Computational Study on the Kinetics and Mechanism for the Unimolecular Decomposition of C6H5NO2and the Related C6H5+ NO2and C6H5O + NO Reactions†

Modulating the Roaming Dynamics for the NO Release in ortho-Nitrobenzenes

The dynamics of NO release upon photodissociation of nitroaromatic compounds is dependent on the nature of the interaction between the NO₂ group and substituent in the ortho position. A bimodal (slow and fast) translational energy distribution of the NO photofragment indicates the presence of two distinct NO elimination channels. The slow-to-fast branching ratio for the NO release is regulated by the hydrogen bonding ability of the ortho substituent and follows the order [OH > NH₂ > CH₃ > OCH₃], indicating that the intramolecular hydrogen bonding plays a pivotal role in NO release dynamics. Further, the topology of the triplet state potential energy surface acts as a doorway to the dissociation pathway switching between the roaming and nonroaming mechanisms, with hydrogen bonding substituents (OH and NH₂) favoring the roaming mechanism.

Predicting the Initial Thermal Decomposition Path of Nitrobenzene Caused by Mode Vibration at Moderate-Low Temperatures: Temperature-Dependent Anti-Stokes Raman Spectra Experiments and First-Principals Calculations

The lack of understanding of the initial decomposition micromechanism of energetic materials subjected to external stimulation has hindered its safe storage, usage, and development. The initial thermal decomposition path of nitrobenzene triggered by molecular thermal motion is investigated using temperature-dependent anti-Stokes Raman spectra experiments and first-principles calculations to clarify the initial thermal decomposition micromechanism. The experiment shows that the symmetric nitro stretching, antisymmetric nitro stretching, and phenyl ring stretching vibration modes are active as increasing temperature below 500 K. The DFT method is used to examine the effects of the three mode vibrations on the initial decomposition of nitrobenzene by relaxed scan for each relevant change in bond lengths and bond angles to obtain the optimal reaction channel leading to initial thermal decomposition of nitrobenzene. The results demonstrate that the initial thermal decomposition is the isomerization of nitrobenzene to phenyl nitrite. The optimal reaction channel leading to the initial isomerization is the increase or decrease of angle O-N-C from the antisymmetric nitro stretching vibration, which causes the torsion of nitro group and the subsequent oxygen atom attacking carbon atom. The scanning energy barrier related to angle O-N-C is about 62.1 kcal/mol, which is very consistent with the calculated activation barrier of isomerization of nitrobenzene. This proves the reliability of our conclusions.

Kinetics of the Reaction of the Cyclopentadienyl Radical with Nitrogen Dioxide

The kinetics of the reaction of the cyclopentadienyl radical (c-C₅H₅) with nitrogen dioxide (NO₂) was studied by laser photolysis/photoionization mass spectroscopy. Overall rate constants were obtained in direct real-time experiments in the temperature region 305-800 K and at bath gas densities of (3.0-12.0) × 10¹⁶ molecules cm^-3. The overall rate constant is independent of temperature between 300 and 400 K but decreases by a factor of approximately 7 above 400 K, without any discernible pressure dependence. A potential energy surface study of the reaction was performed, and an RRKM/master equation model was created. The reaction proceeds via initial addition to one of the two types of atoms of the NO₂ molecule (nitrogen or oxygen). The N-bonded adduct can isomerize and decompose back to the reactants; this channel is significantly affected by falloff above 400 K and, although dominant at room temperature, becomes negligible at 600 K and above. The O-bonded adduct undergoes chemically activated isomerizations and decomposition, with a minor contribution from stabilization at low temperatures; this channel dominates at high temperatures and is effectively pressure-independent. The model provides a quantitative explanation for the observed temperature dependence of the rate constant.

Reply to the comment on “Decomposition mechanisms of trinitroalkyl compounds: a theoretical study from aliphatic to aromatic nitro compounds” by G. Fayet, P. Rotureau, B. Minisini, Phys. Chem. Chem. Phys., 2014, 16, 6614

Decomposition reactions of trinitrobutane.

Decomposition mechanisms and kinetics of novel energetic molecules BNFF-1 and ANFF-1: quantum-chemical modeling

Decomposition mechanisms, activation barriers, Arrhenius parameters, and reaction kinetics of the novel explosive compounds, 3,4-bis(4-nitro-1,2,5-oxadiazol-3-yl)-1,2,5-oxadiazole (BNFF-1), and 3-(4-amino-1,2,5-oxadiazol-3-yl)-4-(4-nitro-1,2,5-oxadiazol-3-yl)-1,2,5-oxadiazole (ANFF-1) were explored by means of density functional theory with a range of functionals combined with variational transition state theory. BNFF-1 and ANFF-1 were recently suggested to be good candidates for insensitive high energy density materials. Our modeling reveals that the decomposition initiation in both BNFF-1 and ANFF-1 molecules is triggered by ring cleavage reactions while the further process is defined by a competition between two major pathways, the fast C-NO₂ homolysis and slow nitro-nitrite isomerization releasing NO. We discuss insights on design of new energetic materials with targeted properties gained from our modeling.

