Dynamics of the N( 2 D) + D 2 Reaction from Crossed-Beam and Quasiclassical Trajectory Studies ² Nadia Balucani, Michele Alagia, ‡ Laura Cartechini, Piergiorgio Casavecchia,* ,§ and Gian Gualberto Volpi Dipartimento di Chimica, UniVersita ` di Perugia, 06123 Perugia, Italy Lisa A. Pederson and George C. Schatz* Department of Chemistry, Northwestern UniVersity, EVanston, Illinois 60208-3113 ReceiVed: October 3, 2000; In Final Form: January 8, 2001 The dynamics of the prototypical insertion reaction N( 2 D) + D 2 have been investigated in a combined experimental and theoretical study. Angular and velocity distributions of the ND product have been obtained in crossed molecular beam experiments with mass spectrometric detection at two collision energies (E c ) 3.8 and 5.1 kcal mol -1 ). The center-of-mass product angular and translational energy distributions have been derived; at both E c ’s, the former is found to be nearly backward-forward symmetric, reflecting an insertion dynamics, and the latter corresponds to a fraction of total available energy released in translation of 32%, indicating that the ND product is highly internally excited. Quasiclassical trajectory (QCT) calculations were performed on an accurate potential-energy surface obtained from large-scale ab initio electronic structure computations, and the results were compared to experiment. Generally good agreement was found between the experimental results and the theoretical predictions; however, small, yet significant, discrepancies point to some inaccuracy of the QCT treatment, calling for a quantum scattering study of the title reaction. I. Introduction Chemical reactions of nitrogen atoms with inorganic and organic molecules are significant in a wide variety of systems: earth-orbital environment, planetary and extraplanetary atmo- spheres, interstellar and circumstellar clouds, hydrocarbon air combustion, and charged particle collisions. 1,2 The study of N atom reactions has always represented a challenge to chemists. Traditionally, these reactions were investigated using “active nitrogen” 3-6 and the reaction mechanism speculated from early product analysis; in those studies, the only reactive species was assumed to be the ground state of atomic nitrogen, N( 4 S). 6,7 More recent and accurate rate constant measurements 8 have established that, when the reactive partner is a closed shell molecule, in most cases the reactivity of the ground state 4 S is practically negligible, whereas the first electronically excited metastable state 2 D (whose energy content is 55.1 kcal mol -1 with respect to the ground state) was found to be the most reactive low-lying state of atomic nitrogen. A deeper understanding of chemical reactions is provided by reaction dynamics studies. Only recently, the experimental investigation of N( 2 D) reactions at the microscopic level has become possible. Important reactions such as N( 2 D) + O 2 , thought to be responsible of the anomalously large concentration of NO (nitric oxide) in the upper atmosphere, 9 can now be tackled, as well as reactions of N( 2 D) with simple hydrocar- bons, 10,11 of great relevance in the atmosphere of Saturn’s moon Titan. 12 In this paper, we report experimental and theoretical results on the dynamics of the simplest N( 2 D) reaction, that with molecular hydrogen Specifically, by carrying out crossed molecular beam (CMB) experiments, we have derived reactive double differential cross sections (DCSs), and by using quasiclassical trajectory (QCT) methods, we have computed the dynamics on a recently developed doublet ground-state potential-energy surface (PES) of NH 2 . 13 By combining experimental findings and theoretical predictions, a clear insight into the micro-mechanism has been gained for the first time on an N( 2 D) reaction. The reason for focusing our attention on this reactive system is that not only does it represents a prototypical case (because of its relative simplicity) for understanding the chemical behavior of atomic nitrogen in the 2 D state but it is also a useful prototype for a more general understanding of the class of reactions usually termed as insertion reactions. The synergism between experiment and theory in the field of reaction dynamics has, indeed, resulted in detailed comparisons between state-of- the-art experiment and state-of-the-art theory for the dynamics of benchmark three-atom reactions (H + H 2 ,F + H 2 , and Cl + ² Part of the special issue “Aron Kuppermann Festschrift”. It is a great pleasure and most appropriate to dedicate, on the occasion of his 75th birthday, this paper to Professor Aron Kuppermann, the pioneer and inspirer of quantum scattering studies on the dynamics of elementary chemical reactions that have stimulated a great amount of experimental dynamics work over the past 30 years. * To whom correspondence should be addressed. Piergiorgio Casavec- chia, Dipartimento di Chimica, Universita ` di Perugia, Via Elce di Sotto, 8, 06123 Perugia, Italy. E-mail: piero@dyn.unipg.it. Tel: +39 075 585 5514. Fax: +39 075 585 5606. http://www.chm.unipg.it/chimgen/mb/exp3/ casavecchia.html ‡ Present address: INFM, Sincrotrone Elettra. 34012, Trieste, Italy. § Visiting Miller Professor, Department of Chemistry, University of California, Berkeley. N( 2 D) + H 2 ( 1 Σ g + ) f NH(X 3 Σ - ) + H( 2 S) ΔH° 0 )-33.2 kcal mol -1 (1) 2414 J. Phys. Chem. A 2001, 105, 2414-2422 10.1021/jp0036238 CCC: $20.00 © 2001 American Chemical Society Published on Web 02/20/2001