Estudio teórico de procesos elementales reactivos: reacciones del O(3P) con CS(X1E+) y CS2(X1Eg+)

[spa] Se ha realizado un estudio teórico de las reacciones elementales en fase gas O(3P)+CS(X1 +) CO*(X1 +) +S(3P) y O(3P) +CS2(X1 G+) CS(X1 +)+SO(X3 -) mediante el método de trayectorias clásicas tridimensionales. Para la primera reacción se ha empleado una superficie de energia potencial (SEP) analítica tipo Sorbie-Murrell que ajusta datos espectroscópicos del sistema y resultados del cálculo cuántico a nivel MNDO-He/CI de la SEP triplete fundamental. Los resultados obtenidos muestran un buen acuerdo con los datos experimentales, especialmente con relación a la inversión de la población vibracional de la molecula CO, que actúa como fuente del láser químico de CO. Para la reacción tetratómica se ha utilizado el mismo tipo de SEP analítica, ajustando en este caso los resultados principales de los cálculos cuánticos realizados sobre la SEP triplete fundamental a nivel MINDO3-UHF, MNDO-UHF y “ab initio” con pseudo-potenciales (a nivel HF con introducción de la energía de correlación según la aproximación MP2 y CIPSI). También se ha construido una segunda SEP que ajusta la energía de activación experimental. La comparación de los resultados de ambas SEP indica que la primera, la cual ajusta un mínimo OSCS entre reactivos y productos, presenta mejores resultados en comparación a la mayoría de datos experimentales

Estudio teórico de procesos elementales reactivos: reacciones del O(3P) con CS(X1E+) y CS2(X1Eg+)

Se ha realizado un estudio teórico de las reacciones elementales en fase gas O(3P)+CS(X1 +) CO*(X1 +) +S(3P) y O(3P) +CS2(X1 G+) CS(X1 +)+SO(X3 -) mediante el método de trayectorias clásicas tridimensionales. Para la primera reacción se ha empleado una superficie de energia potencial (SEP) analítica tipo Sorbie-Murrell que ajusta datos espectroscópicos del sistema y resultados del cálculo cuántico a nivel MNDO-He/CI de la SEP triplete fundamental. Los resultados obtenidos muestran un buen acuerdo con los datos experimentales, especialmente con relación a la inversión de la población vibracional de la molecula CO, que actúa como fuente del láser químico de CO. Para la reacción tetratómica se ha utilizado el mismo tipo de SEP analítica, ajustando en este caso los resultados principales de los cálculos cuánticos realizados sobre la SEP triplete fundamental a nivel MINDO3-UHF, MNDO-UHF y “ab initio” con pseudo-potenciales (a nivel HF con introducción de la energía de correlación según la aproximación MP2 y CIPSI). También se ha construido una segunda SEP que ajusta la energía de activación experimental. La comparación de los resultados de ambas SEP indica que la primera, la cual ajusta un mínimo OSCS entre reactivos y productos, presenta mejores resultados en comparación a la mayoría de datos experimentales

Elementary reaction processes involving atomic and molecular oxygen on ZrB2 (0001)surface

The interaction of atomic and molecular oxygen along with the atomic recombination on thin ZrB2(0001) Zr- and B-terminated surfaces were studied using density functional theory (GGA/PBE) calculations. The adsorption of atomic oxygen is predominantly produced on threefold hollow sites for the Zr-finished surface and on B-B bridge sites for the B-finished surface. The experimental specular HREELS loss peaks and their shifts at high O exposures can be satisfactory explained by the present calculations. The interaction of O2 over both terminated surfaces produces mainly its dissociation through non-activated processes. This fact is in agreement with the observed open dissociation at room temperature. The atomic oxygen recombination over both ZrB2 surfaces shows that the Eley-Rideal reaction will be much more important than the Langmuir-Hinshelwood reaction at all temperatures and even more accessible in the case of the B-finished surface

Ab initio, VTST and QCT study of the 1 2A'' potential energy surface of the N(2D) + O2(X3 Σg-) → O(3P) + NO(X2Π) reaction

