J. F. Castillo

Spin–orbit effects in quantum mechanical rate constant calculations for the F+H2→HF+H reaction

Exact and approximate quantum mechanical calculations of reaction probabilities and cumulative reaction probabilities have been carried out for the F+H2 reaction on the ab initio adiabatic potential energy surfaces by Stark and Werner (SW) and by Hartke, Stark, and Werner (HSW), the latter including spin–orbit corrections in the entrance channel. These data have been employed to obtain thermal rate constants for the title reaction in the temperature range 200–700 K. The exact and approximate results have been compared with experimental determinations and previous theoretical predictions. In particular, the reaction probabilities obtained on the HSW surface are found to be in very good agreement with recent calculations by Alexander et al. [J. Chem. Phys. 109, 5710 (1998)] based on the exact treatment of spin–orbit and Coriolis coupling for this system. However, the rate constants calculated on the HSW PES are systematically lower than the experimental values, which indicates that the height of the adiabat...

A quantum mechanical and quasi-classical trajectory study of the Cl+H2 reaction and its isotopic variants: Dependence of the integral cross section on the collision energy and reagent rotation

Quantum mechanical (QM) and quasi-classical trajectory (QCT) calculations have been performed for the Cl+H2, Cl+D2, Cl+HD→ HCl(DCl)+D(H) reactions in order to determine integral cross sections as a function of collision energy and for different reagent rotational quantum numbers using the recent ab initio BW2 potential energy surface (PES) by Bian and Werner [J. Chem. Phys. 112, 220 (2000)]. The results are compared with experimental data obtained by using the Doppler-selected time-of-flight technique. It has been found theoretically by both the QM and QCT methods that reagent rotation enhances reactivity in agreement with experiment. The QM results are found to be in quantitative agreement with the experimental excitation functions for the Cl+p-H2 and Cl+n-H2 reactions, whereas those obtained quasi-classically fail to reproduce the experimental data. These results are in strong contrast with those reported on the previous G3 PES, in which QM and QCT calculations predicted that reactivity decreases with r...

Semiclassical angular scattering in the F+H2→HF+H reaction: Regge pole analysis using the Padé approximation

Abstract We apply the complex angular momentum analysis to study state-selective forward scattering in the F+H 2 reaction. The scattering matrix calculated by Castillo et al. is used. The S-matrix element is analytically continued into the complex plane of the total angular momentum with the help of the Pade approximation. Resonance (Regge) poles are identified and their residues evaluated. The semiclassical optical model is corrected to account for finite lifeangles of leading resonances. For chosen transitions, the forward scattering enhancement is shown to be a resonance effect due to the decay of a Regge state with angular life of about 32°.

Quantum mechanical and quasi-classical trajectory reaction probabilities and cross sections for the S(1D)+ H2,D2,HD insertion reactions

Time-independent quantum mechanical (QM) and quasi-classical trajectory (QCT) calculations of reaction probabilities at total angular momentum J = 0 as a function of collision energy for the S(1D) + H2(v = 0, j = 0), S(1D) + D2(v = 0, j = 0) and S(1D) + HD(v = 0, j = 0) reactions have been performed on a recent ab initio potential energy surface. In addition, QCT calculations of integral cross sections as a function of collision energy (the excitation functions) have been carried out for the same reactions. The QCT excitation functions and those obtained by applying a capture model to the QM reaction probabilities are compared with the available experimental determinations. The QCT and QM methods reproduce the shape of the measured excitation functions quite satisfactorily. However, the theoretical intramolecular and intermolecular isotope effects are in disagreement with those obtained experimentally.

The dynamics of the O(1D)+HD reaction: A quasiclassical trajectory multisurface study

Integral and differential cross sections for the O(1D)+HD reaction have been obtained from adiabatic and nonadiabatic quasiclassical trajectory calculations performed on new ab initio versions of the 1A′, 1A″ and 2A′ potential energy surfaces at the collision energies of 0.089 and 0.196 eV (2.05 and 4.53 kcal/mol, respectively). Results are reported for both the OH+D and OD+H exit channels of reaction. The new data are compared with those from previous theoretical studies employing other potential energy surfaces, and are also used to simulate experimental differential cross sections obtained from recent molecular beam measurements, which are partially resolved in the internal states of the products. The comparison provides evidence that excited electronic states do participate in the title reaction at 0.196 eV, but that their contribution, particularly that of the A″ state, is overestimated by the quasiclassical trajectory (QCT) calculations employing the latest, and most accurate, potential energy surfaces.

