R. Sayós

Ab initio ground potential energy surface, VTST and QCT study of the O(3P)+CH4(X 1A1)→OH(X 2Π)+CH3(X 2A2″) reaction

An ab initio study of the ground potential energy surface (PES) of the O(3P)+CH4→OH+CH3 reaction has been performed using the second- and fourth-order Mo/ller–Plesset methods with a large basis set. A triatomic analytical ground PES with the methyl group treated as an atom of 15.0 a.m.u. has been derived. This PES has been employed to study the kinetics [variational transition state theory (VTST) and quasiclassical trajectory (QCT) rate constants] and dynamics (QCT method) of the reaction. The ab initio points have also been used directly to calculate the VTST rate constant considering all atoms of the system. The best VTST methods used lead to a good agreement with the experimental rate constant for 1000–2500 K, but QCT rate constant values are about one-third the experimental ones for 1500–2500 K. The cold QCT OH(v=0) rotational distribution arising from the simulation of the reaction with O(3P) atoms produced in the photodissociation of NO2 at 248 nm is in good agreement with experiment, while the very...

Theoretical investigation of the eight low-lying electronic states of the cis- and trans-nitric oxide dimers and its isomerization using multiconfigurational second-order perturbation theory (CASPT2)

In this work we have carried out ab initio electronic structure calculations, CASSCF/CASPT2 and CASSCF/MRCI-SD+Q with several Pople’s and correlation-consistent Dunning’s basis sets, of the planar cis- and trans-NO dimers for the lowest eight electronic (singlet and triplet) states. The geometry, frequencies, dipole moment, binding energy, and vertical excitation energies are predicted with an accuracy close to or even better than the best reported ab initio previous results for some of these properties, and in very good agreement with the available experimental data. CASPT2 optimized geometries show the existence of at least four shallow NO-dimers (i.e., two cis-(NO)2 (1A1 and 3B2) and two trans-(NO)2 (1Ag and 3Au)), although CASSCF optimization with CASPT2 pointwise calculations indicate the existence of other less stable dimers, on the excited states. Vertical excitation energies were calculated for these four dimers. For the cis-NO dimer, the ordering and the energy spacings between the excited states...

Dynamics of the N(4Su) + NO(X 2Π) → N2(X 1Σg+) + O(3Pg) atmospheric reaction on the 3A‘ ground potential energy surface. I. Analytical potential energy surface and preliminary quasiclassical trajectory calculations

The N(4Su)+NO(X 2Π)→N2(X 1Σg+)+O(3Pg) reaction plays an important role in the upper atmosphere chemistry and as a calibration system for discharge flow systems. Surprisingly, very little theoretical and experimental work has been devoted to the characterization of the dynamical features of this system. In this work a Sorbie–Murrell expression for the lowest 3A‘ potential energy surface (PES) connecting reactants in their ground electronic states based upon the fitting of an accurate ab initio CI grid of points has been derived. The PES fitted shows no barrier to reaction with respect to the reactants asymptote in accordance with experimental findings and becomes highly repulsive as the NNO angle is varied away from the saddle point geometry. The results of preliminary quasiclassical trajectory calculations on this surface reproduce very well the experimental energy disposal in products, even though the vibrational distribution derived from trajectories seems to be a bit cooler than the experimental data. ...

Quasiclassical trajectory study of the N(4Su) + O2 (X 3Σ−g)→NO (X 2Π) + O (3Pg) atmospheric reaction on the 2A′ ground potential energy surface employing an analytical Sorbie—Murrell potential

Abstract The reaction between ground state nitrogen atoms and oxygen molecules plays an important role in the chemistry of the upper atmosphere as a source of infrared chemiluminescence and also as a participant in the auroral mechanism. Even though there is considerable experimental stock concerning rate constants over a wide range of temperatures and also on the vibrational distribution of products, as well as theoretical information about the potential energy surfaces of this system, no extensive work is available on its dynamical features. The present paper is concerned with the construction of an analytical Sorbie—Murrell surface for the lowest 2 A′ surface correlating reactants and products in their ground electronic states, based upon the reproduction of a grid of ab initio CI points and a preliminary dynamics on that surface. The analytical potential energy surface fits the corrected ab initio CI barrier of 7.6 kcal/mol, in agreement with the experimental estimates, exhibiting also a repulsive behaviour of the NOO bending at the saddle point configuration. Quasiclassical trajectories lead on the whole to a rather good reproduction of the different experimental properties measured. The calculated vibrational distribution resulting from the present QCT calculations does not however fully agree with any of the existing experimental, mutually conflicting, measurements.

A comparison between experimental, quantum and quasiclassical properties for the N(4S) + O2 (X3Σg) → NO (X2Π) + O(3P) reaction

Abstract A detailed reactive infinite order sudden approximation (R-IOSA) study of the reactivity of the N ( 4 S ) + O 2 system has been carried out in the 0.53–1.5 eV translational energy range. Results have been compared with experimental and quasiclassical trajectory (QCT) data. The general dynamical features already obtained within the QCT framework have been reproduced. In addition, the R-IOSA product vibrational distribution matches almost quantitatively the experimental one. The relative importance of the different ν′ contributions, both to the classical and quantum more averaged quantities has been interpreted with the help of the state-to-state opacity function. Finally, the alternating vibrational distribution and its evolution with translational energy are discussed within the context of a Franck-Condon type model.

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 go...

Quantum reactive scattering calculations of cross sections and rate constants for the N(2D)+O2(X 3Σg−)→O(3P)+NO(X 2Π) reaction

Time-dependent quantum wave packet calculations have been performed on the two lowest adiabatic potential energy surfaces (2 2A′ and 1 2A″) for the N(2D)+O2(X 3Σg−)→O(3P)+NO(X 2Π) reaction. The calculations have been carried out, on these recently published potential energy surfaces, using the real wave packet method together with a new dispersion fitted finite difference technique for evaluating the action of the radial kinetic energy operator. Reaction probabilities, corresponding to the O2 reactant in its ground vibrational-rotational state, have been calculated for both surfaces and for many different values of the total angular momentum quantum number (J), within the helicity decoupling approximation. The reaction probabilities associated with all other relevant J values have been interpolated, and to a smaller extent extrapolated, using a capture model, to yield probabilities as a function of energy. The probabilities have in turn been summed to yield energy dependent cross sections and then used to...

Ab initio ground potential energy surface and quasiclassical trajectory study of the O(1D)+CH4(X 1A1)→OH(X 2Π)+CH3(X 2A2″) reaction dynamics

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 Mo/ller–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 mec...

Quasiclassical dynamics and kinetics of the N+NO→N2+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).

Analysis of product Doppler-broadened profiles generated from photoinitiated bimolecular reactions

This paper, which is the companion to the experimental paper (M. Brouard, S. P. Duxon, P. A. Enriquez and J. P. Simons, J. Chem. Soc., Faraday Trans., 1993, 89, 1435) in this issue, is divided into two parts. In the first, equations for the laboratory (LAB) velocity distribution of products generated via photoionitiated bimolecular reaction are presented. These equations provide the basis for numerical simulation of these distributions from assumed forms for the centre-of-mass (CM) differential cross-section: the results may be compared directly with those derived experimentally via Fourier-transform Doppler 1 + 1 LIF or REMPI spectroscopy. The latter inversion technique is described in detail in the second part of the paper.