An ab initio study based on the CASSCF ~Complete Active Space Self-Consistent Field! and CASPT2 (Second-Order Perturbation Theory on a CASSCF wave function) methods has been carried out on the ground 2A' potential energy surface (PES) involved in the relevant atmospheric reaction between N(2D) and O2 to produce O(3P) and NO. Also, some intersections between PES have been studied. The stationary points have been characterized and a grid of more than 800 points have been fitted to an analytical function. This analytical representation of the PES has been used to obtain kinetic and dynamic properties of the reaction. The rate constant of this reaction has been calculated at different levels of theory [variational transition state theory (VTST) and quasiclassical trajectory (QCT) methods] and has been compared with the experimental values (overall rate constant including physical electronic quenching) obtaining a good agreement. The QCT method has also been employed to study the properties of products from both the abstraction and insertion microscopic mechanisms. The vibrational distribution of NO arising from the reaction at 100 K has also been calculated and compared with the experimental ones. In this case, the agreement between the theoretical and the experimental results is not so good, the experimental vibrational distribution being less excited. Future work is necessary to determine the origin of this differenc

The lowest doublet and quartet potential energy surfaces involved in the N(4S) + O2 reaction. II. Ab initio study of the C2v-symmetry insertion mechanism

In the present work we have carried out ab initio complete active space self-consistent field (CASSCF) and second-order perturbation theory on CASSCF wave function (CASPT2) calculations and also some density functional theory calculations with the aug-cc-pVTZ Dunning's basis set on the lowest A1, B1, A2 and B2 doublet and quartet potential energy surfaces (PESs) that could be involved in the title reaction. Thus, several minima, transition states and surface crossings have been found for the C2v-insertion reaction mechanism. The results agree very well with available experimental data (i.e., for NO2 (2A1), MIN2 (2B2), NO2 (2∏u)) and with other previous ab initio calculations. Six A' / A' and four A' / A' type surface crossings were located and classified for these PESs, whose only one (i.e., 2B2 / 2A1) has been previously reported in theoretical and experimental studies. High-energy barriers were found for the direct C2v-insertion mechanism (3.11 and 2.54 eV for the lowest doublet and quartet PESs at the CASPT2/aug-cc-pVTZ level, respectively), clearly showing that this competitive mechanism is much less favorable than the direct Cs-abstraction or the indirect Cs-insertion reaction mechanisms reported in paper I

A theoretical approach to the O(1D) + H2O (X1A1) reaction: ab initio potential energy surface and quasiclassical trajectory dynamics study

An ab initio study of the ground potential energy surface (PES) of the O(1D)+CH4→OH+CH3 reaction has been performed using the second and fourth order Møller-Plesset methods with a large basis set. From the ab initio data a triatomic analytical ground PES with the methyl group treated as an atom of 15.0 amu has been derived. This PES has been employed to study the dynamics of the reaction by means of the quasiclassical trajectory (QCT) method. A good agreement between the experimental and QCT OH rovibrational distributions at a collision energy of 0.212 eV with the methane molecule at 298 K has been obtained. The analysis of the microscopic reaction mechanism shows that the reaction takes place almost exclusively through the insertion of the O(1D) atom into a C-H bond, due to the presence of the deep (CH3)OH minimum, and the resulting trajectories may be direct or nondirect (short-lived collision complexes mainly) with about the same probability. The OH vibrational distribution arising from the direct mechanism is inverted, while the nondirect mechanism leads to a noninverted one. There is some tendency to give broader OH rotational distributions peaking at higher N′ values, particularly for the vibrational levels v′ = 0-1, in the case of the nondirect trajectories. The PES derived here may be used in dynamics studies under conditions where the methyl group motions are not strongly coupled to the motions leading to reactio

Ab initio, variational transition state theory and quasiclassical trajectory study on the lowest 2A' potential enegy surface involved in the N(2D) + O2(X3Σg-) → O(3P) + NO(X2Π) atmospheric reaction

A CASSCF and CASPT2 ab initio study has been carried out for the lowest 2A′ potential energy surface (2 2A′ PES) that correlates reactants and products of the N(2D)+O2→O(3P)+NO reaction. All the stationary points have been characterized and along with a grid of more than 600 ab initio points have been fitted to an analytical function. Afterwards, this analytical PES has been employed to study the kinetics [variational transition state theory (VTST) and quasiclassical trajectory (QCT) methods] and dynamics (QCT method) of the reaction. Concerning the rate constants, a good agreement with the experimental values corresponding to the global deactivation of N(2D) has been obtained. This suggests that this reaction is responsible of most of the reactivity of the N(2D)+O2 system. NO vibrational distributions have also been calculated. Although there is not a good agreement between the theoretical and experimental values, preliminary results show that they can become quite close by taking into account the contribution of the 1 2A″ PES

