HONO Elimination

The large difference between the values for this barrier obtained from the two methods merits a comment. While investigating the decomposition of nitroethylene [6], we found that the results obtained from the use of B3P86 and B3LYP were similar in almost all of the steps, except the ones which result in HONO formation. Moreover, the B3LYP results come 

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Lewis et al.106 calculated four possible decomposition pathways of the ot-HMX polymorph N-N02 bond dissociation, HONO elimination, C-N bond scission, and concerted ring fission. Based on energetics, it was determined that N-N02 dissociation was the initial mechanism of decomposition in the gas phase, whereas they proposed HONO elimination and C-N bond scission to be favorable in the condensed phase. The more recent study of Chakraborty et al.42 using density functional theory (DFT), reported detailed decomposition pathways of p-HMX, which is the stable polymorph at room temperature. It was concluded that consecutive HONO elimination (4 HONO) and subsequent decomposition into HCN, OH, and NO are the most energetically favorable pathways in the gas phase. The results also showed that the formation of CH20 and N20 could occur preferably from secondary decomposition of methylenenitramine. 

Ab initio density functional calculations have been applied to dimethyhiitramine decomposition in order to calculate the primary deuterium kinetic isotope effect (kH/km =4.21) for HONO elimination Me2NN02 - [TS] - CH2NMe + N02H and the secondary isotope effect (kH/kD6 = 1.4) for N—N bond homolysis, each at 240 °C.64 Since the experimentally observed isotope effect is 1.57, it has been concluded that the latter process may be rate determining. 

Yim and Liu [145] used a combined ab initio molecular dynamics (MD) study to reveal several new mechanistic paths that are energetically favored over dissociation of the C-N02 bond. Their study indicated dissociation of NTO via an aci-nitro tautomer followed by ring scission to a ketinimine intermediate as the most favorable pathway (Scheme IX), requiring only 38 kcal/mol as compared to the 62.5 kcal/mol needed for cleaving the C-N02 bond. Kohno et al. [151] conducted a theoretical study on the decomposition of the NTO dimer. They reported that HONO elimination is the last step of a cascade of reactions with a total barrier of 88 kcal/mol. 

We concluded that the HONO elimination reaction B and nitro-to-nitrite rearrangement D are the lowest energy decomposition channels for nitroethylene with the G2 calculated activation barriers of 57.3 and 57.9 kcal/mol, respectively. This is -15 kcal/mol lower than the amount of energy required for the C-N02 bond cleavage. 

DFT calculations that predict that the dominant initial decomposition pathway is consecutive HONO eliminations. Their calculations also predict that N02 elimination and the concerted ring fission are significant channels. 

Dimethylnitramine has been used as a simple model for cyclic nitramines because it is easier to deal with experimentally and theoretically than RDX and HMX, and it undergoes some of the same kinds of reactions N-NO2 bond fission, nitro-nitrite isomerization (which can be followed by NO elimination), and HONO elimination. However, even though it seemingly presents fewer problems than do the large nitramines, its decomposition mechanism is not fully resolved. 

The focus in the reaction dynamics studies was on the N02 elimination channel, but they also studied the HONO elimination reactions [70]. They based the potential energy surface on experimental data but performed some minimal basis set ab initio calculations to determine geometries, force fields, torsional potentials, and some information about the reaction paths. The representations of the global potential energy surfaces were based on valence force fields for equilibrium structures with arbitrary switching functions operating on the potential parameters to effect smooth and (assumed) proper behavior along the reaction paths. Based on the available experiments [71-73], they assumed that the primary decomposition reaction is simple N-N bond rupture to eliminate N02. 

Rice et al. [99] developed a global potential energy surface based on the Mowrey et al. [103] results and performed extensive classical trajectory calculations to study the dynamics of the CH2NN02 dissociation reactions. They calculated rates for reactions (III) and (IV) with classical barriers of 35 and 37 kcal/mol, respectively. They found that N-N bond fission dominates at low energy but that HONO elimination is competitive. Chakraborty and Lin [104] predict the opposite on the basis of their ab initio barriers and RRKM theory calculations. The two dissociations channels are closely competitive and it is not clear that ab initio methods are sufficiently reliable to distinguish between two reactions that have such similar energy requirements. Also, the Zhao et al. results [33] are not in accord with the theoretical predictions. 

A strong motivation for the Rice et al. [99] simulations was to try to interpret the Zhao et al. [33] observations that the HONO elimination channel dominates while the N-N bond rupture reaction does not occur. A possible explanation is that the nascent CH2NN02 product of the RDX ring fission reaction is highly excited and has a nonstatistical distribution of energy. Sewell and Thompson [35] estimated that it may be formed with 55 to 65 kcal/mol of energy, which is well in excess of the predicted energy... 

Figure 3. Energetics of unimolecular decomposition mechanisms in RDX obtained using the ReaxFF (full lines with filled symbols) and with QM (dashed lines with open symbols). Circles represent the sequential HONO elimination, triangles show the decomposition process following homolytic N-N bond breaking (NO2 elimination), and diamonds represent the concerted ring-opening pathway.

As an illustrative example of the semiclassical reaction path dynamics method, we discuss the HONO elimination pathway for the unimolecular decomposition of methylene nitramine,... 

In an effort to understand this difference in sensitivity between PETN and Si-PETN, Liu et al. analyzed computationally several possible initial steps in the explosive decompositions of these molecules [64]. Eor PETN, the energetically-preferred possibilities were O-NO2 rupture and HONO elimination these were both found to have energy barriers of about 39 kcal/mol. For Si-PETN, the analogous... 

So far we have discussed simple bond fission in which only one bond is broken. In this section we discuss decomposition pathways in which multiple bonds are broken and formed simultaneously In particularly, we present results for five-centered HONO elimination from nitro compounds containing a [3-hydrogen as well as results for concerted decomposition of six-membered ring compounds. 

DFT and ab initio calculations have been used to study the mechanism of the gas-phase oxidation of phenol by HO. Addition of HO to the ort/io-position forms P2, which subsequently combines with O2 at the ip o-position to form adduct P2-1-00. A concerted HO2 elimination from P2-1-00 forms 2-hydroxy-3,5-cyclohexadienone (HCH) as the main product and is responsible for the rate constants for the reaction between P2 and O2 to be about two orders of magnitude higher than those between other aromatic-OH adducts and O2. The HCH subsequently isomerizes to catechol, which is thermodynamically more stable than HCH, possibly through a heterogeneous process. Reaction of P2 with NO2 proceeds by addition to form P2-n-N02 ( = 1, 3, 5) followed by HONO elimination from P2-1/3-N02 to form catechol. The barriers for HONO elimination and catechol formation are below the separate reactants P2 and NO2, being consistent with the experimental observation of catechol in the absence of O2, while H2O elimination from P2-I/3-NO2 forms 2-nitrophenol (2NP). The most likely pathway for 2NP is the reaction between phenoxy radical and N02." ... 

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