Quasiclassical dynamics and kinetics of the N+NO-->N(2)+O, NO+N atmospheric reactions.

The kinetics and dynamics of the title reactions were studied using the quasiclassical trajectory (QCT) method and two ab initio analytical potential energy surfaces (PESs) developed by our group. In addition to the rate constant (T: 10-5000 K), we also considered a broad set of dynamic properties as a function of collision energy (up to 1.0 eV) and the rovibrational state of NO (v=0-2,j=1,8,12). The production of N(2)+O, reaction (1), dominates the reactivity of the N+NO system over the conditions studied, as expected from the large energy barriers associated to the NO+N exchange reaction, reaction (2). Moreover, the ground PES, which is barrierless for reaction (1), plays a dominant role. Most of the results were interpreted according to the properties of the PESs involved and the kinematics of the system. The QCT rate constants of reaction (1) are in agreement with the experimental data (T: 47-3500 K), including very recent low temperature measurements, and also with variational transition state kinetics and most of quantum dynamics calculations. In addition, the QCT average vibrational energy content of the N(2) product also agrees with the experimental and quantum data. The PESs used here could also be useful to determine equilibrium and nonequilibrium reaction rates at very high temperatures (e.g., 5000-15 000 K).

A density functional theory study of atomic oxygen and nitrogen adsorption over alpha-alumina (0001).

The interaction of atomic oxygen and nitrogen on the (0001) surface of corundum (alpha-alumina) is investigated from first-principles by means of periodic density functional calculations within the generalized gradient approximation. A large Al(2)O(3) slab model (18 layers relaxing 10) ended with the most stable aluminium layer is used throughout the study. Geometries, adsorption energies and vibrational frequencies are calculated for several stationary points for two spin states at different sites over an 1 x 1 unit cell. Two stable adsorption minima over Al or in a bridge between Al and O surface atoms are found for oxygen and nitrogen, without activation energies. The oxygen adsorption (e.g., E(ad) = 2.30 eV) seems to be much more important than for nitrogen (e.g., E(ad) = 1.23 eV). Transition states for oxygen surface diffusion are characterized and present not very high-energy barriers. The computed geometries and adsorption energies are consistent with similar adsorption theoretical studies and related experimental data for O, N or alpha-alumina. The present results along with our previous results for beta-cristobalite do not support the assumption of an equal E(ad) for O and N over similar oxides, which is commonly used in some kinetic models to derive catalytic atomic recombination coefficients for atomic oxygen and nitrogen. The magnitude of O and N adsorption energies imply that Eley-Rideal and Langmuir-Hinshelwood reactions with these species will be exothermic, contrary to what happens for beta-cristobalite.

Quantum real wave-packet dynamics of the N(4S)+NO(X2Pi)-->N2(X1Sigma(g)+)+O(3P) reaction on the ground and first excited triplet potential energy surfaces: rate constants, cross sections, and product distributions.

The reaction N+NO-->N(2)+O was studied by means of the time-dependent real wave-packet (WP) method and the J-shifting approximation. We consider the ground 1 (3)A(") and first excited 1 (3)A(') triplet states, which correlate with both reactants and products, using analytical potential energy surfaces (PESs) recently developed in our group. This work extends our previous quantum dynamics study, and probabilities, cross sections, and rate constants were calculated and interpreted on the basis of the different shapes of the PESs (barrierless 1 (3)A(") and with barrier 1 (3)A(') surfaces, respectively). The WP rate constant (k(1)) shows a weak dependence on T(200-2500 K), as the dominant contribution to reactivity is provided by the barrierless ground PES. There is a good agreement of WP k(1) with the measurements and variational transition state theory (VTST) data, and also between the WP and VTST k(1)(1 (3)A(")) results. Nevertheless, there is a large discrepancy between the WP and VTST k(1)(1 (3)A(')) results. Product state distributions were also calculated for the much more reactive 1 (3)A(") PES. There is an excellent agreement with the experimental average fraction of vibrational energy in N(2)(25+/-3%), the only measured dynamics property of this reaction.

Adsorption of atomic oxygen and nitrogen at beta-cristobalite (100): a density functional theory study.

The adsorption of atomic oxygen and nitrogen on the beta-cristobalite (100) surface is investigated from first principles density functional calculations within the generalized gradient approximation. A periodic SiO2 slab model (6 layers relaxing 4 or 6) ended with a layer of Si or O atoms is employed throughout the study. Several adsorption minima and diffusion transition states have been characterized for the two lowest spin states of both systems. A strong chemisorption is found for either O or N in several sites with both slab endings (e.g., it is found an average adsorption energy of 5.89 eV for O (singlet state) and 4.12 eV for N (doublet state) over the Si face). The approach of O or N on top O gives place to the O2 and NO abstraction reactions without energy barriers. Atomic sticking coefficients and desorption rate constants have been estimated (300-1900 K) by using the standard transition state theory. The high adsorption energies found for O and N over silica point out that the atomic recombination processes (i.e., Eley-Rideal and Langmuir-Hinshelwood mechanisms) will play a more important role in the atomic detachment processes than the thermal desorption processes. Furthermore, the different behavior observed for the O and N thermal desorption processes suggests that the published kinetic models for atomic O and N recombination reactions on SiO2 surfaces, based on low adsorption energies (e.g., 3.5 eV for both O and N), should probably be revised.