Journal of Molecular Structure (Theochem) 531 (2000) 283±299
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Comparative ab initio MO investigation on the reactivity of the Ä 2B1) radical species in their three NH(a 1D), NH(X 3S 2) and NH2(X bimolecular abstraction gas-phase reaction with the HN3 molecule E. Henon*, F. Bohr Laboratoire de Chimie-Physique, GSMA, UPRESA 6089 CNRS, Universite de Reims Champagne-Ardenne, U.F.R Sciences Exactes et Naturelles, Moulin de la Housse, B.P. 1039, 51687 Reims Cedex 2, France Received 9 September 1999; received in revised form 28 February 2000; accepted 29 February 2000
Abstract The purpose of this paper is to investigate in a comparative manner the reactivity of the three radical species NH(a 1D), Ä 2B1) in their bimolecular abstraction reaction with HN3, by using ab initio molecular orbital calculations NH(X 3S 2) and NH2(X within the conventional Transition State Theory. The emphasis has been put on the inclusion of the electronic correlation effects at the geometry optimization step, particularly important for azide molecules, by means of the Complete Active Space SCF (CASSCF) method. Energetic information has been obtained by carrying out high-level correlated methods: MRSDCI and CASPT2, with the ANO basis set. The preliminary H 1 NH 3 ! H2 1 NH2 and H 1 HN3 ! NH2 1 N2 reactions have been used as a ®rst test of the behaviour of both CASPT2 and MRCI approachs in the calculation of energy barriers in view of TST rate constant predictions for the three aforementionned reactions. The results of optimized geometries are given and point to a Ä 2B1) and NH(X 3S 2) radicals. Our comparavery similar planar structure (Cs) for the three reactions involving NH(a 1D), NH2(X tive study emphasizes large variations between the three barrier heights in the series: 2.9, 8.7 and 16.3 kcal/mol, respectively for the singlet, doublet and triplet states processes. Finally, the bimolecular rate constants obtained for all the reactions are discussed. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Hydrazoic acid HN3; Boimolecular abstraction; NH or NH2 radicals
1. Introduction For many years, a large number of experimental studies in the area of gaseous detonation have been devoted to hydrazoic acid HN3 [1±5]. The knowledge of the HN3 decomposition mechanism is of great interest in order to test detonation propagation models [6]. Thermal decomposition studies of such a small * Corresponding author. Tel.: 1 33-326-91-33-66; fax: 1 33326-91-33-98. E-mail address:
[email protected] (E. Henon).
energetic molecule should provide a link between chemical kinetic and detonation structure. Indeed, because of the small size of this molecule, the number of chemical species present in the explosive decomposition reaction is small. Thus, the kinetic mechanism should be simpler. Studies of the photochemical decomposition of HN3 have played a great role in the elucidation of the decomposition mechanism [7±14]. However, explosive decomposition modelling meets some problems owing to the lack of kinetic data on elementary reactions and also to uncertainties on rate constant values due to extrapolation at high
0166-1280/00/$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0166-128 0(00)00452-8