Does addition of NO2 to carbon-centered radicals yield RONO or RNO2? An investigation using distonic radical ions

Nitrogen dioxide is used as a "radical scavenger" to probe the position of carbon-centered radicals within complex radical ions in the gas phase. As with analogous neutral radical reactions, this addition results in formation of an [M + NO2](+) adduct, but the structural identity of this species remains ambiguous. Specifically, the question remains: do such adducts have a nitro- (RNO2) or nitrosoxy- (RONO) moiety, or are both isomers present in the adduct population? In order to elucidate the products of such reactions, we have prepared and isolated three distonic phenyl radical cations and observed their reactions with nitrogen dioxide in the gas phase by ion-trap mass spectrometry. In each case, stabilized [M + NO2](+) adduct ions are observed and isolated. The structure of these adducts is probed by collision-induced dissociation and ultraviolet photodissociation action spectroscopy and a comparison made to the analogous spectra of authentic nitro- and nitrosoxy-benzenes. We demonstrate unequivocally that for the phenyl radical cations studied here, all stabilized [M + NO2](+) adducts are exclusively nitrobenzenes. Electronic structure calculations support these mass spectrometric observations and suggest that, under low-pressure conditions, the nitrosoxy-isomer is unlikely to be isolated from the reaction of an alkyl or aryl radical with NO2. The combined experimental and theoretical results lead to the prediction that stabilization of the nitrosoxy-isomer will only be possible for systems wherein the energy required for dissociation of the RO-NO bond (or other low energy fragmentation channels) rises close to, or above, the energy of the separated reactants.

REACTION MECHANISM OF THE CCN RADICAL WITH NITROGEN DIOXIDE

The complex singlet and triplet potential energy surfaces (PESs) of the [ C 2 N 2 O 2] system are performed at the B3LYP and Gaussian-3//B3LYP levels in order to investigate the possibility of the carbyne radical CCN in removal of nitrogen dioxide. Thirty minimum isomers and 36 transition states are located. Starting from the very energy-rich reactant R CCN + NO 2, the terminal C -attack adduct NCCN ( O ) O (singlet at -48.6 and triplet at -48.1 kcal/mol) is first formed on both singlet and triplet PESs. Subsequently, the singlet NCCN ( O ) O takes an O -transfer to form the intermediate singlet cis- NCC ( O ) NO (-120.1), which can lead to the fragments NCCO + NO (-94.4) without barrier. The simpler evolution of the triplet NCCN ( O ) O is the direct N – O rupture to form the final fragmentation NCCNO + 3 O (-31.0). However, the lower lying products 3 NCNO + CO (-103.3) and NCNCO + 3 O (-86.5) are kinetically much less competitive. All the involved transition states for the generation of NCCO + NO and NCCNO + 3 O lie much lower than the reactants, and it indicates that this reaction can proceed effectively even at low temperatures. We expect that the reaction CCN + NO 2 can play a role in both combustion and interstellar processes. Comparison is made between the CCN + NO 2 and CH + NO 2 reaction mechanisms.

Photodissociation dynamics of nitrobenzene and o-nitrotoluene

Photodissociation of nitrobenzene at 193, 248, and 266 nm and o-nitrotoluene at 193 and 248 nm was investigated separately using multimass ion imaging techniques. Fragments corresponding to NO and NO(2) elimination from both nitrobenzene and o-nitrotoluene were observed. The translational energy distributions for the NO elimination channel show bimodal distributions, indicating two dissociation mechanisms involved in the dissociation process. The branching ratios between NO and NO(2) elimination channels were determined to be NONO(2)=0.32+/-0.12 (193 nm), 0.26+/-0.12 (248 nm), and 0.4+/-0.12(266 nm) for nitrobenzene and 0.42+/-0.12(193 nm) and 0.3+/-0.12 (248 nm) for o-nitrotoluene. Additional dissociation channels, O atom elimination from nitrobenzene, and OH elimination from o-nitrotoluene, were observed. New dissociation mechanisms were proposed, and the results are compared with potential energy surfaces obtained from ab initio calculations. Observed absorption bands of photodissociation are assigned by the assistance of the ab initio calculations for the relative energies of the triplet excited states and the vertical excitation energies of the singlet and triplet excited states of nitrobenzene and o-nitrotoluene. Finally, the dissociation rates and lifetimes of photodissociation of nitrobenzene and o-nitrotoluene were predicted and compared to experimental results.

