Fengyi Liu

Density functional theory study on the reaction of triazol-3-one with nitronium: direct nitration versus acidic group-induced nitration

The nitration mechanism as well as the kinetics of triazol-3-one (TO) with nitronium (NO2+) in both a concentrated nitric acid and a nitric–sulfuric acid system was theoretically studied. Firstly, the density functional theory (DFT) with a B3LYP functional was employed to investigate the mechanism of the mentioned reactants towards the targeted product, 5-nitro-2,4-dihydro-1,2,4-triazol-3-one (NTO). An unexpected induction effect, which derived from the coexisting acid group (NO3− and/or HSO4−), was proclaimed. The impact of the induction effect on the nitration of TO was systematically demonstrated. It is found that unlike the nitration of most aromatics, the nitration of TO with NO2+ to form NTO does not follow the typical electrophilic substitution mechanism. Based on the results calculated in each acid system, the nitration mechanisms, including the NO2+ direct nitration (path A), NO3−-induced nitration (paths Bn–Dn) and HSO4−-induced nitration (paths Bs–Ds), were proposed. It is indicated that path A is unlikely or unfavorable due to the high activation barrier in the rate-determining step, whereas paths Bn–Dn and Bs–Ds are favorable, mainly attributed to the significant decrease of the activation energy induced by NO3− and HSO4− during the nitration process, especially for the NTO-oriented path Bn and Bs. Secondly, the canonical variational transition (CVT) state theory with small curvature tunneling (SCT) correction was used and the rate constants of the rate-determining steps for all paths at different temperatures were calculated. It is shown that the nitration rate in either path Bn or path Bs outdistances that in path A, indicating that NO3− and HSO4− accelerate the nitration of TO with NO2+, and ultimately favour the formation of NTO due to the proposed induction effect of each acid group. An enhanced catalytic effect of the nitric acid or/and sulfuric acid is thought to be embodied in not only the acceleration to the formation of NO2+, but also the induction effects of NO3− and HSO4− during the nitration processes. Meanwhile, it is suggested that the concentration of nitric acid and sulfuric acid in each nitration system should be well controlled since the favourable condition to produce NO2+ and NO3−/HSO4− differs in the concentrations of the corresponding acids.

Photochemistry of the Simplest Criegee Intermediate, CH2OO: Photoisomerization Channel toward Dioxirane Revealed by CASPT2 Calculations and Trajectory Surface-Hopping Dynamics

The photochemistry of Criegee intermediates plays a significant role in atmospheric chemistry, but it is relatively less explored compared with their thermal reactions. Using multireference CASPT2 electronic structure calculations and CASSCF trajectory surface-hopping molecular dynamics, we have revealed a dark-state-involved A1A → X1A photoisomerization channel of the simple Criegee intermediate (CH2OO) that leads to a cyclic dioxirane. The excited molecules on the A1A state, which can have either originated from the B1A state via B1A → A1A internal conversion or formed by state-selective electronic excitation, is driven by the out-of-plane motion toward a perpendicular A/X1A minimal-energy crossing point (MECI) then radiationless decay to the ground state with an average time constant of ∼138 fs, finally forming dioxirane at ∼254 fs. The dynamics starting from the A1A state show that the quantum yield of photoisomerization from the simple Criegee intermediate to dioxirane is 38%. The finding of the A1A → X1A photoisomerization channel is expected to broaden the reactivity profile and deepen the understanding of the photochemistry of Criegee intermediates.

Tuning of energetics and reaction mechanism of water-assisted intramolecular proton transfer of 7-azaindole by external electric field applied in various directions: a TD-DFT study

Ground- and excited-state intramolecular proton transfer reactions, as one of the most important processes in biological systems, have been utilized as the basis of artificially designed molecular photoswitch. Taking 7-azaindole as model of study, we report the electric field tuning of water-assisted proton transfer reactions in both the ground and excited states, by applying static electric fields along various directions. The electric fields applied in the direction (or those with component in the direction) of net proton transfer path have remarkable impact on the energetics of reaction, including tuning of thermodynamic and kinetic balance of tautomers, as well as red-/blueshifting the absorption and emission maxima. The electric fields applied in a direction perpendicular to the net proton transfer, although which have been found to play negligible roles in changing the energetics, tune the concerted double-proton transfer reactions from roughly synchronic to asynchronic. The electrostatic origins of such influences are analyzed. The findings of our (TD)-DFT calculations provide insights into fine tuning of both energetic and mechanistic aspects of reversible systems by electric field, and shed light on the nature of proton transfer in natural systems as well as designing novel electric optical switches.

