手性和尺寸对受限于单壁碳纳米管内的硝基甲烷热解反应的影响

采用组合的量子化学ONIOM(B3LYP/6-311++G**:UFF)方法, 研究了不同直径的扶手椅型(CNT(5,5)、CNT(6,6)、CNT(8,8))和锯齿型(CNT(9,0)、CNT(10,0)、CNT(11,0))单壁碳纳米管(CNTs)的限制作用对硝基甲烷分子结构和热解反应的影响. 分子结构分析表明, 与单体硝基甲烷相比, 受限于直径较小的CNT(5,5)和CNT(9,0)碳纳米管内的硝基甲烷构型发生扭转, Cs对称性消失, C—N键长略微缩短; 而受限于CNT(6,6)、CNT(8,8)、CNT(10,0)和CNT(11,0)内的硝基甲烷结构变化不明显. 热解势能面计算发现, 与硝基甲烷单体的热解是一个无过渡态的解离过程明显不同: 硝基甲烷在CNT(5,5)和CNT(9,0)碳纳米管内沿C—N键的解离经历过渡态结构, 所需克服的活化能比单体的解离能分别下降了约71和58 kJ·mol-1; 在CNT(6,6)和CNT(10,0)碳纳米管内, 硝基甲烷的热解活化能略有下降; 而在直径较大的CNT(8,8)和CNT(11,0)碳纳米管内, 热解活化能基本不变. 研究结果表明, 直径小的碳纳米管的限制作用对硝基甲烷热解活化能影响显著, 碳纳米管的手性对硝基甲烷热解反应影响不明显.     

Density-dependent liquid nitromethane decomposition: molecular dynamics simulations based on ReaxFF

The decomposition mechanism of hot liquid nitromethane at various compressions was studied using reactive force field (ReaxFF) molecular dynamics simulations. A competition between two different initial thermal decomposition schemes is observed, depending on compression. At low densities, unimolecular C-N bond cleavage is the dominant route, producing CH(3) and NO(2) fragments. As density and pressure rise approaching the Chapman-Jouget detonation conditions (～30% compression, >2500 K) the dominant mechanism switches to the formation of the CH(3)NO fragment via H-transfer and/or N-O bond rupture. The change in the decomposition mechanism of hot liquid NM leads to a different kinetic and energetic behavior, as well as products distribution. The calculated density dependence of the enthalpy change correlates with the change in initial decomposition reaction mechanism. It can be used as a convenient and useful global parameter for the detection of reaction dynamics. Atomic averaged local diffusion coefficients are shown to be sensitive to the reactions dynamics, and can be used to distinguish between time periods where chemical reactions occur and diffusion-dominated, nonreactive time periods.     

Early chemistry in hot and dense nitromethane: molecular dynamics simulations

We report density functional molecular dynamic simulations to determine the early chemical events of hot (T=3000 K) and dense (rho=1.97 g/cm(3), V/V(0)=0.68) nitromethane (CH(3)NO(2)). The first step in the decomposition process is an intermolecular proton abstraction mechanism that leads to the formation of CH(3)NO(2)H(+) and the aci ion H(2)CNO(2) (-). This event is also confirmed to occur in a fast annealing simulation to a final temperature of 4000 K at rho=2.20 g/cm(3). An intramolecular hydrogen transfer that transforms nitromethane into the aci acid form, CH(2)NO(2)H, accompanies this event. To our knowledge, this is the first confirmation of chemical reactivity with bond selectivity for an energetic material near the Chapman-Jouget state of the fully reacted material. We also report the decomposition mechanism followed up to the formation of H(2)O as the first stable product. We note that similarities in the global features of reactants, intermediates, and products of the reacting fluid seem to indicate a threshold for similar chemistry in the range of high densities and temperatures reported herein.     

