J. Phys. Chem. A 2010, 114, 6235–6249 6235 Formation and Decomposition of Chemically Activated and Stabilized Hydrazine Rubik Asatryan,*,† Joseph W. Bozzelli,†,* Gabriel da Silva,‡ Saartje Swinnen,§ and Minh Tho Nguyen§ Department of Chemistry and EnVironmental Science, New Jersey Institute of Technology, Newark, New Jersey 07102, USA, Department of Chemical and Biomolecular Engineering, The UniVersity of Melbourne, Victoria 3010, Australia, and Department of Chemistry, Katholieke UniVersiteit LeuVen, B-3001 LeuVen, Belgium ReceiVed: February 23, 2010; ReVised Manuscript ReceiVed: April 15, 2010 2 Recombination of two amidogen radicals, NH2 (X B1), is relevant to hydrazine formation, ammonia oxidation and pyrolysis, nitrogen reduction (fixation), and a variety of other N/H/X combustion, environmental, and interstellar processes. We have performed a comprehensive analysis of the N2H4 potential energy surface, using a variety of theoretical methods, with thermochemical kinetic analysis and master equation simulations used to treat branching to different product sets in the chemically activated NH2 + NH2 process. For the first time, iminoammonium ylide (NH3NH), the less stable isomer of hydrazine, is involved in the kinetic modeling of N2H4. A new, low-energy pathway is identified for the formation of NH3 plus triplet NH, via initial production of NH3NH followed by singlet-triplet intersystem crossing. This new reaction channel results in the formation of dissociated products at a relatively rapid rate at even moderate temperatures and above. A further novel pathway is described for the decomposition of activated N2H4, which eventually leads to the formation of the simple products N2 + 2H2, via H2 elimination to cis-N2H2. This process, termed as “dihydrogen catalysis”, may have significant implications in the formation and decomposition chemistry of hydrazine and ammonia in diverse environments. In this mechanism, stereoselective attack of cis-N2H2 by molecular hydrogen results in decomposition to N2 with a fairly low barrier. The reverse termolecular reaction leading to the gas-phase formation of cis-N2H2 + H2 achieves non-heterogeneous catalytic nitrogen fixation with a relatively low activation barrier (77 kcal mol-1), much lower than the 125 kcal mol-1 barrier recently reported for bimolecular addition of H2 to N2. This termolecular reaction is an entropically disfavored path, but it does describe a new means of activating the notoriously unreactive N2. We design heterogeneous analogues of this reaction using the model compound (CH3)2FeH2 as a source of the H2 catalyst and apply it to the decomposition of cis-diazene. The reaction is seen to proceed via a topologically similar transition state, suggesting that our newly described mechanism is general in nature. 1. Introduction Association and disproportionation reactions involving the NH2 radical have long been studied experimentally,4-28 and to a lesser Hydrazine (N2H4) is an important energetic material, the degree by theoretical methods.29-31 main component of diamine-based rocket fuels. Hydrazine and its methyl-substituted homologues are unique endother- The N2H4 potential energy surface (PES) involves several mic compounds forming the class of hypergolic propellants. important transformation pathways closely related to the This type of propellant ignites spontaneously on contact with storage of hydrogen and the fixation of nitrogen (i.e., 1 32-35 an oxidizer, typically nitrogen tetroxide. N2H4 can be reduction to NH3, HCN, etc). It is evident that the gas- stabilized as a flame without any oxidizer, and its decomposi- phase hydrogenation of nitrogen occurs without a catalyst tion can lead to self-ignition or detonation.2 Hydrazine, as only at extreme conditions, and we note that the degree of the simplest diamine, has been the subject of numerous this “extremity” and the corresponding energy paths of the investigations with the earlier literature reviewed compre- process are not well established. hensively in refs 3-9. The overall free energy change (∆G) for the reaction N At adequate pressures, self-reaction of two amidogen radicals 2 + T - -1 generates hydrazine: 3H2 2NH3 is about 8 kcal mol . However, the first step of activating the triple bond is extremely unfavorable, with bond dissociation energy at 225 kcal mol-1. According + + f + to the recent calculations of Hwang and Mebel,30 the lowest NH2 NH2 ( M) N2H4( M) (1) barrier associating H2 and N2 into diimide is about 125 kcal mol-1 at the G2M(MP2)//MP2/6-31G** level of theory. Such * To whom correspondence should be addressed. E-mail: bozzelli@ a high activation barrier results in little or no probability for njit.edu (J.W.B.), [email protected] (R.A.). unimolecular occurrence of this reaction in the gas-phase. † New Jersey Institute of Technology. ‡ University of Melbourne. Hence, chain reactions are considered by the authors as the § University of Leuven. dominant