RESUMEN
Effective dust removal has long been a challenge in the blasting mining of underground metal mine tunnels, and uncontrolled dust diffusion seriously endangers workers' respiratory systems and the underground space safety environment. However, the vast majority of existing numerical studies on dust diffusion are focused on coal mine fully mechanized mining, which is different from metal mine blasting excavation in terms of stope structure and dust properties. Furthermore, the mechanism by which the forced and exhaust ventilation modes affect the diffusion characteristics of inhalable particles is unclear. In this work, gas-solid flow characteristics for dust diffusion in a typical metal mine blasting tunnel were numerically investigated based on the Euler-Lagrange method, where the blasting face instantly released 6.37 × 107 particles with 100 different sizes, ranging from 0.8 to 200 µm. The interphase forces between airflow and dust particles are comprehensively modeled, and the particle diffusion effect caused by fluid turbulence is described by a discrete random walk model. Detailed information for airflow turbulence and particle migration was revealed, and dust removal efficiencies for inhalable particulate matter (PM10) by forced, exhaust, and hybrid ventilation were analyzed. Numerical results predict a complex airflow pattern in the working roadway, including the jet-flow region, return airflow core region, airflow disorder region, and secondary flow region. Dust diffusion temporal characteristics can be divided into three stages, namely, the initial stage of dust generation, the efficient ventilation and dust removal stage, and the later stage of dust diffusion. Dust diffusion spatial characteristics indicate that under the Coanda wall attachment effect, the dust concentration exhibits nonuniform distribution in both vertical and horizontal directions of the return air roadway. The dust removal efficiency of hybrid ventilation on inhalable particles above respiratory height is better than that of forced ventilation, especially in the return air roadway. The additional exhaust air duct based on forced ventilation can discharge more inhalable particles from the tunnel.
RESUMEN
As an ambident nucleophile, CN- has both C and N atoms that can act as the reactive center to facilitate substitution reactions. We investigate in detail the potential energy profiles of CN-(H2O)0-3 with CH3CH2X (X = Cl, Br, I) to explore the influence of solvent molecules on competition between the different nucleophilic atoms C and N involving the SN2 and E2 pathways. The energy barrier sequence for the transition states follows C@inv-SN2 < N@inv-SN2 < C@anti-E2 < N@anti-E2. When two different atoms act as nucleophilic atoms, the SN2 reaction is always preferred over the E2 reaction, and this preference increases with microsolvation. For the ambident nucleophiles CN-(H2O)0-3, C as the reactive center always has stronger nucleophilicity and basicity than N acting as the reactive center. Regarding the leaving group, the height of the energy barrier is positively correlated with the acidity of the CH3CH2X substrate for the E2 pathway and with X-heterolysis for the SN2 pathway. Furthermore, we found that in the gas phase, the energy barrier for different leaving group systems decreases gradually in the order Cl > Br > I, while in the SMD solution, the energy barrier and product energy increase slightly in the system from X = Cl to Br; this change may be due to the significantly weakened transition-state interaction for the X = Br system. Our activation strain, quantitative molecular orbital, and charge analyses reveal the physical mechanisms underlying the various computed trends. In addition, we also demonstrate the two points recently proposed by Vermeeren, P. . Chem. Eur. J. 2020, 26, 15538-15548.
RESUMEN
Bimolecular substitution reactions involving N as the central atom have continuously improved our understanding of substitution dynamics. This work used chemical dynamics simulations to investigate the dynamics of NH2Cl with N as the central atom and the multiatomic nucleophile CH3O- and compared these results with the F- + NH2Cl reaction. The most noteworthy difference is in the competition between proton transfer (PT) and the SN2 pathways. Our results demonstrate that, for the CH3O- + NH2Cl system, the PT pathway is considerably more favorable than the SN2 pathway. In contrast, no PT pathway was observed for the F- + NH2Cl system at room temperature. This can be attributed to the exothermic reaction of the PT pathway for the CH3O- + NH2Cl reaction and is coupled with a more stable transition state compared to the substitution pathway. Furthermore, the bulky nature of the CH3O- group impedes its participation in SN2 reactions, which enhances both the thermodynamic and the dynamic advantages of the PT reaction. Interestingly, the atomic mechanism reveals that the PT pathway is primarily governed by indirect mechanisms, similar to the SN2 pathway, with trajectories commonly trapped in the entrance channel being a prominent feature. These trajectories are often accompanied by prolonged and frequent proton exchange or proton abstraction processes. This current work provides insights into the dynamics of N-centered PT reactions, which are useful in gaining a comprehensive understanding of the dynamics behavior of similar reactions.
RESUMEN
Nucleophilic substitution (SN2) and elimination (E2) reactions between ambident nucleophiles have long been considered as typical reactions in organic chemistry, and exploring the competition between the two reactions is of great importance in chemical synthesis. As a nucleophile, CN- can use its C and N atoms as the reactive centers to undergo E2 and SN2 reactions, but related research is currently limited. This study uses the CCSD(T)/pp/t//MP2/ECP/d electronic structure method to perform detailed investigations on the potential energy profiles for SN2 and E2 reactions between CN- and CH3CH2I in gaseous and aqueous media. The potential energy profiles reveal that the energy barriers for SN2 and E2 reactions with the C atom as the reactive center are consistently lower than those with the N atom, indicating that the C atom has a stronger nucleophilic ability and stronger basicity. Furthermore, the potential energy profiles in both gas and aqueous environments show that the barriers of SN2 reactions are lower than those for E2 reactions with both C and N as the attacking atom. By using the frontier molecular orbital and activation strain models to explain the interesting phenomenon, the transition from the gas phase to solution was investigated, specifically in the gas-microsolvation-water transition. The results show that water molecules reduce the nucleophilicity and basicity of CN-, while strain energy (ΔEstrain) causes a greater increase in the energy barrier for E2 reactions. This study provides new insights and perspectives on the understanding of CN- as a nucleophile in SN2 reactions and serves as theoretical guidance for organic synthesis.
RESUMEN
Bimolecular nucleophilic substitution (SN2) plays a vital role in organic synthesis. Compared with nucleophiles with one reactive center, ambident nucleophiles can form isomer products. Determining the isomer branching ratios through experiments is difficult, and research on related dynamics characteristics is limited. This study uses dynamics trajectory simulations to explore the dynamics characteristics of the SN2 reaction of ambident nucleophiles CN- and CH3I. The calculated rate constants reproduce the experimental results at room temperature. The dynamics simulations reveal the mechanism of the competition between isomer products CH3CN and CH3NC with a ratio of 0.93 : 0.07. This mechanism is attributed to the height of the central barrier, which strongly stabilizes the transition state of the CH3CN product channel of the formed C-C bond. The product internal energy partitionings and the velocity scattering angle distributions are calculated based on the trajectory simulations, and are in almost agreement with the experimental results obtained at a low collision energy. The dynamics of the title reaction with the ambident nucleophile CN- are also compared with the SN2 dynamics of one reactive center F- and the substrate CH3Y (Y = Cl, I) reactions. This intensive review shows the competition of isomer products for the SN2 reaction of the ambident nucleophile CN- in the current study. This work provides unique insights into reaction selectivity for organic synthesis.