New analytical (2A',4A') surfaces and theoretical rate constants for the N(4S) + O2 reaction

We report two new analytical fits of the ground potential energy surface (PES) (2A') and the first excited PES (4A') involved into the title reaction and its reverse, using ab initio electronic structure calculations from papers I and II along with new grids of ab initio points by means of the Second-Order Perturbation Theory on CASSCF wave function (CASPT2 (17,12) G2/aug-cc-pVTZ) reported here (1250 points for the 2A' PES and 910 points for the 4A' PES). Some experimental data were also introduced to better account for the exoergicity and the experimental rate constant at 300 K. The final root-mean-square deviations of the fits were 1.06 and 1.67 kcal/mol for 2A' and the 4A' PESs, respectively, for the NOO Cs-abstraction and insertion regions of the PESs. Thermal rate constants were calculated (300-5000 K) for both the direct and reverse reactions by means of the variational transition state theory with the inclusion of a microcanonical optimized multidimensional tunneling correction, obtaining a very good agreement with the experimental data within all the temperature range. The new analytical 2A' PES presents several stationary points not introduced in previous analytical surfaces, and describes accurately the NO2 (X 2A1) minimum, which seems to be very accessible according to the trajectories run in a preliminary quasiclassical trajectory study. The new analytical 4A' PES has a lower energy barrier than the previous one, which increases significantly the contribution of this PES to the total rate constant at high temperatures. Moreover, the new analytical PESs not only describe accurately the Cs-regions of the NOO system but also the ONO C2v- or near C2v-regions

Ab initio CASPT2//CASSCF study of the O(1D) + H2O(X1A1) reaction

The ground potential energy surface (PES) of the O(1D) + H2O system was studied with the CASPT2//CASSCF ab initio method. We analyzed the degree of validity of an earlier ab initio study by us that used the Møller-Plesset (MP) method. Both the present CASPT2//CASSCF calculations and the highest level MP calculations [PUMP4//UMP2] showed that the main reaction channel (OH + OH) has no energy barrier along the minimum energy path. This result is consistent with the absence of experimental activation energy. The CASPT2//CASSCF and PUMP4//UMP2 results, however, show important differences, mainly concerning the energy, due to the dominant open-shell singlet character of the ground PES. To make an accurate general description of this system, ab initio calculations using multireference methods like the one discussed here are required. Nevertheless, the earlier PUMP4//UMP2 calculations can be taken as a reasonable starting point for characterizing the ground PES of this system. Moreover, the pseudotriatomic (O(1D) + H-(OH)) analytical potential energy surface derived in the previous work to interpret the experimental resultsis a reasonable model for describing the O(1D) + H2O → 2OH reaction

DFT and kinetics study of O/O2 mixtures reacting over a graphite (0001) basal surface

Spin-polarised density functional calculations were used to investigate the interaction of atomic and molecular oxygen on the basal graphite surface at several atomic coverages. Two carbon layers were enough to represent the graphite surface. Oxygen atoms bind mainly over C-C bridge sites forming an epoxide-like structure with a two carbon puckering and with adsorption energies in the 0.95-1.28 eV range, depending on the atomic coverage. Molecular oxygen only shows a very weak physisorption. Atomic adsorption and diffusion along with atomic recombination via Eley-Rideal and Langmuir-Hinshelwood mechanisms were studied. All surfaces processes were activated with energy barriers that decreased for lower atomic coverages. Relaxation effects were non-negligible. A microkinetic model with six elementary surface processes was proposed to see the overall behavior of several initial O/O2 mixtures flowing over a graphite surface at 300-1000 K. Thermal rate constants were derived from Density Functional Theory (DFT) data and standard Transition State Theory. A very low steady-state atomic coverage ( < 0.5 %) was predicted and also very low atomic recombination coefficients were observed (gamma_O < 5x10-4). The Eley-Rideal together with the adsorption and desorption processes were much more important than the Langmuir-Hinshelwood reaction