Substituent effects in carbon-nitrogen cleavage of thiamin derivatives. Fragmentation pathways and enzymic avoidance of cofactor destruction

The combination of thiamin and benzaldehyde can produce benzoin but also destroys thiamin. The destruction comes from fragmentation of the conjugate of thiamin and benzaldehyde undergoing a process that produces a phenyl thiazole ketone and pyrimidine. The key step in this process is cleavage of the C-N bond between the heterocycles, which occurs by an unknown mechanism. Enzymes that utilize similar intermediates do not fragment the cofactor although fragmentation is inherent to the structure. To analyze the nature of the C-N cleavage step, the rates of fragmentation of a series of phenyl-substituted N1'-methyl-2-(1-hydroxybenzyl)thiamin derivatives were determined under two sets of conditions: (1) where proton transfer in the step prior to C-N bond breaking is rate-determining and (2) where C-N bond breaking is rate-determining. The resulting rho values are 1.6 and 1.8, respectively, leading to the conclusion that C-N cleavage is insensitive to substituent effects. On the basis of these results, we conclude that cleavage occurs by a facile process that resembles the outcome of a [1,5] sigmatropic rearrangement. An enzyme may avoid the fragmentation by holding the intermediate in a conformation that prevents such a process, allowing the normal catalytic process to proceed.     

Mechanisms of acid decomposition of dithiocarbamates. 3. Aryldithiocarbamates and the torsional effect

The acid decomposition of some p-substituted aryldithiocarbamates (arylDTCs) was observed in 20% aqueous ethanol at 25 degrees C, mu = 1.0 (KCl, for pH > 0). The pH-rate profiles showed a dumbell shape with a plateau where the observed first-order rate constant k(obs) was equal to k(o), the rate constant of the decomposition of the dithiocarbamic acid species. The acid dissociation constants of the dithiocarbamic acids (pK(a)) and their conjugate acids (pK(+)) were calculated from the pH-rate profiles. Comparatively, k(o) was more than 10(4)-fold faster than alkyldithiocarbamates (alkDTCs) with similar pK(N) (the acid dissociation constant of the parent amine). It was observed that the values of pK(a) and pK(+)were 5 and 8 units of pK, respectively, higher than the expected values from the pK(N) of alkylDTCs. The higher values were attributed to the inhibition of the delocalization of the nitrogen electron pair into the benzene ring because of the strong electron withdrawal effect of the thiocarbonyl group. Comparison of the activation parameters showed that the rate acceleration was due to a decrease in the enthalpy of activation. Proton inventory indicated the existence of a multiproton transition state, and it was consistent with an S to N proton transfer through a water molecule. There are two hydrogens contributing to a secondary SIE, and there are also two protons that are being transferred at the transition state to form a zwitterion followed by fast C-N bond cleavage. The mechanism could also be a concerted asynchronic process where the N-protonation is more advanced than the C-N bond breakdown. The kinetic barrier is similar to the torsional barrier of thioamides, suggesting that the driving force to reach the transition state is the needed torsion of the C-N bond that inhibits the resonance with the thiocarbonyl group and the aromatic moiety, increasing the basicity of the nitrogen and making the proton transfer thermodynamically favorable.     

Dimethyl sulfoxide-aided copper(0)-catalyzed intramolecular decarbonylative rearrangement of N-aryl isatins leading to acridones

Described here is the first example of Cu(0)-catalyzed intramolecular decarbonylative rearrangements of readily available N-aryl isatins assisted by solvent dimethyl sulfoxide(DMSO) under air atmosphere and additive-free conditions leading to various biologically important acridones in good to excellent yields.This novel transformation is proposed to go through a sequential DMSO-aided Cu insertion into the amide C-N bond,CO extrusion,Cu migration,reductive elimination and DMSO-aided proton migration processes,involving multiple types of bond cleavage and formation in a single chemical step.     

Simulation of the photodeactivation of formamide in the nO-pi* and pi-pi* states: an ab initio on-the-fly surface-hopping dynamics study

The short-time photodynamics (1 ps) of formamide in its low-lying singlet excited n(O)-pi(*) and pi-pi(*) states have been investigated by the direct trajectory surface-hopping method based on multiconfigurational ab initio calculations. The simulations showed that in both states, the primary deactivation process is C-N bond dissociation. In the ground state, the energy is transferred to (a) translational motion of the HCO and NH(2) fragments, (b) additional C-H dissociation from the vibrationally hot HCO fragment, or (c) formation of NH(3) and CO. In addition to the C-N dissociation pathway, C-O bond fission is found to be an additional primary deactivation path in the pi-pi(*) dynamics. From fractional occupations of trajectories, lifetimes of formamide were estimated: tau(S(1))=441 fs and tau(S(2))=66 fs.     

