Jordi Hernando

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...

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...

Theoretical study of the dynamics, stereodynamics, and microscopic mechanism of the O(1D)+CH4(X 1A1)→OH(X 2Π)+CH3(X 2A2″) reaction

A previously reported potential energy surface (PES) and a new barrierless PES (both based on ab initio data and describing the CH3 group as a pseudoatom) were used to study the O(1D)+CH4→OH+CH3 reaction with the quasiclassical trajectory (QCT) method. The new PES accurately reproduces the experimental rate constant values, in contrast to the previous PES. The QCT study was mainly performed at the relative translational energy (ET) resulting from the photodissociation of N2O at 193 nm (〈ET〉=0.403 eV), although the collision energy obtained from the photodissociation of O3 at 248 nm (〈ET〉=0.212 eV) was also considered. Good agreement between theory and experiment was obtained for the OH vibrational populations and for the OH rotational populations for the v′⩾2 vibrational levels, while the rotational distributions for v′=0–1 are more excited than in the experiment. The QCT results at ET=0.403 eV satisfactorily reproduce the experimental kk′ angular distribution of the state-specific channel OH(v′=4, N′=8) ...

Ab initio study of the O(1D)+CH4(X 1A1)→OH(X 2Π)+CH3(X 2A2″) reaction: Ground and excited potential energy surfaces

The two potential energy surfaces (1 1A and 2 1A PESs) adiabatically correlating the reactants and products asymptotes of the title reaction were studied by means of the CASSCF and CASPT2 ab initio methods. The minimum energy path determined for the ground PES evolved through the barrierless insertion of the O(1D) atom into a C–H bond. The OH+CH3 products result from the dissociation of the CH3OH methanol intermediate formed. Reactivity on the excited 2 1A PES was found to proceed via an abstraction pathway. The energy barrier involved is low enough to expect the 2 1A PES to play a non-negligible role in the title reaction, even at the usual conditions attained in the experiments. The crossing between the 1 1A and 3 1A PESs was also investigated, the latter surface correlating with the excited OH(A 2Σ+) product.

An analytical potential energy surface of the HClF (2A′) system based on abinitio calculations. Variational transition state theory study of the H+ClF→F+HCl, Cl+HF and F+HCl→Cl+HF reactions and their deuterium isotope variants

In this work we have carried out abinitio electronic structure calculations on the ground (2A′) potential energy surface (PES) involved in the H(2S)+ClF and the F(2P)+HCl reactions. Transition states and van der Waals minima have been characterized and have been used along with a grid of approximately 3400 abinitio [PUMP2/6-311G(3d2f,3p2d)] points to derive an analytical PES. The global root-mean-square deviation of the fit (2.66 kcal mol-1) is within the range of the estimated abinitio accuracy. The saddle-point energies of this fitted PES were locally scaled to reproduce the thermal rate constants at 300 K of these reactions considering the H isotope. Calculated variational transition state theory rate constants with the inclusion of a microcanonical optimized multidimensional tunneling correction are in good accord with experiments at different temperatures, both for reactions with H and D isotopes. A small H/D kinetic isotope effect is predicted to have a similar extension (kH/kD≈1–2) for the three reactions depending on the temperature and according to the available experimental results.

Collision energy effects on the dynamics of the reaction O(3P)+CH4(X1A1)→OH(X2Π)+CH3(X2A2″)

A study of the collision energy effects on the dynamics of the title reaction was performed using the quasi-classical trajectories (QCT) method and an analytical triatomic potential energy surface recently derived by our group. Scalar and two-vector properties of the reaction were analysed in terms of the collision energy. The results obtained can be rationalised in terms of the coexistence of reactive trajectories with rebound and non-rebound features, both corresponding to an abstraction reaction mechanism. Future work should account for both the full dimensionality of the system and the possibility of quantum effects.

Quasiclassical trajectory study of the H+ClF→F+HCl, Cl+HF and F+HCl→Cl+HF reactions and their deuterium isotope variants on a new (2A′) ab initio potential energy surface

In this work we present a 3D quasiclassical trajectory (QCT) study of the H+ClF→F+HCl (1), Cl+HF (2) and F+HCl→Cl+HF (3) reactions on a recent ab initio ground 2A′ potential energy surface, mainly for reactants at 300 K. Rate constants, vibrational and rovibrational distributions, angular distributions and mean energy fractions disposed into products were analysed. Deuterated reactions were also considered. Internal distributions were in close agreement with the experimental data, especially for reactions (1) and (3). Reaction (2) exhibited major discrepancies due to the existence of a double microscopic mechanism, direct or migratory plus insertion, which gives rise to very different reaction attributes in each mechanism. The migratory collisions, which are favored by the van der Waals minima, correlate with large impact parameters, produce mainly forward scattering, and furnish a high internal excitation of the products. The direct collisions show exactly the contrary behaviour. In general, the calculated reaction properties can be accounted for in terms of the known L+HH and H+LH dynamics (L: light and H: heavy). QCT rate constants agree very well with experimental data, and a small isotope effect (i.e. kH/kD<2) is found for the three reactions, even smaller in other reaction properties [e.g. angular distributions, P(v′) or P(v′, J′) energy distributions].

Influence of collision energy on the N(2D)+O2→O(3P)+NO reaction dynamics: A quasiclassical trajectory study involving four potential energy surfaces

The influence of collision energy (ET) on the dynamics of the N(2D)+O2→O(3P)+NO atmospheric reaction was studied by means of the quasiclassical trajectory method. The four lowest potential energy surfaces (PESs) involved in the process were used in the calculations (2 2A′, 3 2A′, 1 2A″, and 2 2A″ PESs), and the nonadiabatic couplings between them were neglected. The dependence of the scalar and two-vector properties of the reaction with ET was analyzed. Moreover, the different modes of reaction taking place were investigated. Although only one type of microscopic mechanism (abstraction) was found for the 2 2A′, 3 2A′, and 2 2A″ PESs, two different modes of reaction (abstraction and insertion) were observed to coexist for the 1 2A″ PES. For this PES, the abstraction mechanism is the most important one at room temperature, while the insertion mechanism increases its contribution to reactivity with ET (it accounts for about half of the reactivity above 0.5 eV).

Influence of collision energy on the dynamics of the reaction O(1D) + CH4(X1A1) → OH(X 2Π) + CH3(X 2A2″)

We studied the effects of collision energy (ET) on the dynamics of the title reaction using the quasiclassical trajectory method on an analytical triatomic potential energy surface that we had derived for this system. We compared the dependence of the scalar and two-vector properties of the reaction on ET with experimental data and obtained a quite good agreement. The results can be explained in terms of the coexistence of two microscopic reaction mechanisms: insertion and abstraction. The former mechanism is the most important one, although the contribution of the latter increases with ET .

Nascent OH(X2Π) product state distributions from the reaction of O(1D) with ethylene.: A laser-induced fluorescence study

Abstract The full characterization of the OH( X 2 Π , v″=0–3, N″, J″, Λ″) product state distributions for the O ( 1 D )+ C 2 H 4 → OH + C 2 H 3 reaction was experimentally performed using the laser-induced fluorescence (LIF) technique. Statistical spin–orbit distributions were obtained, while some preference for the formation of the Π(A′) Λ-doublet level was observed. The rovibrational populations obtained suggest that the reaction preferentially evolves via insertion, yielding rovibrationally cold OH through slow decomposition of an alcohol-type collision complex and rovibrationally excited OH by fast decomposition. Moreover, some evidences were found about the implication of an abstraction mechanism, which would produce rotationally cold and highly vibrationally excited OH.