Insight into the acidic group-induced nitration mechanism of 2-methyl-4,6-dihydroxypyrimidine (MDP) with nitronium

The strong desire for developing promising candidates of insensitive nitro energetic materials has spurred numerous attempts to discern the nitration details. The nitration mechanism of 2-methyl-4,6-dihydroxypyrimidine (MDP) with nitronium (NO2+) to form 2-dinitromethylene-5,5-dinitropyrimidine-4,6-dione is studied via DFT-B3LYP/6-311G(d,p) method. The possible nitration pathways are excavated and illustrated. Herein, a unique/incredible induction/enhancement of the co-existing acidic group of HSO4− to the targeted nitration is definitively proposed. The impact of the introduction order of four nitro groups (–NO2) on the title nitration is systematically demonstrated. It is suggested that the proposed induction of HSO4− may effectively promote/catalyze not only the NO2+ attack, but the H-transfer and H-abstraction as well, and thus dramatically decreases the activation free energy of the nitration system. Moreover, the NO2+ attack may be dynamically affected by the pre-introduced –NO2 and the resulted fluctuation of charge distribution in the pre-intermediates, which has been primarily supported via the calculated atomic charge, Coulomb attraction and Fukui function. It is indicated that the preferential dinitration on –CH3 with the induction of HSO4− (path A) is the most likely pathway. It is optimistically expected that the present study may provide a theoretical basis to the research and engineering tests of the title nitration, and promote the exploration of related insensitive energetic materials.

Kinetic and mechanistic investigations of the thermal decomposition of methyl-substituted cycloalkyl radicals

A systematic theoretical study on the thermal decomposition of 2-Me-cyclobutyl, 2-Me-cyclopentyl and 2-Me-cyclohexyl radicals is performed using the high-level ab initio CBS-QB3 and CCSD(T) quantum chemical calculations. The calculation reveals that the detailed reaction mechanism of the thermal decomposition of these cyclic alkyl radicals incorporates ring opening, vinyl rearrangements (exocyclization), beta-site C–C bond cleavage and H-elimination processes. The standard reaction enthalpies (ΔrH0298) and Gibbs free energies (ΔrG0298) for each elementary reaction involved in the 2-Me-cyclohexyl radical reactive system are also determined with the composite CBS-QB3 method. All the investigated vinyl rearrangements reactions are exothermic and spontaneous, while the ring opening, C–C bond scission and H-elimination processes are endothermic and nonspontaneous. Among all the investigated elementary reactions, the exocyclization processes are kinetically accessible and readily proceed (due to their significantly lower barrier and they are highly exothermic). Compared with the barrier heights for the distinct vinyl rearrangement pathways in these cyclic alkyl radicals, it can be found that they decrease in the order of 1,3- > 1,2- > 1,4-vinyl transfer. The branching ratios are evaluated at different temperatures on the basis of the quasi-steady state approximation (QSSA). The calculated result shows that the 1,2-, 1,3- and 1,4-vinyl rearrangement reactions are advantaged at low temperature, while the formation of a cycloalkene is favoured at high temperature.

Impact of the acidic group on the hydrolysis of 2-dinitromethylene-5,5-dinitropyrimidine-4,6-dione

The hydrolysis mechanism and the kinetics of using 2-dinitromethylene-5,5-dinitropyrimidine-4,6-dione (NMP) to prepare the representative insensitive energetic material 1,1-diamino-2,2-dinitroethylene (FOX-7) in a nitric–sulfuric acid system are systematically investigated via a density functional theory (DFT) method. The impact of the co-existing acidic group of HSO4− as well as the solvent effects of the mixed acids on the hydrolysis of NMP are elucidated and discerned, and the proposed catalysis and promotion of the hydrolysis of NMP with HSO4− are verified. The HSO4−-catalyzed hydrolysis pathway is more favorable than the direct pathway as well as the H2O-catalyzed hydrolysis, indicating that HSO4− may be a promising catalyst for the preparation of FOX-7 in a mixed acid system. The present study is expected to provide a better understanding of the hydrolysis of NMP, and will significantly help with better preparation of FOX-7 and other nitro-energetic materials.

Quantum chemical studies of the OH-initiated oxidation reactions of propenols in the presence of O2

ABSTRACT The atmospheric oxidation mechanisms of 1- and 2-propenol initiated by OH radical have been theoretically investigated at the CCSD(T)//BH&HLYP/6-311 + +G(d,p) level of theory. Conventional transition state theory was employed to predict the rate constants for the initial reaction channels. The calculations clearly indicate that OH-addition channels contribute maximum to the total reaction, both for 1- and 2-propenol, while H-abstraction channels can be neglected at the temperature range of 220–520 K. The calculated total rate constants at 298 K are 1.66 × 10−11 and 7.69 × 10−12 cm3•molecule−1•s−1 respectively for 1- and 2-propenol, which are in reasonable agreement with the experimental values of similar systems (vinyl ethers + OH reactions). The deduced Arrhenius expressions are k(OH + 1-propenol) = 1.43 × 10−12 exp[(743.7 K)/T] and k(OH + 2-propenol) = 2.86 × 10−12 exp[(310.5 K)/T] cm3•molecule−1•s−1. Under atmospheric condition, the OH-addition intermediates (CH3C•HCH(OH)2, CH3CH(OH)C•H(OH), CH3CH(OH)2•CH2, CH3•C(OH)CH2(OH)) are likely to react rapidly with O2, the theoretically identified major products for 1-propenol are HCOOH, CH3CHO and CH3CH(OH)CHO, and the dominant products for 2-propenol are CH3COOH, HCHO and CH3COCH2OH, both companied with the regeneration of OH and HO2 radicals (crucial reactive radicals in the atmosphere). GRAPHICAL ABSTRACT