A novel mechanism for the isomerization of N₂O₄ and its implication for the reaction with H₂O and acid rain formation

Abstract

We have discovered, by high-level quantum-chemical calculations, a new and predominant isomerization mechanism for N 2 O 4 ! ONONO 2 via a roaming-like transition state occurring unimolecularly or bimolecularly during collision with H 2 O. The new mechanism allows N 2 O 4 to react with H 2 O with a significantly lower barrier (< 13.1 kcal/mol) than the commonly known tight transition state (30-45 kcal/mol) by concurrent stretching of the NAN bond and rotation of one of the NO 2 groups to form trans-ONONO 2 , which then undergoes a rapid metathetical reaction with H 2 O in the gas phase and in aqueous solution. The results have a significant implication for the hydrolysis of N 2 O 4 in water to produce HONO and HNO 3 . Rate constants for the isomerization and hydrolysis reactions have been predicted for atmospheric modeling applications. K E Y W O R D S HONO and HNO 3 formation, N 2 O 4 hydrolysis, N 2 O 4 isomerization and reaction, roaming-like transition state | I N TR ODU C TI ONNitrogen oxides play important roles in a wide variety of reactions in upper and lower atmospheric systems. Symmetric N 2 O 4 (commonly referred to as nitrogen tetroxide [NTO]) is among the most stable dinitrogen oxides in the gas phase, with a substantial concentration at low temperature under atmospheric pressure conditions. N 2 O 4 is the key precursor for the formation of HONO, which is the major source of OH radicals in the troposphere. [1] The less stable nitrosonium nitrate (ONONO 2 ), a structural isomer of N 2 O 4 (D 2h ), is known to be much more reactive than the latter symmetrical one. ONONO 2 has been proposed to be responsible for the hydrolysis of NTO to produce HONO and HNO 3 in laboratory experiments, [2] in which ONONO 2 was assumed to form via the interface-catalyzed isomerization of NTO. [2][3][4][5][6][7][8] The high reactivity of ONONO 2 toward H 2 O has been confirmed by high-level quantum chemical calculations by Zhu et al. [9] ; the predicted potential energy surface (PES) for the H 2 OAONONO 2 reaction was able to quantitatively account for the experimentally determined third-order kinetics for the 2NO 2 1 H 2 O ! HONO 1 HNO 3 reaction in the gas phase [10][11][12] involving the ONONO 2 directly formed by the barrierless association of 2NO 2 molecules. A similar very fast reaction of ONONO 2 with hydrazine was proposed earlier by Lai et al. [13] to account for the hypergolic ignition of the well-known hydrazine-NTO propellant system.Several authors have theoretically investigated the isomerization of NTO to ONONO 2 . [14][15][16] Pimentel et al. [14] reported the isomerization barrier as 31.3 kcal/mol in the gas phase and 21.1 kcal/mol in aqueous solution at 298 K using the B3LYP/13s8p(2d,1f) method. Lai et al. [15] obtained an isomerization barrier of 42.2 kcal/mol in the gas phase at the G2M(CC1)//B3LYP/6-31111G(3df,2p) level of theory. More recently, Liu and Goddard [16] also located a transition state for isomerization at the RCCSD(T)/CBS level as high as 45.6 kcal/mol. The major focus of the present work ...

Ab Initio Chemical Kinetics for Nitrogen Tetroxide Reactions with 1,1‐ and 1,2‐Dimethylhydrazines

“…The difference could be attributed to the much stronger H-OH bond (118 kcal/mol) vs. the HÀ N (H)R bonds (R=H 2 N and CH 3 NH,~81 and 77 kcal/mol, respectively). The primary products formed in the hydrazine reactions were predicted to be HNO 3 + RNO [11][12][13][14]. Notably, the experimental work of Christos and co-workers [7] on the ignition of HZ with NTO in the solid HZ at 218 K could be explained by our new isomerization mechanism [15].…”

“…The isomerization of NTO in the solid HZ within its lattice was found to occur with a 13.1 kcal/mol barrier predicted at the DFT (projected augmented plane wave) level of theory with a similar loose TS to ONONO 2 , which then reacted rapidly with a small (1.4 kcal/mol) barrier to form NO 3 À + H 2 NN(H)NO + N 2 H 5 + in the lattice (which prevented the direct formation of HNO 3 ) [15]. Most significantly, the barriers for the isomerization reactions in the gas phase or in condensed phases from NTO to ONONO 2 taking place unimolecularly [12] or bimolecularly with H 2 O or HZ as a spectator were predicted to be around 13�1 kcal/mol at different levels of theory [11,12,15].…”

“…Because of the extremely powerful hypergolic ignition property of these systems, numerous studies had been carried out since the 1960's [1][2][3][4][5][6][7][8][9][10]. However, at the molecular level, the dominant ignition mechanisms of these hypergols have not been understood until our recent investigation on the mechanisms and kinetics of NTO/ HZ and NTO/MHZ systems [11] based on a new mechanism discovered for the direct hydrolysis reaction of NTO in water solution [12].…”

“…The new mechanism involved in the NTO + H 2 O reaction was found to take place in the gas phase and in the solution phase via a very loose transition state (TS) converting NTO to ONONO 2 in the presence of the H 2 O molecule as a bystander within the collision complex with a barrier of 14 kcal/mol; the reaction of the more polar [ON + ONO 2 À ] with H 2 O producing HNO 3 and HONO was predicted to have a 10 kcal/mol barrier above the reactants [12]. Significantly, a similar mechanism was found to be operative in the gas phase for the reactions of NTO with HZ and MHZ [11], in which the barriers for the isomerization of NTO to ONONO 2 in the presence of HZ or MHZ as a spectator in each of these reactions was also found to be about 13 kcal/ mol predicted at the CCSD(T)//B3LYP/6-311 + G(3df,2p) level [11].…”

“…Practically, SDMHZ has not been employed for propulsion applications because of its high melting point (see Table S1 in the Electronic Supporting Information (ESI) section) which effectively limits its application at low temperature in the outer space, for example [1]. The mechanisms of the NTO-UDMHZ/SDMHZ systems were first elucidated by high-level ab initio molecular orbital methods similar to those employed in our recent investigation of the NTO hydrolysis [12] and the NTO-HZ/MHZ reactions in the gas phase [11] as alluded to above. On the basis of the calculated potential energy surfaces (PESs), we have also predicted the kinetics for these reactions in the temperature range of 200-2000 K; these new results are presented and compared, in particular, on the dissimilarity in the two isomeric reactions.…”