A Computational Study on the Kinetics and Mechanism for the Unimolecular Decomposition of o-Nitrotoluene

The kinetics and mechanism for the unimolecular decomposition of o-nitrotoluene (o-CH(3)C(6)H(4)NO(2)) have been studied computationally at the G2M(RCC, MP2)//B3LYP/6-311G(d, p) level of theory in conjunction with rate constant predictions with RRKM and TST calculations. The results of the calculations reveal 10 decomposition channels for o-nitrotoluene and its six isomeric intermediates, among them four channels give major products: CH(3)C(6)H(4) + NO(2), C(6)H(4)C(H)ON (anthranil) + H(2)O, CH(3)C(6)H(4)O (o-methyl phenoxy) + NO, and C(6)H(4)C(H(2))NO + OH. The predicted rate constants in the 500-2000 K temperature range indicate that anthranil production, taking place initially by intramolecular H-abstraction from the CH(3) group by NO(2) followed by five-membered ring formation and dehydration, dominates at temperatures below 1000 K, whereas NO(2) elimination becomes predominant above 1100 K and CH(3)C(6)H(4)O formation by the nitro-nitrite isomerization/decomposition process accounts for only 5-11% of the total product yield in the middle temperature range 800-1300 K. The branching ratio for CH(2)C(6)H(4)NO formation by the decomposition process of CH(2)C(6)H(4)N(O)OH is negligible. The predicted high-pressure-limit rate constants with the rate expression of 4.10 x 10(17) exp[-37000/T] s(-1) for the NO(2) elimination channel and 9.09 x 10(12) exp[-25800/T] s(-1) for the H(2)O elimination channel generally agree reasonably with available experimental data. The predicted high-pressure-limit rate constants for the NO and OH elimination channels are represented as 1.49 x 10(14) exp[-30000/T] and 1.31 x 10(15) exp[-38000/T] s(-1), respectively.

Ultrafast Electron Diffraction: Structural Dynamics of Molecular Rearrangement in the NO Release from Nitrobenzene

Nitro compounds release NO, NO2, and other species, but neither the structures during the reactions nor the time scales are known. Ultrafast electron diffraction (UED) allowed the study of the NO release from nitrobenzene, and the molecular pathways and the structures of the transient species were identified. It was observed, in contrast to previous inferences, that nitric oxide and phenoxyl radicals are formed dominantly and that the time scale of formation is 8.8+/-2.2 ps. The structure of the phenoxyl radical was determined for the first time, and found to be quinoid-like. The mechanism proposed involves a repulsive triplet state, following intramolecular rearrangement. This efficient generation of NO may have important implications for the control of by-products in drug delivery and other applications.

Cyanomethylidyne: A Reactive Carbyne Radical

The cyanomethylidyne radical (CCN) has been a long-standing subject of extensive structural and spectroscopic studies. However, its chemical reactivity has received rather little attention. Recently, we studied the reaction of CCN with the simplest alkane, CH4, which follows a mechanism of carbyne insertion-dissociation rather than that of direct H abstraction proposed by a recent experimental study. However, we are aware that alkanes like CH4 bear no electron lone pairs and thus are not ideal diagnostic molecules for distinguishing between the carbyne-insertion and H-abstraction mechanisms. Hence, we chose a series of sigma-bonded molecules HX (X=OH, NH2, and F) which bear electron lone pairs and are better diagnostics for carbyne-insertion behavior. The new results at the CCSD(T)/6-311+G(2df,p)//B3LYP/6-311G(d,p)+ZPVE, CCSD(T)/aug-cc-pVTZ//B3LYP/6-311G(d,p)+ZPVE, G2M(CC1), and MC-QCISD//B3LYP/6-31G(d)+ZPVE levels definitively confirm the carbyne-insertion behavior of the CCN radical towards HX. In addition, we make the first attempt to understand the reactivity of the CCN radical toward pi-bonded molecules, using the CCN+C2H2 model reaction. This reaction involves carbenoid addition to the C[triple chemical bond]C bond without a potential-energy barrier to form a C3 three-membered cyclic intermediate followed by H extrusion. Therefore, the reactions of CCN with both sigma- and pi-bonded molecules conclusively show that CCN is a reactive carbyne radical and may be more reactive than the well-known CN radical. Future experimental studies, especially on product characterization, are strongly desired to test our proposed carbyne mechanism. The studied reactions of CCN with CH4, NH3, H2O, and C2H2 could be of interest to combustion science and astrophysics, and they could provide efficient routes to form novel cyano-containing molecules in interstellar space.