Some peculiarities of the influence of molecular structure on the activation energy of radical gas-phase decomposition of aliphatic nitro compounds

Geometric parameters of nitro and fluoronitro derivatives of nitromethane, nitroethane, I-nitropropane, and I-nitrobutane (for which experimental data on kinetics of radical gas-phase decomposition are available) have been determined by the MINDO/3 method. Correlations between changes in logarithm of the activation energy and those in the lengths of C-N and C-H bonds as well as in the ionization potentials have been found. The major changes in the C-N- and C-H- bonds occur when bulky atoms and groups (N02, Cl, Br, and 1) are introduced into the molecules. Nonempirical calculations of energies of dissociation of the C-N bonds in the molecules of nitromethane and its halogen derivatives have been carried out. An equation was proposed which allows one to perform a high-accuracy determination of the activation energy for radical gas-phase decomposition of nitroalkanes using the coefficients of steric interactions in the molecules calculated by the methods of molecular mechanics.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 2438–2444, October, 1996.     

Identification of decomposition reactions for HMDSO organosilicon using quantum chemical calculations

Hexamethyldisiloxane [HMDSO, (CH₃)₃-SiOSi-(CH₃)₃] is an important precursor for SiO₂ formation during flame-based silica material synthesis. As a result, HMDSO reactions in flame have been widely investigated experimentally, and many results have indicated that HMDSO decomposition reactions occur very early in this process. In this paper, quantum chemical calculations are performed to identify the initial decomposition of HMDSO and its subsequent reactions using the density functional theory at the level of B3LYP/6-311+G (d, p). Four reaction pathways—(a) Si O bond dissociation of HMDSO, (b) Si C bond dissociation of HMDSO, (c) dissociation and recombination of Si O and Si C bonds, and (d) elimination of a methane molecule from HMDSO—have been examined and identified. From the results, it is found that the barrier of 84.38 kcal/mol and Si O bond dissociation energy of 21.55 kcal/mol are required for the initial decomposition reaction of HMDSO in the first pathway, but the highest free energy barrier (100.69 kcal/mol) is found in the third reaction pathway. By comparing the free energy barriers and reaction rate constants, it is concluded that the most possible initial decomposition reaction of HMDSO is to eliminate the CH₃ radical by Si C bond dissociation.     

Proton dissociation and transfer in hydrated phosphoric acid clusters

The dynamics and mechanisms of proton dissociation and transfer in hydrated phosphoric acid (H₃PO₄) clusters under excess proton conditions were studied based on the concept of presolvation using the H₃PO₄–H₃O⁺–nH₂O complexes (n = 1–3) as the model systems and ab initio calculations and Born–Oppenheimer molecular dynamics (BOMD) simulations at the RIMP2/TZVP level as model calculations. The static results showed that the smallest, most stable intermediate complex for proton dissociation (n = 1) is formed in a low local‐dielectric constant environment (e.g., ε = 1), whereas proton transfer from the first to the second hydration shell is driven by fluctuations in the number of water molecules in a high local‐dielectric constant environment (e.g., ε = 78) through the Zundel complex in a linear H‐bond chain (n = 3). The two‐dimensional potential energy surfaces (2D‐PES) of the intermediate complex (n = 1) suggested three characteristic vibrational and ¹H NMR frequencies associated with a proton moving on the oscillatory shuttling and structural diffusion paths, which can be used to monitor the dynamics of proton dissociation in the H‐bond clusters. The BOMD simulations over the temperature range of 298–430 K validated the proposed proton dissociation and transfer mechanisms by showing that good agreement between the theoretical and experimental data can be achieved with the proposed rate‐determining processes. The theoretical results suggest the roles played by the polar solvent and iterate that insights into the dynamics and mechanisms of proton transfer in the protonated H‐bond clusters can be obtained from intermediate complexes provided that an appropriate presolvation model is selected and that all of the important rate‐determining processes are included in the model calculations. © 2015 Wiley Periodicals, Inc.     