reaction channels for nitrogen hydrogenation:30 10.1021/jp101640p 2010 American Chemical Society Published on Web 05/12/2010 6236 J. Phys. Chem. A, Vol. 114, No. 21, 2010 Asatryan et al. + f N2 H N2H is made in some kinetic models between isomers of N2H2 and + f + they are often combined into one reaction by use of a single N2H H2 HNNH H + f average N2H2 species. Analysis of the principal energetic, HNNH H N2H3 + f + structural, and kinetic differences between trans-N2H2, cis-N2H2, N2H3 H2 H2NNH2 H + f + and NNH2 (also called iso-N2H2) show significant distinctions. H2NNH2 H NH2 NH3 Kinetic analysis in this study suggests that the specific isomer + f + NH2 H2 NH3 H that forms or dominates in a given process may well play a crucial role in the final integrated results. Dean et al. proposed10 + f that the recombination reaction NH + NH had two channels Net: N2 3H2 2NH3 2 2 to N2H3 + H and N2H4 to better explain their radical profile measurements in rich ammonia flames. Formation of an N2H4 Recent studies substantiate that N2H4 is an intermediate product appears to dominate at lower-temperatures in agreement reaction product in the thermal decomposition of ammonia (at with literature data8 with wide variation in the reported rate ° 38 800 C). Chain-mediated decomposition of hydrazine is known constants. Other reaction channels, however, are less consistent. f + + from classic literature: 2N2H4 2NH3 N2 H2 with chain Miller and Bowman suggested7 that the high-temperature 4 steps N2H4 f H + N2H3 f H + N2H2 f N2 + H2. 25 products are N2H2 + H2. The measurements by Stothard et al. There are a number of unresolved problems in the mechanism at room temperature and low pressures indicate that H2 of nitrogen fixation in biological media (reduction of N2 to NH3 production is the dominant channel. They deduced a rate by nitrogenase via bacterial enzymes, nitrogen metabolism in coefficient for the bimolecular channel of amidogen recombina- 33 archaea, etc.). Notably, hydrogen is always produced when 11 3 -1 -1 tion forming N2H2 as 7.8 × 10 cm mol s . However, the nitrogenase reduces N2 to NH3 (vide infra). Most N2 fixing mass-spectrometric method used by these authors did not allow bacteria contain hydrogenase to recycle hydrogen.34 There is for assigning an observed experimental value to one of the N2H2 an agreement that N2 binds end-on to the Fe[H]2 complex and coproduct isomers. Dean and Bozzelli have further examined releases H2, while the bound N2 is reduced to N2H2. Hydrazine this system6 to try to account for the observations of Stothard as an intermediate has been detected experimentally whereas et al.25 at ambient temperatures and have interpreted their results N2H2 is assumed to be a very unstable intermediate that tends 33 in terms of a new reaction channel to H2 and singlet H2NN. To to decompose back to N2 + H2. account for reactions of H2NN in nitrogen mechanisms, several It is also believed that the primary initiation step in the thermal rate constants for a series of related reactions have been de-NOx processes is the reaction of ammonia with hydroxyl to 6 8 6,36 proposed. Recent modeling of Konnov and De Ruyck dem- form amidogen (NH2). The temperature dependence of the onstrates that reactions forming N2H2, as well as their unimo- thermal de-NOx process depends on the behavior of NH2. lecular decomposition and bimolecular radical reactions, are Recombination and cross-reactions of NH2 radicals with simple important in hydrazine flames, since N2H2 is a crucial open and closed-shell species such as O2, NO, O3,SO2, and intermediate. CO, along with the oxidation of NH3, are also of principal importance in combustion media, environmental processes, and Decomposition of gaseous hydrazine behind a reflected shock 28,37 wave is found to be pressure dependent; the falloff behavior in atmospheric chemistry. 27 The amidogen radical self-reaction involves several dispro- depends on the nature of diluents. The main products of portionation channels hydrazine decomposition are ammonia, hydrogen, and nitrogen. Relative yields of the products in hydrazine decomposition flames closely correspond to the overall equation 2N2H4 f 2 + 2 f 2 ′ + NH2(X B1) NH2(X B1) N2H3(X A ) H (2) 2NH3 + N2 + H2, and their relative yields were found to be 11,39 f + unaffected by pressure. Notably, less ammonia is formed trans-N2H2 H2 in hydrazine decomposition at higher temperatures.40 An (3) inclusive review on the kinetic modeling of decomposition of f cis-N H + H (4) hydrazine using different estimated rate constants is contained 2 2 2 in the Konnov and De Ruyck article,8 while some early models f + 41 NNH2 H2 (5) are described by Adams. The overall recombination process of amidogen radicals have 3 - f + 28 NH3 NH(X Σ ) been considered quite recently by Bahng and Macdonald at (6) 293 ( 2 K over a pressure range from 2 to 10 Torr. The low pressure rate constant showed a linear dependence on pressure.