The mechanism and kinetics of decomposition of 5-aminotetrazole

The pathway and ab initio direct kinetics of the decomposition 5-aminotetrazole (5-ATZ) to HN₃ and NH₂CN was investigated. Reactant, products and transition state were optimized with MP2 and B3LYP methods using 6–311G** and aug-cc-pVDZ basis sets. The intrinsic reaction coordinate curve of the reaction was calculated using the MP2 method with 6–311G** basis set. The energies were refined using CCSD(T)/6–311G**. Rate constants were evaluated by conventional transition-state theory (CVT) and canonical variational transition-state theory (TST), with tunneling effect over 300 to 2,500 K. The results indicated that the tunneling effect and the variational effect are small for the calculated rate constants. The fitted three-parameter expression calculated using the CVT and TST methods are and , respectively. Figure The mechanism of the decomposition process of 5-ATZ to HN₃ and NH₂CN     

Theoretical studies on the tautomerism and intramolecular hydrogen shifts of 5-Amino-tetrazole in the gas phase

The tautomerism and intramolecular hydrogen shifts of 5-amino-tetrazole in the gas phase were studied in the present work. The minimum energy path (MEP) information of 5-amino-tetrazole was obtained at the CCSD(T)/6–311G**//MP2/6–311G** level of theory. The six possible tautomers of 1H, 4H-5-imino-tetrazole (a), 1H-5-amino-tetrazole (b), 2H-5-amino-tetrazole (c), 1H, 2H-5-imino-tetrazole (d), the mesoionic form (e) and 2H, 4H-5-imino-tetrazole (f) were investigated. Among these tautomers, there are 2 amino- forms, 3 imino- forms, and 1 mesoionic structure form. In all the tautomers, 2-H form (c) is the energetically preferred one in the gas phase. In the imino- tautomers, the energy value of the compound d is similar as that of the compound f but it is higher than the energy value of the compound a. The potential energetic surface (PES) and kinetics for five reactions have been investigated. Reaction 2 (b→c) was hydrogen shifts only in which the 1-H and 2-H rearrangement. This means that the reaction 2 (b→c) is energetically favorable having an activation barrier of 45.66 kcal·mol⁻¹ and the reaction energies (ΔE) is only 2.67 kcal·mol⁻¹. However, the reaction energy barrier for tautomerism of reaction 1 (b→e) is 54.90 kcal·mol⁻¹. Reaction 1 (b→a), reaction 3 (c→d), and reaction 5 (c→f) were amino- →imino- tautomerism reactions. The energy barriers of amino- →imino- tautomerism reactions required are 59.39, 65.57, 73.61 kcal·mol⁻¹ respectively in the gas phase. The calculated values of rate constants using TST, TST/Eckart, CVT, CVT/SCT and CVT/ZCT methods using the optimized geometries obtained at the MP2/6–311G** level of theory show the variational effects are small over the whole temperature range, while tunneling effects are big in the lower temperature range for all tautomerism reactions. Graphical Abstract Figure (DOC 45.0 KB)     

Theoretical study of the decomposition mechanisms and kinetics of the ingredients RDX in composition B

RDX as a component in composition B (TNT + RDX) was first studied by us on its mechanism and kinetics of decomposition reactions in this paper. We have pointed out three possible pathways and found a new low-energy process of its decomposition. The N-N bond cleavage in composition B has higher dissociation energies than the monomer, but it is also the initial step. The optimized structures and the frequencies of all the stationary points were calculated at the B3LYP/6-31G(d) level. The minimum-energy paths were obtained by using the intrinsic reaction coordinate (IRC) theory, and the reaction potential energy curve was corrected with zero-point energy. Finally, the rate constants were calculated in a wide temperature region from 200 to 2500 K using TST, TST/Eckart theories. The obtained results also indicate that the tunneling effects are remarkable at low temperature (200 K 相似文献   

Kinetics investigation of the hydrogen abstraction reaction between CH<Subscript>3</Subscript>SS and CN radicals