Theoretical study of the nitroalkane thermolysis. 1. Computation of the formation enthalpy of the nitroalkanes, their isomers and radical products

The gas phase enthalpies of formation of mono-, di-, tri-, tetranitromethane and nitroethane, as well as of their nitrite and aci-form isomers were calculated using different multilevel (G2, G3, G2M(CC5)) and density functional theory (DFT)-based (B3LYP, MPW1B95 and MPWB1K) techniques. The enthalpies of the C-N bond dissociation and isomerization of these nitroalkanes were also calculated. The calculated values of the formation and reaction enthalpies were compared with the experimental data when these data were available. It was found that only the G3 procedure gave accurate (within 1 kcal/mol) results for the formation enthalpy of nitroalkanes, their isomers, and radical products. The G3 procedure and two new hybrid meta DFT methods proposed by Truhlar's group (Zhao, Y.; Truhlar, D. J. Phys. Chem. A 2004, 108, 6908) showed good results for the reaction enthalpies of the nitromethane isomerization and the C-N bond dissociation. Our calculation results were used to analyze thermodynamics of the dissociation and isomerization reactions of the poly nitro-substituted methanes.     

Initial nucleation of platinum clusters after reduction of K(2)PtCl(4) in aqueous solution: a first principles study

The initial nucleation of platinum clusters after the reduction of K(2)PtCl(4) in aqueous solution is studied by means of first principles molecular dynamics simulations. A reaction mechanism leading to a Pt dimer is revealed both by gas-phase simulations and by simulations which model the solution environment. The key step of the observed reaction process is the formation of a Pt-Pt bond between a Pt(I) complex and an unreduced Pt(II) complex. In light of this result, we discuss the reduction process leading to the formation of platinum nanoparticles. In the generally accepted model, the nucleation of Pt particles starts only when a critical concentration of Pt(0) atoms is reached. Here, we discuss a complementary mechanism where metal-metal bonds form between Pt complexes in higher oxidation states. This is consistent with a number of experimental results which show that a high concentration of zerovalent atoms is not necessary to start the nucleation.     

Ab initio molecular dynamics simulations study on initial decompositions of β‐HMX at high temperature coupled with high pressures

We performed ab initio molecular dynamics simulations to investigate initial decomposition mechanisms and subsequent chemical processes of β‐HMX (cyclotetramethylene tetranitramine) (octahydro‐1,3,5,7‐tetranitro‐1,3,5,7‐tetrazocine) crystals at high temperature coupled with high pressures. It was found that the initial decomposition step is the simultaneous C–H and N–NO₂ bond cleavage at 3,500 K. When the pressure (1–10 GPa) is applied, the first reaction steps are primarily the C–N and C–H bond fission at 3,500 K. The C–H bond cleavage is a triggering decomposition step of the HMX crystals at 3,500 K coupled with 16 GPa. This indicates that the C–H bonds are much easier to be broken and the hydrogen radicals are much more active. The applied pressures (1–10 GPa) accelerate the decompositions of HMX at 3,500 K. The decomposition pathways and time evolution of the main chemical species demonstrate that the temperature is the foremost factor that affects the decomposition at high pressures (1–10 GPa). However, the decomposition of HMX is dependent on both the temperature (3,500 K) and the pressure (16 GPa). This work will enrich the knowledge of the decompositions of condensed energetic materials under extreme conditions.     

硝基甲烷异构化反应势能面的ab initio研究

在B3LYP/6-311+ +G(2d,2p)水平上,优化得到硝基甲烷CH3NO2的10种异构体和23个异构化反应过渡态,并用G2MP2方法进行了单点能计算.根据计算得到的G2MP2相对能量,探讨了CH3NO2势能面上异构化反应的微观机理.研究表明,反应初始阶段的CH3NO2异构化过程具有较高的能垒,其中CH3NO2的两个主要异构化反应通道,即CH3NO2→CH3ONO和CH3NO2→CH2N(O)OH的活化能分别为270.3和267.8 kJ/mol,均高于CH3NO2的C-N键离解能.因而,从动力学角度考虑, CH3NO2的异构化反应较为不利.     

A multistate empirical valence bond description of protonatable amino acids

The multistate empirical valence bond (MS-EVB) model, which was developed for molecular dynamics simulations of proton transport in water and biomolecular systems, is extended for the modeling of protonatable amino acid residues in aqueous environments, specifically histidine and glutamic acid. The parameters of the MS-EVB force field are first determined to reproduce the geometries and energetics of the gas phase amino acid-water clusters. These parameters are then optimized to reproduce experimental pK(a) values. The free energy profiles for acid ionization and the corresponding pK(a) values are calculated by MS-EVB molecular dynamics simulations utilizing the umbrella sampling technique, with the center of excess charge coordinate chosen as the dissociation reaction coordinate. A general procedure for fitting the MS-EVB parameters is formulated, which allows for the parametrization of other amino acid residues with protonatable groups and the subsequent use of the MS-EVB approach for molecular dynamics simulations of proton transfer processes in proteins involving protonation/deprotonation of the protonatable amino acid groups.     