Energy dependence of forward scattering in the differential cross section of the H+D2→HD(v′=3,j′=0)+D reaction

Extensive time-independent quantum mechanical scattering calculations for the H+D2(v=0,j=0) reaction have been performed in the collision energy range 1.39–2.20 eV on the Boothroyd–Keogh–Martin–Peterson potential energy surface. The theoretical differential cross sections (DCS) obtained for the H+D2→HD(v′=3,j′=0)+D channel of the reaction have been compared with recent measurements by Zare and co-workers over the collision energy range 1.39–1.85 eV using the photoloc technique [S. C. Althorpe et al., Nature (London) 416, 67 (2002)]. An excellent agreement between experiment and theory has been found for most of the collision energies studied. In particular, the appearance and evolution of forward scattering with collision energy observed experimentally has been quantitatively reproduced by the theoretical calculations. An analysis of the theoretical results, including a semiclassical complex angular momentum analysis, have been performed in order to ascertain the origin of the sharp forward peaks in the DCS.

The stereodynamics of the O(1D)+HD reaction on the ground 1 1A′ and excited 1 1A″ potential energy surfaces

The stereodynamics of the O(1D)+HD(v=0,j=0) reaction has been studied theoretically on the Dobbyn–Knowles (DK) ab initio potential energy surfaces (PES). Quasiclassical trajectory (QCT) calculations have been carried out on the 1 1A′ ground state PES and both QCT and quantum mechanical (QM) calculations have been performed on the 1 1A″ excited state PES. The product rotational angular momentum (j′) polarization in the (k,k′) scattering frame (i.e., the k–k′–j′ vector correlation) has been determined for selected rovibrational states of the OH(v′,j′)+D and OD(v′,j′)+H product channels at the collision energy of 0.196 eV. The agreement between the QM and QCT results is very good and indicates that the “abstraction” reaction occurring on the excited 1 1A″ PES yields OH and OD products strongly polarized, in strong contract with the “insertion” reaction occurring on the ground 1 1A′ PES, which leads to an essentially isotropic distribution of the product rotational angular momentum. The differentiated stereod...

Differential cross sections and Regge trajectories for the F + H2 → HF + H reaction

We analyse reactive angular scattering in the F+H2→HF+H reaction using accurate S-matrix elements. With the help of the Pade approximation we obtain positions of the resonance poles of the S-matrix in the complex planes of both energy (E) and total angular momentum (J). We reconstruct the Regge trajectories and correlate the Regge poles affecting angular scattering at higher energies with a resonance pole at J=0. Even though the lifetime of the resonance is short, the lifeangle of the corresponding Regge state is large enough to enhance forward scattering.

Accurate Time-Dependent Wave Packet Study of the Li + H2+ Reaction and Its Isotopic Variants

The dynamics and kinetics of the Li + H₂⁺ reaction and its isotopic variants (D₂⁺ and T₂⁺) have been studied by using a time-dependent wave packet (TDWP) coupled-channel (CC) method on the ab initio potential energy surface (PES) of Martinazzo et al. [J. Chem. Phys. 2003, 119, 21]. Total initial v = 0, j = 0 state-selected reaction probabilities for the Li + H₂⁺ reaction and its isotopic variants have been calculated from the threshold up to 1 eV for total angular momenta J from 0 to 90. Integral cross sections have been evaluated from the reaction probabilities at collision energies from threshold (≈0.2 eV) up to 1.0 eV collision. The calculated rate constants as a function of temperature show an Arrhenius type behavior in the 200 ≤ T ≤ 1000 K temperature interval. It has been found to be a considerable large intermolecular kinetic isotope effect. The TDWP-CC results are in overall good agreement with those obtained applying the TDWP Centrifugal-Sudden (CS) approximation, showing that the CS approximation is rather accurate for the title reaction.

Quantum Mechanical Wave Packet and Quasiclassical Trajectory Calculations for the Li + H2+ Reaction†

The dynamics and kinetics of the Li + H2(+) reaction have been studied by means of quantum mechanical (QM) real wave packet, wave packet with flux operator, and quasiclassical trajectory (QCT) calculations on the ab initio potential energy surface of Martinazzo et al. [J. Chem. Phys., 2003, 119, 21]. Total initial state-selected reaction probabilities for the title reaction have been calculated for total angular momentum J = 0 at collision energies from threshold up to 1 eV. Wave packet reaction probabilities at selected values of the total angular momentum up to J = 60 are obtained using the centrifugal sudden approximation (CSA). Integral cross sections and rate constants have been calculated from the wave packet reactions probabilities by means of a refined J-shifting method and the separable rotation approximation in combination with the CSA for J > 0. The calculated rate constants as function of temperature show an Arrhenius type behavior. The QM results are found to be in overall good agreement with the corresponding QCT data.