Theoretical Studies of Water’s and Methanol’s Effects on Alcoholysis of N-Benzyl-3-oxo-β-sultam

The mechanisms about the water’s and methanol’s effects on the alcoholysis of N-benzyl-3-oxo-β-sultam together with their differences have been studied by using density func- tional theory at the B3LYP/6-31G* level. The results, in comparison with a previous study on the relative reaction without the assistance of water and methanol, show that the added water or methanol can remarkably reduce the energy barrier of alcoholysis reaction of N-benzyl-3-oxo- β-sultam and the most favorite pathway is the breaking of C–N bond instead of S–N. It is also found that the reaction energy barrier of methanol-assisted alcoholysis is a little higher than that of the water-assisted one.     

Decomposition Mechanism of 5,5′-Bis(tetrazole)-1,1′-dioIate(TKX-50) Anion Initiated by Intramolecular Oxygen Transfer

Density functional theory calculations at the B3LYP/6-31+G^** and B3LYP/6-311++G ^** levels were perfonned on thermal decomposition of 5,5′-bis(tetrazole)-1 ,r-diolate(TKX-50) anion with an intramolecular oxygen transfer being an initial step. Tlie results show that the intramolecular oxygen transfers are the rate-limiting steps for the decomposition of title anion with activation energies being in the range of 287-328 kJ/mol. Judged by the nucleus- independent chemical shift values, the formation of antiaromatic ring in transition state or the decrease of aromaticity of the tetrazole ring of the reactant makes somewhat contribution to the high potential energies of the rate-limiting transition states. However, the activation energies of the following N2 elimination tlirough various pathways are in a low range of 136-166 kJ/mol. The tetrazole ring acts as an electron donor or acceptor in difierent pathways to assist the bond nipture or group elimination. The rate constants in a temperature range of 500-2000 K for all the intramolecular oxygen transferring reactions were obtained. The corresponding linear relationships between InA and 1/T were established.     

Theoretical Study on the Reaction of Thymine with Iodomehane in the Water Phase

The reaction mechanisms of carcinogenic methylating agent iodomethane (MeI) with keto and enol tautomers of thymine (K- and E-thymine) were studied by using the B3LYP/6-311+G (d, p) method in water phase. The solvent effects were examined using the polarizable continuum model (PCM). Specifically, PCM single-point calculations at the same level of theory were performed in acetone and CCl4 that represent a range in nonpolarity. The calculated results show that the reaction of K-thymine with MeI is a two-step mechanism, whereas that of E-thymine is a one-step mechanism. Our calculations reveal that K-thymine is appreciably more stable than the enol form in the water phase or in the two solvents. The K- and E-form reaction barriers are 135.6 and 222.1 kJ/mol, respectively in water phase. These findings indicate that the reactions mentioned above could not occur efficiently in biological media in the absence of catalyst. Our conclusions are in agreement with the previous studies on the reactions of guanine with methyl chloride and methyl bromide.     

Density Functional Study on the Mechanism of Amadori Rearrangement Reaction

The reaction mechanism of amadori rearrangement in the initial stage of Maillard reaction has been investigated by means of density functional theory calculations in the gaseous phase and aqueous solution.Cyclic ribose and glycine were taken as the model in the amadori rearrangement.Reaction mechanisms have been proposed,and possibility for the formation of different compounds has been evaluated through calculating the relative energy changes for different steps of the reaction by following the total mass balance.The calculations reveal that the amadori rearrangement initialized via the intramolecular rearrangement,transferring one proton from N(3) to O(4) atom.In the next step,the second proton is also transferred from N(3) to O(4) atom,corresponding to the cleavage of C(4)-O(4) bond and the release of one water molecule.Then another proton is transferred from N(3) to C(5) atom via TS3 with the reaction barrier of 58.3 kcal·mol-1 after tunneling the effect correction calculated at the B3LYP/6-31+G(d) level of theory,and this step is rate limiting for the whole catalytic cycle.Ultimately,the product is generated via keto-enolic tautomerization.Present calculation could provide insights into the reaction mechanism of Maillard reaction since experimental evaluation of the role of intermediates in the Maillard reaction is quite complicated.     

Theoretical Studies on the Reaction Mechanism of 1-Chloroethane with Hydroxyl Radical

The reaction mechanism of 1-chloroethane with hydroxyl radical has been inves- tigated by using density functional theory (DFT) B3LYP/6-31G (d, p) method. All bond dissociation enthalpies were computed at the same theoretical level. It was found that hydrogen abstraction pathway is the most favorable. There are two hydrogen abstraction pathways with activation barriers of 0.630 and 4.988 kJ/mol, respectively, while chlorine abstraction pathway was not found. It was observed that activation energies have a more reasonable correlation with the reaction enthalpy changes (△Hr) than with bond dissociation enthalpies (BDE).     