The reaction mechanisms and rates for the H abstraction reactions between CH₃SS and CN radicals in the gas phase were investigated with density functional theory (DFT) methods. The geometries, harmonic vibrational frequencies, and energies of all stationary points were obtained at B3PW91/6-311G(d,p) level of theory. Relationships between the reactants, intermediates, transition states and products were confirmed, with the frequency and the intrinsic reaction coordinate (IRC) analysis at the same theoretical level. High accurate energy information was provided by the G3(MP2) method combined with the standard statistical thermodynamics. Gibbs free energies at 298.15 K for all of the reaction steps were reported, and were used to describe the profile diagrams of the potential energy surface. The rate constants were evaluated with both the classical transition state theory and the canonical variational transition state theory, in which the small-curvature tunneling correction was included. A total number of 9 intermediates (IMs) and 17 transition states (TSs) were obtained. It is shown that IM1 is the most stable intermediate by the largest energy release, and the channel of CH₃SS?+?CN?→?IM3?→?TS10?→?P1(CH₂SS?+?HCN) is the dominant reaction with the lowest energy barrier of 144.7 kJ mol^?1. The fitted Arrhenius expressions of the calculated CVT/SCT rate constants for the rate-determining step of the favorable channel is k =7.73?×?10⁶? T ^1.40exp(?14,423.8/T) s^?1 in the temperature range of 200–2000 K. The apparent activation energy E _a(app.) for the main channel is ?102.5 kJ mol^?1, which is comparable with the G3(MP2) energy barrier of ?91.8 kJ mol^?1 of TS10 (relative to the reactants).     

Theoretical Ab initio study of the effects of methylation on the nature of hydrogen bonding in A:T base pair

We report the results of a theoretical ab initio study of methylation in Watson-Crick A:T base pairs. Equilibrium geometries were obtained without symmetry restrictions by the gradient procedure at DFT level of theory with the standard 6-31G(d) basis set. Each local minima was verified by energy second derivative calculations. Single-point calculations for the DFT geometries have been performed at the MP2/6-31G(d,p), MP2/6-31++G(d,p), and MP2/6-311++G(2d,2p) levels of theory. The geometrical parameters, relative stabilities and counterpoise corrected interaction energies are reported. In addition, using a variation-perturbation energy decomposition scheme, we have found the important contributions to the total interaction energy.     

EVALUATION OF SEVERAL ECONOMICAL COMPUTATIONAL METHODS FOR GEOMETRY OPTIMISATION OF PHOSPHORUS ACID DERIVATIVES

Several economical methods for geometry optimisation, applicable to larger molecules, have been evaluated for phosphorus acid derivatives. MP2/cc-pVDZ and B3LYP/6-31+G(d) geometry optimisations are used as reference points, results from geometry optimisations for other methods and their subsequent single point energy calculations are compared to these references. The geometries from HF/MIDI! optimisations were close to those of the references and subsequent single point energies with B3LYP/6-31+G(d,p) or EDF1/6-31+G(d) gave a mean average deviation (MAD) of less than 0.5 kcal mol^?1 from those obtained with the reference geometries.     

A density functional theory study of the decomposition mechanism of nitroglycerin

The detailed decomposition mechanism of nitroglycerin (NG) in the gas phase was studied by examining reaction pathways using density functional theory (DFT) and canonical variational transition state theory combined with a small-curvature tunneling correction (CVT/SCT). The mechanism of NG autocatalytic decomposition was investigated at the B3LYP/6-31G(d,p) level of theory. Five possible decomposition pathways involving NG were identified and the rate constants for the pathways at temperatures ranging from 200 to 1000 K were calculated using CVT/SCT. There was found to be a lower energy barrier to the β-H abstraction reaction than to the α-H abstraction reaction during the initial step in the autocatalytic decomposition of NG. The decomposition pathways for CHOCOCHONO₂ (a product obtained following the abstraction of three H atoms from NG by NO₂) include O–NO₂ cleavage or isomer production, meaning that the autocatalytic decomposition of NG has two reaction pathways, both of which are exothermic. The rate constants for these two reaction pathways are greater than the rate constants for the three pathways corresponding to unimolecular NG decomposition. The overall process of NG decomposition can be divided into two stages based on the NO₂ concentration, which affects the decomposition products and reactions. In the first stage, the reaction pathway corresponding to O–NO₂ cleavage is the main pathway, but the rates of the two autocatalytic decomposition pathways increase with increasing NO₂ concentration. However, when a threshold NO₂ concentration is reached, the NG decomposition process enters its second stage, with the two pathways for NG autocatalytic decomposition becoming the main and secondary reaction pathways.     

