Unimolecular Decomposition Reactions of Picric Acid and Its Methylated Derivatives─A DFT Study

Atmospheric chemistry of 2-nitrobenzaldehyde: Initiated by photo-excitation, OH-oxidation, and small TiO2 clusters adsorption catalysis

The fate of 2-nitrobenzaldehyde (2-NBA) is of interest in atmospheric chemistry as it is a semi-volatile organic compound with high photosensitivity. This study presents a quantum chemical study of the gas-phase reactions of 2-NBA photo-excitation and OH-oxidation in the absence and presence of small TiO2 clusters. To further understand the unknown photolysis mechanism, the photo-reaction pathways of ground singlet state and the lying excited triplet state of 2-NBA were investigated including the initial and subsequent reactions of proton transfer, direct CO, NO2, and HCO elimination routes in the presence of O2 and NO. Meanwhile, the OH-mediated degradation of 2-NBA proceeded via five H-extraction and six OH-addition channels by indirect mechanism, which follows a succession of reaction steps initiated by the formation of weakly stable intermediate complexes. The H-extraction from the -CHO group was the dominant pathway with a negative activation energy of -1.22 kcal/mol. The calculated rate coefficients at 200-600 K were close to the experimental data in literature within 308-352 K, and the kinetic negative temperature independence was found in both experimental literature and computational results. Interestingly, 2-NBA was favored to be captured onto small TiO2 clusters via six adsorption configurations formed via various combination of three types of bonds of Ti···O, Ti···C, and O···H between the molecularly adsorbed 2-NBA and TiO2 clusters. Comparison indicted that the chemisorptions of aldehyde oxygen have largest energies. The results suggested adsorption conformations have a respectable impact on the catalysis barrier. This study is significant for understanding the atmospheric chemistry of 2-nitrobenzaldehyde.

Thermochemistry, Tautomerism, and Thermal Stability of 5,7-Dinitrobenzotriazoles

Nitro derivatives of benzotriazoles are safe energetic materials with remarkable thermal stability. In the present study, we report on the kinetics and mechanism of thermal decomposition for 5,7-dinitrobenzotriazole (DBT) and 4-amino-5,7-dinitrobenzotriazole (ADBT). The pressure differential scanning calorimetry was employed to study the decomposition kinetics of DBT experimentally because the measurements under atmospheric pressure are disturbed by competing evaporation. The thermolysis of DBT in the melt is described by a kinetic scheme with two global reactions. The first stage is a strong autocatalytic process that includes the first-order reaction (Ea1I = 173.9 ± 0.9 kJ mol−1, log(A1I/s−1) = 12.82 ± 0.09) and the catalytic reaction of the second order with Ea2I = 136.5 ± 0.8 kJ mol−1, log(A2I/s−1) = 11.04 ± 0.07. The experimental study was complemented by predictive quantum chemical calculations (DLPNO-CCSD(T)). The calculations reveal that the 1H tautomer is the most energetically preferable form for both DBT and ADBT. Theory suggests the same decomposition mechanisms for DBT and ADBT, with the most favorable channels being nitro-nitrite isomerization and C–NO2 bond cleavage. The former channel has lower activation barriers (267 and 276 kJ mol−1 for DBT and ADBT, respectively) and dominates at lower temperatures. At the same time, due to the higher preexponential factor, the radical bond cleavage, with reaction enthalpies of 298 and 320 kJ mol−1, dominates in the experimental temperature range for both DBT and ADBT. In line with the theoretical predictions of C–NO2 bond energies, ADBT is more thermally stable than DBT. We also determined a reliable and mutually consistent set of thermochemical values for DBT and ADBT by combining the theoretically calculated (W1-F12 multilevel procedure) gas-phase enthalpies of formation and experimentally measured sublimation enthalpies.

Complete and selective nitration of tyrosine residue in peptides caused by ultraviolet matrix-assisted laser desorption/ionization

Complete and highly selective nitration of tyrosine (Tyr) as a residue-specific modification in peptides was found without side reactions, using ultraviolet matrix-assisted laser desorption/ionization (UV-MALDI) with a nitroaromatic reagent 3, 5-dinitrosalicylic acid (3,5-DNSA). The tyrosine nitration supported two propositions, namely, the UV-induced. NO₂ attack reaction mechanism by Long et al. and the C-NO₂ homolysis as a thermal process by Wiik et al. and Furman et al. With the UV-MALDI of peptides, a residue-specific reaction was observed in glycine (Gly) residue, i.e., an oxidation of the alpha-carbon of Gly due to attack of hydroxyl radical (.OH).

Understanding Explosive Sensitivity with Effective Trigger Linkage Kinetics

We present a simple linear model for ranking the drop weight impact sensitivity of organic explosives that is based explicitly on chemical kinetics. The model is parameterized to specific heats of explosion, Q, and Arrhenius kinetics for the onset of chemical reactions that are obtained from gas-phase Born-Oppenheimer molecular dynamics simulations for a chemically diverse set of 24 molecules. Reactive molecular dynamics simulations sample all possible decomposition pathways of the molecules with the appropriate probabilities to provide an effective reaction barrier. In addition, the calculations of effective trigger linkage kinetics can be accomplished without prior physical intuition of the most likely decomposition pathways. We found that the specific heat of explosion tends to reduce the effective barrier for decomposition in accordance with the Bell-Evans-Polanyi principle, which accounts naturally for the well-known correlations between explosive performance and sensitivity. Our model indicates that sensitive explosives derive their properties from a combination of weak trigger linkages that react at relatively low temperatures and large specific heats of explosion that further reduce the effective activation energy.