Theoretical Study on the Isomerization Mechanism of Enol Ester from 2-Acyl-1, 3-cyclohexanediones

The present paper employed density function theory to investigate two reaction pathways for isomerization of enol ester proposed by Yang(path a) and the present authors(path a), respectively. The base catalytic effects of solvent triethylamine on these two reactions were also evaluated. It is demonstrated that path B is more preferable than path a due to low barrier height for the rate-determining step.     

Reaction Mechanism and Kinetics for HCCO Radical with NO

The mechanism and dynamical properties for the reaction of HCCO radicals with NO were investigated theoretically. The minimum energy paths(MEP) of the reaction were calculated by using the density functional theory(DFT) at the B3LYP/6-311 G^** level, and the energies along the MEP were further refined at the QCISD(T)/6-311 G^** level. It is found that the reaction mechanism of the title reaction involves three channels, producing HCNO CO, HONC CO and HCN CO2 products, respectively. Channel 1 is the most favorable path. The rate constant for channel 1 were calculated over a temperature range of 800-2500 K by using the canonical variational transition-state theory(CVT). The rate constant for the main path is negatively dependent on temperature, which is a characteristic of radical reactions with negative activation energy, and the variational effect for the rate constant calculation is small in the whole temperature range.     

COS Activation by Zr~＋ in the Gas Phase：A Case of Two-state Reactivity Process

To elucidate the mechanisms of Zr + reacting with COS,both the quartet and doublet potential energy surfaces (PESs) for reactions of Zr + (4 F,2 D) with COS in the gas phase have been investigated in detail by means of density functional method (B3LYP).To obtain more accurate results,the coupled cluster single-point calculations (CCSD(T)) using B3LYP optimized geometries were performed.For the C-O bond activation,the calculated results indicate that both the quartet and doublet states proceed via an insertion-elimination mechanism.For the C-S bond activation,the quartet reaction has an insertion-elimination mechanism,but the doublet reaction is a direct abstraction of the sulfur atom by Zr +.The C-S bond activation is found to be energetically more favorable than the C-O bond activation.It is found that the reaction of the 4 F gound state of Zr + to yield ZrO + is spin-forbidden (Zr + (4 F) + COS (1 Σ) → ZrO + (2) + CS (1 Σ)) and the crossing points were approximately determined.All the results have been compared with the existing experimental and theoretical data.     

Density functional theory study of 1:1 glycine–water complexes in the gas phase and in solution

We present a systematic study of 1:1 glycine-water complexes involving all possible glycine conformers. The complex geometries are fully optimized for the first time both in the gas phase and in solution using three DFT methods (B3LYP, PBE1PBE, X3LYP) and the MP2 method. We calculate the G3 energies and use them as the reference data to gauge hydrogen bond strength in the gas phase. The solvent effects are treated via the integral equation formalism-polarizable continuum model (IEF-PCM). Altogether, we loca...     

DFT Calculations and NBO Analysis of 2-Chloroethylethyldichlorosilane Unimolecular Elimination Kinetics in the Gas Phase

Ab initio molecular orbital calculations have been used to investigate the thermal decomposition kinetics of 2-chloroethylethyldichlorosilane at the B3LYP/6-311+G**,B3PW91/6-311+G**,and MPW1PW91/6-311+G** levels of theory.Among these methods,the results(activation parameters) obtained using the B3LYP/6-311+G** level are in good agreement with the available experimental data.The calculated data imply that in the unimolecular β-elimination reactions of the studied compound in the gas phase,the polarization of C(1)-Cl(3) and C(1)-H(4) bonds in the sense of C(1)δ+-Cl(3)δ-and C(1)δ+-H(4)δ-,respectively,is a determining factor in the gas phase elimination reactions 1,2 and 3.Analysis of bond order,natural bond orbital charges,bond indexes,synchro-nicity parameters,and IRC calculations suggest the elimination of 2-chloroethylethyldichlorosilane via reactions 1～3 can be described as concerted and slightly asynchronous.The transition state structures of these reactions are a four-membered cyclic structure.     