Evaluation of several economical computational methods for geometry optimization of phosphorus acid derivatives

Several economical methods for geometry optimisation, applicable to larger molecules, have been evaluated for phosphorus acid derivatives. MP2/cc-pVDZ and B3LYP/6-31+G(d) geometry optimizations are used as reference points, results from geometry optimizations for other methods and their subsequent single point energy calculations are compared to these references. The geometries from HF/MIDI! optimizations were close to those of the references and subsequent single point energies with B3LYP/6-31+G(d,p) or EDF1/6-31+G(d) gave a mean average deviation (MAD) of less than 0.5 kcal mol-1 from those obtained with the reference geometries.     

Theoretical ab initio study of the effects of methylation on structure and stability of G:C Watson-Crick base pair

Methylation of DNA occurs most readily at N(3), N(7), and O(6) of purine bases and N(3) and O(2) of pyrimidines. Methylated bases are continuously formed through endogenous and exogenous mechanisms. The results of a theoretical ab initio study on the methylation of G:C base pair components are reported. The geometries of the local minima were optimized without symmetry restrictions by the gradient procedure at DFT level of theory and were verified by energy second derivative calculations. The standard 6-31G(d) basis set was used. The single-point calculations have been performed at the MP2/6-31G(d,p), MP2/6-31++G(d,p), and MP2/6-311++G(2d,2p) levels of theory. The geometrical parameters, relative stability and counterpoise corrected interaction energies are reported. Also, using a variation-perturbation energy decomposition scheme we have found the vital contributions to the total interaction energy.     

Theoretical ab Initio Study of the Effects of Methylation on Structure and Stability of G:C Watson-Crick Base Pair

Abstract

Methylation of DNA occurs most readily at N(3), N(7), and O(6) of purine bases and N(3) and O(2) of pyrimidines. Methylated bases are continuously formed through endogenous and exogenous mechanisms. The results of a theoretical ab initio study on the methylation of G:C base pair components are reported. The geometries of the local minima were optimized without symmetry restrictions by the gradient procedure at DFT level of theory and were verified by energy second derivative calculations. The standard 6–31G(d) basis set was used. The single-point calculations have been performed at the MP2/6–31G(d,p), MP2/6–31₊₊G(d,p), and MP2/6–311₊₊G(2d,2p) levels of theory. The geometrical parameters, relative stability and counterpoise corrected interaction energies are reported. Also, using a variation-perturbation energy decomposition scheme we have found the vital contributions to the total interaction energy.     

Ab initio studies on the reactivity of the CF3OCH2O radical: Thermal decomposition vs. reaction with O2

