion, HOCO-. It is possible to form stable and/or long-lived HOCO complexes, as well as the dissociative products OH+CO and H+CO2. A variety of observables are determined: the electron kinetic energy (eKE) distributions associated with the OH+CO and H+CO2 channels, the correlated eKE and product translational energy distribution for the OH+CO channel, and product branching ratios. Most of our results are in good accord with the experimental results of Clements, Continetti, and Francisco [J. Chem. Phys. 117, 6478 (2002)], except that the calculated eKE distribution for the H+CO2 channel is noticeably colder than experiment. Reasons for this discrepancy are suggested.
RESUMEN
The complex-forming CH + H2 --> CH2 + H reaction is studied employing a recently developed global potential energy function. The reaction probability in the total angular momentum J = 0 limit is estimated with a four-atom quantum wave packet method and compared with classical trajectory and statistical theory results. The formation of complexes from different reactant internal states is also determined with wave packet calculations. While there is no barrier to reaction along the minimum energy path, we find that there are angular constraints to complex formation. Trajectory-based estimates of the low-pressure rate constants are made and compared with experimental results. We find that zero-point energy violation in the trajectories is a particularly severe problem for this reaction.
RESUMEN
A six-dimensional wave packet study of the unimolecular decay of vibrationally activated OH-CO reactant channel complexes is presented. The ab initio based Lakin-Troya-Schatz-Harding potential energy functions for the A' and A" states are employed. Good agreement with the experimental product distributions and lifetimes of Pond and Lester is found. We are able to confirm that complexes with two vibrational quanta of excitation in OH, vOH=2, and no vibrational excitation in CO, vCO=0, decay through two pathways. One pathway leads to products (vOH=1, vCO=0) with relatively high OH rotational energy and the other leads to products (vOH=1, vCO=1) with relatively low OH rotational energy. We also find that the lifetime of the A" state is less than the A' state and that there is a propensity for A" products.
RESUMEN
We study the OH+CO-->H+CO2 reaction with both six-dimensional quantum wave packets (QM) and quasiclassical trajectories (QCT), determining reaction probabilities and thermal rate constants (or coefficients), and studying the influence of the reactant channel hydrogen-bonded complex well on the reaction dynamics. The calculations use the recently developed Lakin-Troya-Schatz-Harding (LTSH) ground electronic state potential energy surface, along with a modified surface developed for this study (mod-LTSH), in which the reactant channel well is removed. Our results show that there can be significant differences between the QM and QCT descriptions of the reaction for ground-state reactants and for energies important to the thermal rate constants. Zero-point energy violation plays an important role in the QCT results, and as a result, the QCT reaction probability (for ground-state reactants and zero impact parameter) is much higher than its QM counterpart at moderate to low reagent translational energies. The influence of the reactant channel well in the QCT results is to enhance reactivity at moderate energies and to suppress reactivity at the very lowest collision energies. The QM results also show the enhancement at moderate energies but, while the very lowest translational energies cannot be adequately converged, they do not indicate any tendency toward suppression as energy is reduced. QCT calculations for excited rotational states of the reactants show that the suppression of reactivity associated with the reactant channel well is less important when the reactants are rotating, and as a result, the influence of the reactant channel well on the thermal rate coefficients is relatively small, being important below 200 K. Our results indicate that there still remain important discrepancies between experiment and theory in this low temperature regime and that further improvements of the potential are needed.