Comparative ab initio MO investigation on the reactivity of the three NH(a1Δ), NH(X3Σ-) and NH2((X)over-tilde2B1) radical species in their bimolecular abstraction gas-phase reaction with the HN3 molecule

The purpose of this paper is to investigate in a comparative manner the reactivity of the three radical species NH(a(1)Delta), NH(X(3)Sigma (-)) and NH2((X) over tilde B-2(1)) in their bimolecular abstraction reaction with HN3, by using ab initio molecular orbital calculations 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 + NH3 --> H-2 + NH2 and H + HN3 --> NH2 + N-2 reactions have been used as a first 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 very similar planar structure (C-s) for the three reactions involving NH(a(1)Delta), NH2((X) over tilde B-2(1)) and NH(X(3)Sigma (-)) radicals. Our comparative 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. (C) 2000 Elsevier Science B.V. All rights reserved.