Hydrofluoroethers are being considered as potential candidates for third generation refrigerants. The present investigation involves the ab initio quantum mechanical study of the decomposition mechanism of CF₃OCH₂O radical formed from a hydrofluoroether, CF₃OCH₃ (HFE-143a) in the atmosphere. The geometries of the reactant, products and transition states involved in the decomposition pathways are optimized and characterized at the DFT (B3LYP) level of theory using 6-311G(d,p) basis set. Energy calculations have been performed at the G2(MP2) and G2M(CC,MP2) level of theory. Two prominent decomposition channels, C-O bond scission and reaction with atmospheric O₂ have been considered for detailed investigation. Studies performed at the G2(MP2) level reveals that the decomposition channel involving C-O bond scission occurs with a barrier height of 23.8 kcal mol⁻¹ whereas the oxidative pathway occurring with O₂ proceeds with an energy barrier of 7.2 kcal mol⁻¹. On the other hand the corresponding values at G2M(CC,MP2) are 24.5 and 5.9 kcal mol⁻¹ respectively. Using canonical transition state theory (CTST) rate constants for the two pathways considered are calculated at 298 K and 1 atm pressure and found to be 5.9 × 10⁻⁶ s⁻¹ and 2.3 × 10⁻⁵ s⁻¹ respectively. The present study concludes that reaction with O₂ is the dominant path for the consumption of CF₃OCH₂O in the atmosphere. Transition states are searched and characterized on the potential energy surfaces involved in both of the reaction channels. The existence of transition state on the corresponding potential energy surface is ascertained by performing intrinsic reaction coordinate (IRC) calculation.     

Theoretical study of the reaction of CH2XO (X = F, Cl, Br) radicals with the NO radical

In this paper, we focus on the multiple-channel reactions of CH₂XO (X = F, Cl, Br) radicals with the NO radical by means of direct dynamic methods. All structures of the stationary points were obtained at the MP2/6-311+G(d,p) level and vibrational frequency analysis was also performed at this level of theory. The minimum energy path (MEP) was obtained via the intrinsic reaction coordinate (IRC) theory at the MP2/6-311+G(d,p) level, and higher-level energetic information was refined by the MC-QCISD method. The rate constants for the three hydrogen abstraction reaction channels over the temperature range 200–1,500 K were calculated by the improved canonical variational transition state theory (ICVT) with a correction for small-curvature tunneling (SCT). The rate constants calculated in this manner were in good agreement with the available experimental data, and the three-parameter rate–temperature formulae for the temperature range 200–1,500 K were $ {k_{1{\text{a}} }}(T)=0.32\times {10^{-18 }}{T^{1.83 }}\exp \left( {1748.54/T} \right) $ , $ {k_{2{\text{a}} }}(T)=0.22\times {10^{-19 }}{T^{2.19 }}\exp \left( {1770.19/T} \right) $ , $ {k_{3{\text{a}} }}(T)=0.88\times {10^{-20 }}{T^{2.20 }}\exp \left( {1513.82/T} \right) $ (in units of cm³ molecule^?1?s^?1).     

Formation of the Vilsmeier-Haack complex: the performance of different levels of theory

Because of discrepancies in the available experimental data, an extensive theoretical investigation of the formation of the Vilsmeier-Haack (VH) complex has been carried out. The barriers to complex formation calculated using eight different density functional methods (BLYP, B2-PLYP, B3LYP, B3PW91, MPW1K, M06-2X, and PBE1PBE), MP2, and extrapolation techniques (CBS-QB3, G3B3) with several basis sets (6 − 31 + G**, 6 − 311++G**, 6 − 311 + (3df,2p), aug-cc-pVDZ, and aug-cc-pVTZ) were compared with experimental data. For the overall reaction, MP2/aug-cc-pVDZ and M06-2X/6−31 + G(d,p) perform best compared to the CBS techniques. The results help clarify some open mechanistic questions.     

Theoretical Study On The Reaction Mechanism Of Gecl3ch2ch2cooh+h2o→gecl2ohch2ch2cooh+hcl

The mechanism and dynamical properties for the title reaction have been investigated theoretically. Three reaction pathways have been found. Geometries, vibrational frequencies, infra-red (IR) intensities and relative energies for various stationary points in the three reaction channels have been determined respectively. The corresponding rate constants at the B3LYP/6-31++G(2d,2p) level have been deduced over a wide temperature range of 200–2000 K by using canonical variational transition state theory with small curvature tunnelling effect. Solvent effects are taken into account via the Onsager model of self-constant reaction field at the same level of theory. This preliminary study shows that the complex formation is favoured by the use of water solvent.     

