Combined Crossed Molecular Beam and Theoretical Studies of the N( 2 D) + CH 4 Reaction and Implications for Atmospheric Models of Titan † Nadia Balucani,* Astrid Bergeat, ‡ Laura Cartechini, § Gian Gualberto Volpi, and Piergiorgio Casavecchia Dipartimento di Chimica, UniVersita ` degli Studi di Perugia, 06123 Perugia, Italy Dimitris Skouteris Dipartimento di Matematica e Informatica and Dipartimento di Chimica, UniVersita ` degli Studi di Perugia, 06123 Perugia, Italy Marzio Rosi Dipartimento di Ingegneria CiVile e Ambientale and ISTM-CNR, c/o Dipartimento di Chimica, UniVersita ` degli Studi di Perugia, 06123 Perugia, Italy ReceiVed: May 8, 2009; ReVised Manuscript ReceiVed: July 8, 2009 The dynamics of the H-displacement channel in the reaction N( 2 D) + CH 4 has been investigated by the crossed molecular beam (CMB) technique with mass spectrometric detection and time-of-flight (TOF) analysis at five different collision energies (from 22.2 up to 65.1 kJ/mol). The CMB results have identified two distinct isomers as primary reaction products, methanimine and methylnitrene, the yield of which significantly varies with the total available energy. From the derived center-of-mass product angular and translational energy distributions the reaction micromechanisms, the product energy partitioning and the relative branching ratios of the competing reaction channels leading to the two isomers have been obtained. The interpretation of the scattering results is assisted by new ab initio electronic structure calculations of stationary points and product energetics for the CH 4 N ground state doublet potential energy surface. Differently from previous theoretical studies, both insertion and H-abstraction pathways have been found to be barrierless at all levels of theory employed in this work. A comparison between experimental results on the two isomer branching ratio and RRKM estimates, based on the new electronic structure calculations, confirms the highly nonstatistical nature of the N( 2 D) + CH 4 reaction, with the production of the CH 3 N isomer dominated by dynamical effects. The implications for the chemical models of the atmosphere of Titan are discussed. 1. Introduction Chemical reactions involving atomic nitrogen are of relevance in a variety of natural environments, such as the upper terrestrial atmosphere 1 and the atmospheres of other planets, 2 and applied processes, where molecular nitrogen is deliberately introduced or present because it is the main component of air. Notable examples are plasma-induced chemical vapor deposition of nitrogen-doped diamond 3 and metal nitrocarburizing 4 using afterglow plasma or combustion in air. 5 Although kinetic studies are available 1,5 and reaction mechanisms have been speculated from rate constants, a better knowledge of the reactive behavior of atomic nitrogen requires an investigation at the level of reaction dynamics. The capability achieved in our laboratory to generate intense continuous supersonic beams of atomic nitrogen 6 has opened up the possibility of studying the reactive scattering of this species in crossed molecular beam (CMB) experiments with mass-spectrometric (MS) detection. 7-12 In recent years, we have investigated 7-12 several reactions of N atoms in their first excited metastable state, 2 D 5/2,3/2 (energy content: 230.0 kJ/mol; radiative lifetimes of 2 D 3/2 and 2 D 5/2 are 6.1 × 10 4 and 1.4 × 10 5 s, respectively 13 ). Notwithstanding its high energy content, the metastable character of N( 2 D) and the much larger rate constants of its reactions with respect to those of N( 4 S) 13,14 (especially when the reactive partner is a closed- shell molecule) suggest a potential role of this species in the above-mentioned chemical environments, similarly to the case of O( 1 D) as opposed to O( 3 P). For instance, the presence of N( 2 D) in the flame front has been invoked to explain chemi- ionization in ammonia/oxygen flames. 15 N( 2 D) was found to be the main product of the combustion reaction O( 3 P) + CN 16 and similar chemical production of N( 2 D) can probably occur to a significant extent in combustion and plasma systems since few reactants adiabatically correlate with the quartet state. For instance, the initiation chain reaction of the prompt NO mechanism, CH + N 2 f HCN + N, can form N( 4 S) only through a doublet-quartet intersystem-crossing (ISC), 17 which was found to have a low probability, 18 while the same reactants adiabatically correlate with N( 2 D). In particular, the reactions of N( 2 D) with simple hydrocarbons can be relevant in several discharge-induced processes, especially in chemical vapor deposition of nitrogen-doped diamond starting from N 2 /CH 4 mixtures, 3 and in the chemistry of nitrogen dominated planetary † Part of the special section “Chemistry: Titan Atmosphere”. * Corresponding author. Tel.: +39 075 585 5513. Fax: +39 075 585 5606. E-mail: nadia.balucani@unipg.it. ‡ Present address: Institut des Sciences Mole ´culaires, Universite ´ Bordeaux 1, 33405 Talence Cedex, France. § Present address: ISTM-CNR, c/o Dipartimento di Chimica, Universita ` degli Studi di Perugia, 06123 Perugia, Italy. J. Phys. Chem. A 2009, 113, 11138–11152 11138 10.1021/jp904302g CCC: $40.75  2009 American Chemical Society Published on Web 07/30/2009 Downloaded by UNIVERSITA DI PERUGIA on October 16, 2009 | http://pubs.acs.org Publication Date (Web): July 30, 2009 | doi: 10.1021/jp904302g