On the Mechanism for Nitrate Formation via the Peroxy Radical + NO Reaction Jieyuan Zhang, ² Tim Dransfield, ‡ and Neil M. Donahue* ,§ Departments of Chemistry and Chemical Engineering, Carnegie Mellon UniVersity, Pittsburgh, PennsylVania 15213, and HarVard UniVersity, Cambridge, Massachusetts 02138 ReceiVed: May 3, 2004; In Final Form: August 4, 2004 We present a master equation study of organic nitrate formation from the peroxy radical (RO 2 ) + NO reaction. The mechanism is constrained by both quantum chemical calculations of the potential energy surface and existing yield data. This mechanism displays heretofore unrecognized features of the system, including distinct conformers of a critical peroxynitrite (ROONO) intermediate that do not interconvert and a dual falloff behavior driven by collisional stabilization in multiple wells. These features have significant implications for atmospheric chemistry; in particular, only a fraction of the ROONO intermediates may easily isomerize to nitrates, resulting in a limit to total nitrate production. Existing mechanisms, extrapolated to low temperature and high pressure, produce nitrate almost exclusively. As a consequence, hydrocarbon oxidation sequences based on these mechanisms do not propagate radical chemistry, which is inconsistent with available experimental data. To reproduce observed nitrate yields, we model a transition state from the ROONO intermediate to RONO 2 that differs considerably from the few found in computational studies. Specifically, the data require that this transition state energy lie well below the energy of separated radical products (RO + NO 2 ), while computational studies find the transition state at higher energies. A second feature of yield data is difficult to model; to enable collisional stabilization of C 5 systems, as observed, we reduce the unimolecular decomposition rate constants from the ROONO intermediate by a factor that is at the far end of the plausible range. However, with these experimental constraints in place, the model successfully reproduces multiple features of existing data quantitatively, including both high- and low-pressure asymptotes to nitrate production as well as the observed shifting of pressure falloff curves with carbon number. Consequently, we present a new parametrization of nitrate yields, providing interpolation equivalent to existing parametrizations but dramatically improved extrapolation behavior. 1. Introduction If one heretofore unobserVed reaction intermediate cannot explain published data, why not inVoke two? The reaction of peroxy radicals (RO 2 ) with nitric oxide (NO) sits in the very center of atmospheric chemistry. On one hand, it may yield radical products, alkoxy radicals (RO) and NO 2 , effectively producing ozone after NO 2 photolysis. On the other hand, it may yield nitrates (RONO 2 ), effectively terminating the radical chain reaction in the atmosphere. It is widely accepted that both channels share a common intermediate, a peroxy nitrite (ROONO), whose subsequent decomposition governs the branch- ing between radicals and nitrate, as shown in the following sequence: 1,2 What has not been recognized is that two generally distinct conformers of the ROONO intermediate have quite separate chemical behaviors and fates. We will refer to these conformers as cis- and trans-ROONO (describing the O-O-N-O con- formation). In particular, the isomerization between these two forms is quite slow, compared to the other reactions open to each conformer, and only one conformer can easily isomerize to become a nitrate. Thus one conformer connects only to the radical products, while the other connects to both radicals and nitrates. We shall argue that this easily isomerized ROONO is the trans form, though the only important conclusion is that the conformers behave differently. The first purpose of this paper is to establish this assertion and to discuss its implications. The second purpose of this paper is to build the case that the observed pressure dependence for nitrate yields (for all but the very smallest species) is due to the collisional stabilization of these ROONO intermediates and their subsequent, rapid, thermal decomposition. Nitrate yields are thus controlled by three factors: (1) the initial branching of the RO 2 + NO reaction between cis- and trans-ROONO; (2) the ratio of nitrate to radical formation for trans-ROONO at high (reactant) energy (which controls the low-pressure production of nitrate); and (3) the critical energy difference between the nitrate and radical pathways out of trans-ROONO (which controls the high- pressure limit and the temperature dependence of nitrate formation). An intriguing consequence of this is that the puzzling variation of nitrate yields among different classes of peroxy radicals (primary, secondary, and tertiary alkyl-peroxy 3 ; -hy- droxy-peroxy radicals 4-7 ) may be governed in large measure by the initial branching between cis- and trans-ROONO. * To whom correspondence should be addressed. E-mail: nmd@ andrew.cmu.edu. ² Carnegie Mellon University, Department of Chemical Engineering. ‡ Harvard University, Department of Chemistry and Chemical Biology. § Carnegie Mellon University, Department of Chemistry. RO 2 + NO f ROONO (1) ROONO f RO + NO 2 (2) f RONO 2 (3) 9082 J. Phys. Chem. A 2004, 108, 9082-9095 10.1021/jp048096x CCC: $27.50 © 2004 American Chemical Society Published on Web 09/25/2004