Theoretical study on the thermal decomposition of model compounds for Poly (dialkyl fumarate)

The thermal decomposition of model compounds for poly (dialkyl fumarate) was studied by using ab initio and density functional theory (DFT) calculations. To determine the most favorable reaction pathway of thermal decomposition, geometries, structures, and energies were evaluated for reactants, products, and transition states of the proposed pathways at the HF/6-31G(d) and B3LYP/6-31G(d) levels. Three possible paths (I, II and III) and subsequent reaction paths (IV and V) for the model compounds of poly (dialkyl fumarate) decomposition had been postulated. It has been found that the path (I) has the lowest activation energy 193.8 kJ mol⁻¹ at B3LYP/6-31G(d) level and the path (I) is considered as the main path for the thermal decomposition of model compounds for poly (dialkyl fumarate).      

Hybrid density functional theory with a specific reaction parameter: hydrogen abstraction reaction of difluoromethane by the hydroxyl radical

Accurate potential energy surfaces for the OH + CH2F2 --> H2O + CHF2 reaction are constructed using hybrid and hybrid meta density functional theory methods (mPW1PW91, B1B95, and mPW1B95) with specific reaction parameters in conjunction with the 6-31 + G(d,p) basis set. The accuracy of a surface is examined by comparing the calculated rate constants with the experimental ones. The rate constants are calculated over the temperature range 200-1,500 K using variational transition state theory with multidimensional tunneling contributions. The hybrid density functional theory methods with specific-reaction-parameter Hartree-Fock exchange contributions (39.2-41.0% for mPW1PW91, 41.0-42.2% for B1B95, and 44.9-46.3% for mPW1B95, respectively) provide accurate rate constants over an extended temperature range. The classical barrier height for the hydrogen abstraction reaction on these potential energy surfaces is determined to be 5.0-5.3 kcal mol(-1), and the best estimate value is 5.14 kcal mol(-1).     

Density functional computations of enantioselective alkynylation of aldehyde catalyzed by chiral zinc(II)-complexes

The enantioselective alkynylation of aldehyde catalyzed by chiral zinc(II)-complexes was studied by means of the density functional theory (DFT). All the structures were optimized completely at the B3LYP/6-31G(d,p) level. To obtain more exact energies, single-point energy calculations at B3LYP/6-31+G(d,p) level were carried out on the B3LYP/6-31G(d,p) geometries. As shown, this enantioselective alkynylation was endothermic. The chirality-determining step for the alkynylation was the formation of the catalyst–ethanol complexes and the transition states for this step involved a six-membered ring. The dominant products predicted theoretically were of (R)-chirality, in good agreement with experiment.      

Theoretical study of N<Subscript>4</Subscript>X (X = O,S, Se) systems

A series of N₄X (X = O, S, Se) compounds have been examined with ab initio and density functional theory (DFT) methods. To our knowledge, these compounds, except for the C_2v ring and the C_3v towerlike isomers of N₄O, are first reported here. The ring structures are the most energetically favored for N₄X (X = O and S) systems. For N₄Se, the cagelike structure is the most energetically favored. Several decomposition and isomerization pathways for the N₄X species have been investigated. The dissociation of C_2v ring N₄O and N₄S structures via ring breaking and the barrier height are only 1.1 and −0.2 kcal mol⁻¹ at the CCSD(T)/6-311+G*//MP2/6-311+G* level of theory. The dissociation of the cagelike N₄X species is at a cost of 12.1–16.2 kcal mol⁻¹. As for the towerlike and triangle bipyramidal isomers, their decomposition or isomerization barrier heights are all lower than 10.0 kcal mol⁻¹. Although the C_S cagelike N₄S isomer has a moderate isomerization barrier (18.3–29.1 kcal mol⁻¹), the low dissociation barrier (−1.0 kcal mol⁻¹) indicates that it will disappear when going to the higher CCSD(T) level. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.