A Quasiclassical Trajectory Study of H + N₂O (ν₁, ν₂, ν₃)

Abstract

We present a quasiclassical trajectory study of state resolved cross sections, rate coefficients, and product energy partitioning for the reaction H + N2O using a potential surface which is based on ab initio calculations. This surface allows for hydrogen attack on either end of N2O, with N-atom attack giving an intermediate complex HNNO, which can then dissociate into NH + NO or undergo 1,3-hydrogen migration to produce OH + N2. O-atom attack, which involves a higher barrier than N-atom attack, leads only to direct reaction to form OH + N2. We find that the dominant mechanism for the production of OH + N2 changes with energy, with N-atom attack being dominant near the threshold and O-atom attack becoming more important when that pathway becomes energetically accessible. At reagent translational energies of 2 eV, the OH + N2 product is dominant by a factor of 15 over NH + NO, in reasonable agreement with experimental results. Our product OH and NH vibrational distributions also match the experiment, but the partitioning of energy between N2 vibration−rotation and relative translation of OH + N2 is seriously different. Factors which control this difference in energy partitioning are examined, and it is concluded that the difference in energy partitioning between O-atom attack and N-atom attack is not sufficient to explain the discrepancy. We also examine cross sections for different N2O initial states, and we find that N−N stretch is more effective than N−O stretch in enhancing overall reactivity near the OH + N2 reactive threshold, while bend excitation is ineffective. At higher energies, N−O excitation becomes more effective as a result of the change in reaction mechanism from N-atom attack to O-atom attack, and N−N stretch excitation is ineffective. These results are in qualitative accord with recent mode-specific rate measurements. Our calculated thermal rate coefficients for formation of OH + N2 are below measured values, but the agreement is much better for a slightly modified surface in which the barriers for O- and N-atom attack are lowered by 0.1 eV. The trajectory results indicate that O-atom attack rather than N-atom attack is the predominant reaction mechanism at temperatures above 700 K.

The H+N2O→OH(2Π3/2,v′,N′)+N2 reaction at 1.5 eV: New evidence for two microscopic mechanisms

“…19 Although the QCT calculations favor reaction at the O atom end of NNO above ϳ1.0 eV, it is striking that the pattern of energy disposals for the ''indirect'' mechanism below ϳ1.0 eV is predicted to be little different from that arising from the direct process. 15 Thus, the calculated product energy disposals for neither the direct nor the indirect pathways reproduce the high N 2 internal energy release observed experimentally. As noted above, if reaction does proceed significantly via H atom attack at the terminal N atom at 1.5 eV, there must be large errors in the barrier heights and the shape of the PES employed by Bradley and Schatz 15 ͑and in the ab initio points on which they are based 16 ͒, particularly in the H-atom migration and exit channel regions.…”

“…Although proof of the production of rotationally excited N 2 in the bimolecular reaction must await direct experimental verification, the comparison between the two data sets is very striking. It suggests that the potential energy surface describing the N-N-OH bending region in the exit channel of the bimolecular reaction must be very different from that described by the PES employed by Bradley and Schatz in their QCT study, 15 and probably resembles that for the photodissociation of N 2 O via its first absorption band.…”

“…To complement these experimental investigations, the most detailed theoretical work thus far undertaken has been the quasiclassical trajectory ͑QCT͒ study of the reaction dynamics by Bradley and Schatz, 15 employing a global semiempirical potential energy surface ͑PES͒ obtained from a fit to the ab initio data of Walch. 16,17 The energies of some of the stationary points calculated by Walch 16,17 are shown in Fig.…”

Quasiclassical trajectory study of the dynamics of the H+N2O reaction on a new potential energy surface

“…Theoretical studies of the dynamics of the HϩN 2 O and its reverse NHϩNO reactions have been performed using the quasiclassical trajectory method ͑QCT͒ by Bradley et al 8,9 The dynamical calculations were mainly performed using a global PES, hereafter denoted as the BS PES, based on the ab initio calculations of Walch 6 and on the calculation and addition of more, less accurate, ab initio data for a better description of the reaction path for 1,3-hydrogen migration and for the H addition to N 2 O via the H-ONN transition state. 8 The energetics of the stationary points of the BS PES was modified with respect to the original calculations of Walch by fixing the difference between NHϩNO and H a͒ Permanent address: Departamento de Química Física I, Facultad de Química, Universidad Complutense, 28040 Madrid, Spain.…”

“…Electronic mail: jfc@legendre.quim.ucm.es ϩN 2 O at the experimental exoergicity. 8 The QCT study of the HϩN 2 O reaction on that PES by Bradley and Schatz 9 showed that the direct mechanism for production of OH ϩN 2 was dominant for collision energies above 0.8 eV. However, these calculations predicted that OH and N 2 products were born with low rotational and vibrational excitation, with most of the available energy going into translation, in clear disagreement with the experimental observations of Böhmer et al 4 Recently, Fletcher and Wocjick 10 have measured the total reaction cross section, as a function of the collision energy, R (E t ), of the photon initiated HϩN 2 O→OHϩN 2 reaction in a bulk experiment over collision energies in the range 0.87-1.95 eV.…”