J. M. Alvariño

An improvement of the Li+HF PES based on a 3D quasiclassical trajectory test

A quasiclassical trajectory test of some potential energy surfaces designed for the Li+HF reaction is reported. A comparison of scattering quantities with experimental data has allowed the selection of a surface sufficiently accurate for reproducing reactive cross sections and detailed experimental data.

A quasiclassical trajectory test for a potential energy surface of the Li+HF reaction

A three‐dimensional quasiclassical trajectory study of the Li→HF→LiF+H reaction has been performed on a recently proposed analytical potential energy surface (PES) fitted to ab initio points. The results of the calculations are compared with the experiment. Previous related work on a semiempirical PES is noted.

Exact quantum stereodynamics: The steric effect for the Li+HF→LiF+H reaction

To study the stereodynamics of atom diatom reactions, we apply the formalism developed by Aquilanti et al. [J. Phys. Chem. 95, 8184 (1991)]. As a case study the prototype Li+HF→LiF+H reaction at zero total angular momentum J is considered. For this reaction we calculated the scattering SJ-matrix in the standard |lj〉 representation and transformed it into a stereodirected representation. In this way it is possible to investigate the effect on the reaction probability of the orientation of the target HF molecule with respect to Li attack. In the investigated collision energy range (0.45–0.54 eV) propensity is found for Li attack on the side of H atom.

A quantum mechanical collinear study of the reactions Li + FX → LiF + X (X = H, D, T)

Abstract Total and state-to-state quantum-mechanical collinear reactive probabilities for the title reactions have been computed on a recently proposed potential energy surface and compared with previous work. Isotope effects and the dynamical implications resulting from changes in the surface shape are investigated.

Dynamics of the D+ + H2 and H+ + D2 reactions: a detailed comparison between theory and experiment

An extensive set of experimental measurements on the dynamics of the H(+) + D(2) and D(+) + H(2) ion-molecule reactions is compared with the results of quantum mechanical (QM), quasiclassical trajectory (QCT), and statistical quasiclassical trajectory (SQCT) calculations. The dynamical observables considered include specific rate coefficients as a function of the translational energy, E(T), thermal rate coefficients in the 100-500 K temperature range. In addition, kinetic energy spectra (KES) of the D(+) ions reactively scattered in H(+) + D(2) collisions are also presented for translational energies between 0.4 eV and 2.0 eV. For the two reactions, the best global agreement between experiment and theory over the whole energy range corresponds to the QCT calculations using a gaussian binning (GB) procedure, which gives more weight to trajectories whose product vibrational action is closer to the actual integer QM values. The QM calculations also perform well, although somewhat worse over the more limited range of translational energies where they are available (E(T) < 0.6 eV and E(T) < 0.2 eV for the H(+) + D(2) and D(+) + H(2) reactions, respectively). The worst agreement is obtained with the SQCT method, which is only adequate for low translational energies. The comparison between theory and experiment also suggests that the most reliable rate coefficient measurements are those obtained with the merged beams technique. It is worth noting that none of the theoretical approaches can account satisfactorily for the experimental specific rate coefficients of H(+) + D(2) for E(T)≤ 0.2 eV although there is a considerable scatter in the existing measurements. On the whole, the best agreement with the experimental laboratory KES is obtained with the simulations carried out using the state resolved differential cross sections (DCSs) calculated with the QCT-GB method, which seems to account for most of the observed features. In contrast, the simulations with the SQCT data predict kinetic energy spectra (KES) considerably cooler than those experimentally determined.

Double-well structure and microscopic branching in the O(1D)+HCl reaction

Abstract By performing a quasiclassical trajectory study, we have confirmed that the title reaction can take place via different paths even when leading to the same product (microscopic branching). However, contrary to a previous hypothesis, no collinear collisions were found to contribute to the reaction. Therefore, the backward–forward structure of the ClO product angular distribution obtained in the crossed-beam experiment has to be ascribed to the combination of a backward peak associated with insertion-like collisions and a forward peak associated with side attachment of O on Cl. Proper combinations of insertion and attachment contributions also rationalize other properties of the title reaction.

Direct versus indirect microscopic mechanisms in the Li + HF reaction

Quasiclassical trajectory calculations on the chemical reactions Li + XF (v, J = 0) → LiF + X (X = Mu, 1H, 3H, 10H) at selected initial atom-diatom orientations, v = 0, 1 and 2 and 15 kcal mol-1 of collision energy have been carried out. A potential-induced rotation of the target molecule, favoured by the great angular forces of the system, is, as a rule, necessary for the reactive success of the collision if ν = 0 or 1, but not if ν = 2. The ease or difficulty of rotation according to whether the molecule is light (MuF or 1HF) or heavy (3HF or 10HF) explains most of the observed orientational dependence of reactivity.

Quasiclassical trajectory simulation of the O(1D)+HCl→OH+Cl, ClO+H reactions on an improved potential energy surface

The CASSCF (complete active space self-consistent field) and MRCI (multi-reference configuration interaction) ab initio points of Hernandez et al. (J. Chem. Phys., 1996, 105, 2710) have been supplemented with new calculations of the same level of theory at selected geometries, and the whole set was used to fit an improved ground (11A′) potential energy surface (PES) for the O(1D)+HCl system. The points were fitted by a polynomial expansion in bond order coordinates, and the electronic excitation of the isolated oxygen atom was taken care of by a proper, smooth lowering of the HCl potential at long internuclear distances. This ensures a common dissociation to all ground state atoms. On the fitted surface quasiclassical trajectories have been run under conditions similar to those of the experiment. A very good agreement with beam data of Balucani et al. (Chem. Phys. Lett., 1991, 180, 34) was found for the LAB (laboratory frame) angular distribution and the time-of-flight spectra of ClO. A good agreement with the infrared chemiluminescence and laser induced fluorescence data of Kruus et al. (J. Chem. Phys., 1988, 88, 985) was found for the inverted vibrational distribution of the OH product.

Ab initio calculations and dynamical tests of a potential energy surface for the Na+FH reaction

Ab initio calculations of the potential energy surface for the ground state Na+FH reaction were performed. Calculated potential energy values were fitted using a polynomial in bond order coordinates. Quasiclassical trajectories integrated on the fitted surface were used to calculate reactive properties of the system. Calculated quasiclassical properties agree with available experimental information. Quasiclassical trajectories allowed also a rationalization of the reactive dynamics of the system.

Two‐vector correlations and microscopic branching in chemical dynamics: Alignment and orientation effects for the Mg+HF→MgF+H reaction

Correlations between vector parameters characterizing the Mg+HF→MgF+H reaction have been investigated by carrying out extended trajectory calculations. For this study, use has been made not only of the traditional trajectory output, but also of vector distributions and related Legendre moments. Particular attention has been paid to the correlation of the direction of the final rotational angular momentum to that of the reagents’ relative velocity. For reactive events, the final rotational angular momentum was found to be perpendicularly polarized. The degree of alignment is high for trajectories taking a direct path from the saddle to the product asymptote, while the alignment is partially disrupted for those sampling the intermediate HMgF well. These results are compared with predictions from the constrained product orbital angular momentum (CPOAM) model.