Pathways of liquefied petroleum gas pyrolysis in hydrogen plasma:A density functional theory study

A density functional theory(DFT) study has been conducted in this work to investigate the pyrolysis pathways of propane and n-butane,which are the main components of liquefied petroleum gas(LPG),for better understanding the pyrolysis behavior of LPG in hydrogen thermal plasma. Over 60 possible reactions are considered.The reaction enthalpies and activation energies of these reactions are calculated and analyzed with a Gaussian method of B3LYP and basic set of 6-31G(d,p).A most possible reaction pathway is brought up.According to this reaction pathway, the main products of LPG pyrolysis are acetylene,ethylene,methane,ethane and extra hydrogen.Acetylene mainly comes from the pyrolysis of propylene and ethylene,and hydrogen ion reactions are the main source of extra hydrogen gas.Active H·radicals are found to play a very important role in many reactions,and they can remarkably lower the energies needed for reactions.     

Theoretical Study on the Photodegradation Mechanism of Pyrethroid Insecticide Fenvalerate in Water

The photodegradation mechanism of fenvalerate in water has been investigated by density functional theory(DFT).The geometries of reactants,transition states,intermediates and products are optimized at the B3LYP/6-31G* level.The calculated results indicate that the reaction process mainly includes the nucleophilic attack and the substitution reaction by hydroxyl radical to the carbonyl group.By vibrational frequency analysis and intrinsic reaction coordinate(IRC) method,the transition state and its reaction pathway are confirmed.Moreover,the changes of natural population analysis(NPA),calculated using the Natural bond orbital(NBO) method,are analyzed along with the degradation reaction which can explain the variation of chemical bonds.Additionally,the solvent effect is also investigated and the results show that the reaction preferably takes place in water.     

Reaction mechanism and chemoselectivity of gold(Ⅰ)-catalyzed cycloaddition of 1-(1-alkynyl) cyclopropyl ketones with nucleophiles to yield substituted furans

The mechanisms of gold(Ⅰ)-catalyzed cycloaddition of 1-(1-alkynyl) cyclopropyl ketones with nucleophiles have been investigated using density functional theory calculations at the B3LYP/6-31G(d,p) level of theory.A polarizable continuum model(PCM) has been established in order to evaluate the effects of solvents on the reactions.The results of the calculations indicate that the first step of the catalytic cycle is the cyclization of the carbonyl oxygen onto the triple bond which forms a new and stable resonance structure of an oxonium ion and a carbocation intermediate.The subsequent ring expansion step results in the formation of the final product and regeneration of the catalyst.Furthermore,the regioselectivity and effect of substituents has been discussed,including an analysis of energy,bond length,and natural bond orbital(NBO) charge distributions in the rate-determining step.Our computational results are consistent with earlier experimental observations.     

Water effect on peroxy radical measurement by chemical amplification: Experimental determination and chemical mechanism

The water effect on peroxy radical measurement by chemical amplification was determined experimentally for HO2 and HO2 OH, respectively at room temperature (298±2) K and atmospheric pressure (1×105 Pa). No significant difference in water effect was observed with the type of radicals. A theoretical study of the reaction of HO2·H2O adduct with NO was performed using density functional theory at CCSD(T)/6-311 G(2d, 2p)//B3LYP/6-311 G(2d, 2p) level of theory. It was found that the primary reaction channel for the reaction is HO2·H2O NO→HNO3 H2O (R4a). On the basis of the theoretical study, the rate constant for (R4a) was calculated using Polyrate Version 8.02 program. The fitted Arrenhnius equation for (R4a) is k = 5.49×107 T 1.03exp(?14798/T) between 200 and 2000 K. A chemical model incorporated with (R4a) was used to simulate the water effect. The water effect curve obtained by the model is in accordance with that of the experiment, suggesting that the water effect is probably caused mainly by (R4a).     

（E）-1-[4-（2-hydroxy-5-methoxybenzylideneamino）-phenyl]ethanone and （E）-1-[4-（2-hydroxy-4-methoxybenzylideneamino） phenyl]ethanone：X-ray and DFT-calculated structures

The isomeric structures of(E)-1-[4-(2-hydroxy-5-methoxybenzylideneamino)-phenyl] ethanone(I) and(E)-1-[4-(2-hydroxy-4-methoxybenzylideneamino) phenyl]ethanone(Ⅱ) ,both C16H15NO3,have been determined using X-ray diffraction techniques and characterized by IR,and their molecular structures have also been optimized at the B3LYP/6-31G(d,p) level using density functional theory(DFT) . The energetic behaviors of the title compounds in solvent media have been examined using B3LYP method with the 6-31G(d,p) basis set by applying the polarizable continuum model(PCM) . The total energies of the title compounds decrease with the increasing polarity of the solvent. In addition,DFT calculations of the title compounds' molecular electrostatic potentials(MEP) were performed at the B3LYP/6-31G(d,p) level of theory. X-ray study shows that the title compounds both have strong intramolecular O-H…N hydrogen bonds. The molecules of Ⅰ are linked into a one-dimensional framework structure by C-H…π interactions,while in Ⅱ,intermolecular π···π interactions result in the formation of infinite chains running along the [010].     