Force‐Induced Reversal of β‐Eliminations: Stressed Disulfide Bonds in Alkaline Solution

Understanding the impact of tensile forces on disulfide bond cleavage is not only crucial to the breaking of cross‐linkers in vulcanized materials such as strained rubber, but also to the regulation of protein activity by disulfide switches. By using ab initio simulations in the condensed phase, we investigated the response of disulfide cleavage by β‐elimination to mechanical stress. We reveal that the rate‐determining first step of the thermal reaction, which is the abstraction of the β‐proton, is insensitive to external forces. However, forces larger than about 1 nN were found to reshape the free‐energy landscape of the reaction so dramatically that a second channel is created, where the order of the reaction steps is reversed, turning β‐deprotonation into a barrier‐free follow‐up process to C?S cleavage. This transforms a slow and force‐independent process with second‐order kinetics into a unimolecular reaction that is greatly accelerated by mechanical forces.     

Thermal Decomposition of 3-Nitro-1-nitromethyl-1,2,4-1H-triazole in Solution

The thermal decomposition of 3-nitro-1-nitromethyl-1,2,4-1H-triazole in 1% solution in phenyl benzoate proceeds homolytically with initial rupture of the CH₂-NO₂ bond. Activation parameters of the process were E _a = 172.6 kJ/mol, log A = 14.25. The initial basic pathway of fragmentation of the molecule under electron impact coincides with the first step of thermal decomposition, which is in agreement with X-ray structural and calculated quantum chemical data on bond stability in the molecule.     

Connecting structure to infrared spectra of molecular and autodissociated HCl--water aggregates

The properties of perdeuterated HCl(H2O)n aggregates with n=1, 2, ..., 6 water molecules are studied by means of ab initio molecular dynamics simulations. The specific focus is on the phenomenon of autodissociation of the acid HCl as a function of the microsolvation environment size. The calculations provide a basis for characterization in terms of autodissociation energetics as well as in terms of the impact of thermal fluctuations on structure including proton fluxionality and in terms of anharmonic infrared vibrational spectra. Structure stabilization is dominated by strong hydrogen bonds resulting in distinct topologies, which, in turn, heavily influence acid dissociation. The latter is favored for the first time when n = 4. In that case, three hydrogen bonds can be donated toward the chlorine while at the same time a hydronium core is perfectly solvated according to the eigencomplex motif. Hydrogen-bonding interactions between DCl and its solvating molecules affect the dynamical behavior of the D-Cl bond significantly. This can be seen by the onset of fluxionality and an emerging tendency toward proton transfer for the larger clusters. Connecting IR spectra to structural information is possible by exploiting the following observations. Zwitterionic species show characteristic differences in the hydronium region, whereas the D-Cl stretching regime is useful to distinguish neutral aggregates. Furthermore, in the case of fluxional protons large-amplitude motion leads to characteristic band shifts and significant band broadening effects.     

A computational study of ultrafast acid dissociation and acid-base neutralization reactions. II. The relationship between the coordination state of solvent molecules and concerted versus sequential acid dissociation

We investigate the role played by the coordination state of pre-existing water wires during the dissociation of moderately strong acids by means of first-principles molecular dynamics calculations. By preparing 2,4,6-tricyanophenol (calc. pKa～0.5) in two different initial states, we are able to observe sequential as well as concerted trajectories of dissociation: On one hand, equilibrium dissociation takes place on a ～50 ps timescale; proton conduction occurs through three-coordinated water wires in this case, by means of sequential Grotthus hopping. On the other hand, by preparing 2,4,6-tricyanophenol in a hydration state inherited from that of equilibrated phenol (calc. pKa=7.6), the moderately strong acid finds itself in a presolvated state from which dissociation can take place on a ～1 ps timescale. In this case, concerted dissociation trajectories are observed, which consist of proton translocation through two intervening, four-coordinated, water molecules in 0.1-1.0 ps. The present results suggest that, in general, the mechanism of proton translocation depends on how the excess proton is injected into a hydrogen bond network. In particular, if the initial conditions favour proton release to a fourfold H-bonded water molecule, proton translocation by as much as 6-8 A? can take place on a sub-picosecond timescale.