Elsevier

Chemical Physics

Volume 308, Issue 3, 31 January 2005, Pages 201-210
Chemical Physics

A multiproperty analysis of the OH + H2(D2,HD) potential energy surface

https://doi.org/10.1016/j.chemphys.2004.05.029Get rights and content

Abstract

Dynamical and kinetic properties of the OH + H2(D2,HD) reaction have been investigated in detail by carrying out extended quasiclassical trajectory calculations on the most recently proposed potential energy surface. Computed values of the reactive properties of H2, D2 and HD colliding with OH are compared with measured data and with results obtained on other potential energy surfaces. In most cases the new surface provides excellent estimates of the measured quantities. To better evaluate the accuracy of the proposed surface and the implications of some assumptions made when performing the calculations, the comparison has been extended to available quantum results.

Introduction

The reaction OH(vOH,jOH) + H2(vH2,jH2)  H2O(ν1,ν2,ν3) + H has been extensively investigated in the past both theoretically and experimentally (see for example the recent review [1]). For this system Collins and collaborators produced a potential energy surface (PES) based on an extended set (more than 2000) of high level ab initio values [2]. The PES (YZCL2) is fitted using the modified Shepard method [3], [4] in terms of the inverse internuclear distances. Since ab initio potential energy values were calculated at high level and also gradients and hessians were evaluated, the YZCL2 PES is believed to be quite accurate and to lead to an excellent agreement between calculated and measured dynamical properties.

Quantum (QM) dynamical calculations for the direct OH + H2(D2,HD) [2], [5], [6], [7] and the reverse H + H2O(D2O) [8], [9], [10], [11], [12] reaction have been already performed on the YZCL2 PES using reduced and full dimensional time-dependent techniques. The theoretical investigation has also been extended to the photodetachment processes of OH3 [13]. As to quasiclassical trajectory (QCT) calculations, only the reverse H + H2O(D2O) reaction has been investigated on the YZCL2 surface [11], [12], [14]. For this reaction the integral cross-sections, the OH(OD) and H2(HD) product rotational distributions, the rotational alignment parameters, the rovibrational quantum state-resolved centre of mass angular scattering distributions and the H2(HD) internal energy release distributions have been determined using QCT methods. In general, theoretical results were found to be in good agreement with experimental data.

In this paper, we carry out a multiproperty analysis of the direct OH + H2(D2,HD) reactions on the YZCL2 PES by comparing several calculated and measured quantities. The comparison is extended to results obtained on the OC PES [15] using not only (full dimensional) QCT methods [16], [17], [18] but also reduced and full (though at zero total angular momentum J=0) dimensionality QM ones [16]. The comparison is also extended to similar calculations performed on another PES proposed by Schatz and coworkers [19], [20], [21] (called WSLFH). Finally, a comparison is also made with quantum results obtained by a partial averaging over the initial internal states. This makes the multiproperty analysis of the proposed PES more rigorous and the evaluation of the assumptions made when performing the quantum calculations easier.

In Section 2, a comparison of quasiclassical properties with measured values is given. A detailed examination of the rate coefficients and cross-sections is made. In Section 3 the comparison is extended to quantum calculations by focusing on the effect of rotational excitation of the reactants on the system reactivity.

Section snippets

Quasiclassical versus measured data

Quasiclassical calculations were performed using a customized version of the Venus program [22]. The YZCL2 potential energy routine was kindly provided by the authors of [2] while the related analytical derivatives were worked out in our laboratory.

Important parameters of the trajectory calculations are the integration time step (whose value is critical for the balance between the accuracy achieved and the computing time used) and the maximum value of the bond length considered for its break up

Quasiclassical versus quantum properties

In this section, we carry out a comparison between fully converged time dependent QM1 and quasiclassical properties both calculated on the YZCL2 surface. The purpose of the comparison is not only to further investigate the accuracy of the YZCL2 PES but also to analyse of the suitability of QCT calculations for the title system and the validity of the approximations introduced to evaluate the QM

Conclusions

In this paper, we present a quasiclassical trajectory investigation of the OH + H2  H + H2O reaction and its isotopic variants OH + D2 and OH + HD on the YZCL2 PES [2] using its analytical derivatives. The study has focused on the validation of the YZCL2 PES by comparing kinetics and dynamics calculated QCT properties of the system with experimental data. A comparison with values calculated on other available surfaces is also made.

The first considered quantity is the thermal rate coefficient of the OH + H2

Acknowledgements

Partial financial support from MCyT, MIUR, ASI and CNR is acknowledged. Part of this work has been performed within the activities of the D23 Action of the COST in Chemistry European Initiative.

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