DFT Investigation on the Enantioselectivity of Olefin Carboacylation Catalyzed by a Rh(Ⅰ) Complex

The C–C bond activation and recyclization of benzocyclobutenone to poly-fused rings catalyzed by the [Rh(R,S-L)]+ complex producing the R,S-, S,R-, R,R-and S,S-product were investigated systematically at the BP86/6-31 G(d,p) level in gas phase and THF, and the R,S-and S,R-reaction pathways were revisited at the M062 X/6-31 G(d,p) level in THF. The computational results reveal that THF only marginally alters the free-energy barriers, but elevates the relative energies of all species. The BP86 functional testifies that in both gas phase and THF, the activation of strained C–C bonds bears relatively low free-energy barriers, and the rate-determining steps of S,R-and R,R-channels are different from those of R,S-and S,S-channels. The BP86 functional also predicts that the R,S-channel is energetically most favorable in gas phase, but the S,R-product is dominant in THF. The change of NPA charges can mirror the variation of molecular structures to elucidate reaction mechanisms.     

Theoretical Study on the Inclusion Complex of β-Cyclodextrin and Nitrobenzene

Quantum chemical calculations were carried out to investigate the structures and properties for the inclusion complexes of nitrobenzene (NB) into β-cyclodextrin. Two low-energy conformations of β-cyclodextrin (A and B) in the gas phase were initially investigated by the PM3 and B3LYP/6-31+G(d,p) calculations, respectively. Three different orientations were considered in the inclusion process of A and B with NB to form 1:1 complexes. Potential energy scan by PM3 calculations indicated that the phenyl orientation Ab for conformation A and the equator orientation Bc for conformation B are more favorable in energy, respectively. We also considered the 2:1 inclusion complexes of host A or B with guest NB in the gas phase. PM3 calculation indicated that the host-guest interaction energies to form 1:1 complexes are more negative than those to form 2:1 NB/B complexes. Finally, we studied the solvent effect of NB/CD complex, and PM3 results show that the influence of water molecules on the inclusion process is very important. The driving forces for the inclusion process and the geometries of complexes were discussed in detail.     

Mechanism and Kinetics for the Reaction of NCS and OH Radicals

The mechanism and dynamical properties for the reaction of NCS and OH radicals have been investigated theoretically. The minimum energy paths (MEP) of the reaction were calculated using the density functional theory(DFT) at the B3LYP/6-311 G^** level, and the energies along the MEP were further refined at the QCISD(T)/6-311 G^** level. As a result, the reaction mechanism of the title reaction involves three channels, producing HCS NO and HNC SO products, respectively. Path Ⅰ and path Ⅱ are competitive, with some advantages for path Ⅰ in kinetics. As for path Ⅲ, it looks difficult to react for its high energy barrier. Moreover, the rate constant have been calculated over the temperature range of 8190-2500K using canonical variational transition-state theory (CVT). It was found that the rate constants for both path Ⅰ and path Ⅱ are negatively dependent on temperature, which is similarwith the experimental results for reactions of NCS with NO and NO2, and the variational effect for the rate constant calculation olavs an important role in whole temperature range.     

Inhibition Mechanism of Cholinesterases by Carbamate: A Theoretical Study

The density functional theory at the B3LYP/6-311G（d, p） level was applied to exploring the inhibition mechanism of cholinesterases by carbamate. The results indicate that the inhibition reactions with or without the catalytic effect of the catalytic triad in eholinesterases underwent a two-step addition-elimination mechanism, which is in good agreement with the proposed mechanism. The solvent has a strong effect on the inhibition reactions and the reaction with the catalytic triad in the solvent phase is close to the real reaction under biological condition.     

Mechanisms and Kinetics of Radical Reaction of O（^1D,^3P） ＋ HCN System

The reaction of HCN with O（^1D, ^3p） radical has been investigated by density functional theory （DFT） and ab initio methods. The stationary points on the reaction paths （reactants, intermediates and products） were optimized at the （U）B3LYP/aug-cc-pVTZ level. Single-point calculations were performed at the （U）QCISD（T）/aug-cc-pVTZ level for the optimized structures and all the total energies were corrected by zero-point energy. It is shown that there exist three competing mechanisms of oxygen attacking nitrogen O→N, oxygen attacking carbon O→C and oxygen attacking hydrogen O→H. The rate constants were obtained via Eyring transition-state theory in the temperature range of 600～2000 K. The linear relationship between lnk and 1/T was presented. The results show that path 1 is the main reaction channel and the product of NCO ＋ H is predominant.