A Quantum Wave Packet Dynamics Study of the N( 2 D) + H 2 Reaction † Tian-Shu Chu, ‡ Ke-Li Han, ‡ and Anto ´ nio J. C. Varandas* ,§ State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China, and Departamento de Quı ´mica, UniVersidade de Coimbra, 3004-535 Coimbra, Portugal ReceiVed: August 15, 2005; In Final Form: October 24, 2005 We report a dynamics study of the reaction N( 2 D) + H 2 (V)0, j)0-5) f NH + H using the time-dependent quantum wave packet method and a recently reported single-sheeted double many-body expansion potential energy surface for NH 2 (1 2 A′′) which has been modeled from accurate ab initio multireference configuration- interaction calculations. The calculated probabilities for (V)0, j)0-5) are shown to display resonance structures, a feature also visible to some extent in the calculated total cross sections for (V)0, j)0). A comparison between the calculated centrifugal-sudden and coupled-channel reaction probabilities validate the former approximation for the title system. Rate constants calculated using a uniform J-shifting scheme and averaged over a Boltzmann distribution of rotational states are shown to be in good agreement with the available experimental values. Comparisons with other theoretical results are also made. 1. Introduction Compared with the case of ground-state atomic nitrogen, N( 4 S), the reaction of N( 2 D) with molecular hydrogen is significantly more reactive and hence accessible to experimental measurement. Although evidence of an abstraction mechanism for this reaction has been reported in an early experimental work 1 on the vibrational distribution of the NH product as well as in several calculations, 2,3 it is now widely recognized that such a reaction belongs to the family of insertion reactions occurring on potential energy surfaces with deep wells. For this reason, experimental and theoretical work 4-15 has recently been diverted to it from the prototype O( 1 D) + H 2 reaction which has been the most extensively studied thus far. 16-19 The first global potential energy surface for the 1 2 A′′ state of NH 2 favoring an insertion mechanism for the title reaction has been reported by Pederson et al. 4 using the reproducing kernel Hilbert space (RKHS) interpolation method based on high level ab initio points. Using this surface, the same group performed detailed quasiclassical trajectory (QCT) studies 4,10 of the title reaction, having found that the calculated forward- backward symmetric angular distribution and vibrational dis- tributions in the products were in good agreement with the experimental measurements of Umemoto et al. 5,6 This ab initio surface has also been utilized for quantum mechanical studies 7-9 of the title reaction with good results. In particular, time- independent quantum studies 7 led to differential cross sections characteristic of a forward-backward symmetry, in good agreement with the available measurements. 6 Such an observa- tion is indicative of insertion dynamics, thus disputing earlier experimental results 1 that suggested a population inversion in the product nascent vibrational distribution. Further confirmation comes from a combined experimental plus theoretical investiga- tion by Balucani et al., 8 who have found good agreement between the calculated quantum time-independent and experi- mental differential cross sections. This group 8 has also discussed quantum effects in the title reaction by comparing their results with those obtained from QCT calculations. In turn, Alagia et al. 9 extended such studies to the N( 2 D) + D 2 reaction having found that both the experimental and QCT results show nearly backward-forward symmetric product angular distributions. Since there are five electronic doublet states correlating with the N( 2 D) + H 2 reactants, the title reaction can also serve as a prototype for studying nonadiabatic effects, an issue recently addressed by Schatz and co-workers 10,11 who have performed QCT calculations using the two surfaces of 2 A′′ and 2 A′ symmetries that form a Renner-Teller coupled pair of 2 Π symmetry. Although the 2 A′ state correlates adiabatically with the a 1 ∆ state of NH, rather than with the 3 Σ - state of interest in the present work, it may still contribute to production of NH( 3 Σ - ) + H due to the Renner-Teller degeneracy with 2 A′′ (the locus of degeneracy corresponds to a one-dimensional seam in the three-dimensional configuration space of the molecule) for linear HNH geometries. Further theoretical efforts focusing on the refinement and improvement of the 1 2 A′′ potential energy surface of NH 2 should also be mentioned at this stage, in particular the work of Ho et al., 12 Varandas and Poveda, 20 and Qu et al. 13 The former group reported a new RKHS potential energy surface for the 1 2 A′′ state of NH 2 based on 2715 multireference configuration- interaction (MRCI) points of similar quality as those reported in their earlier study, 4 while the other two groups have almost simultaneously calculated potential energy surfaces for the same system from internally contracted MRCI calculations using an aug-cc-pVQZ basis set. By interpolating their MRCI points with three-dimensional cubic splines and using an additional potential energy surface for the lowest quartet surface of NH 2 , Qu et al. 13 have further run QCT calculations of the rate constant for the reactions NH + D f ND + H and NH + D f N + HD and compared the results with their own experimental measurements. In turn, Varandas and Poveda 20 modeled an accurate DMBE (double many-body expansion) potential energy surface for NH 2 - (1 2 A′′) by using their own 1498 MRCI/aug-cc-pVQZ points † Part of the special issue “William Hase Festschrift”. * Corresponding author e-mail address: varandas@qtvs1.qui.uc.pt. ‡ Chinese Academy of Sciences. § Universidade de Coimbra. 1666 J. Phys. Chem. A 2006, 110, 1666-1671 10.1021/jp054572n CCC: $33.50 © 2006 American Chemical Society Published on Web 12/06/2005