Kinetic analysis of the oxidative degradation of polyaryl-orthophosphate

Summary Thermogravimetric study of poly-aryl-orthophosphate indicates that in presence of air the decomposition is a two step process. The first step is the chain scission to form quinone and the subsequent step the oxidation of quinone to maleic anhydride. The kinetic parameters were determined for the oxidative thermal degradation process.With 4 figures     

Kinetic Study of the Heck Reaction by the Method of Competing Reactions

The method of competing reactions is used to measure the isotope effect in styrene phenylation by iodobenzene. The kinetics of styrene, methyl acrylate, and methyl methacrylate phenylation is studied. Experimental data suggest that the step of alkene insertion is relatively fast, while the subsequent step that involves C–H bond dissociation is rate-limiting. The kinetic parameters of the process point to the dependence of the rate-limiting step of the catalytic cycle on the concentrations of base and alkene. These results are consistent with the existing hypothesis that the step of -elimination of palladium hydride is present in the mechanism.     

Investigation of the catalytic activity of a Pd/biphenyl-based phosphine system in the Ullmann homocoupling of aryl bromides

We described Ullmann homocoupling promoted by a Pd/biphenyl-based phosphine system using DMF as solvent. Using Hammett equation it is found that the rate determining step of the reaction depends on the electronic nature of substituents of aryl bromides. Increase the rate of reaction with decreasing the electron donating of the substituent from NH₂ to H suggesting an oxidative addition step as the rate determining step. Decrease the rate of reaction with increasing the electron-withdrawing ability of the substituent from H to NO₂ indicating a reductive elimination step as the rate determining step.     

臭氧化物生成机理研究综述

以Criegee三步反应机理为核心对烯烃臭氧化反应中的臭氧化物生成机理进行了综述。论述了初级臭氧化物的生成与分解和臭氧化物生成过程的选择性和立体化学。讨论了烯烃结构和反应各步骤的选择性和立体化学的关系。     

On the restricted step method coupled with the augmented Hessian for the search of stationary points of any continuous function

In any optimization using the augmented Hessian technique, the step is not restricted to any length. Since the restriction of the step at each iteration is very important in order to achieve good convergence, we present a coupled method such that the augmented Hessian automatically gives both the adequate length of the step and the correct Hessian structure. The method is showed for the minima and saddle points of any order. © 1997 John Wiley & Sons, Inc.     

Density functional study of the mechanism of the Beckmann rearrangement catalyzed by H-ZSM-5: a cluster and embedded cluster study

The mechanism of the Beckmann rearrangement (BR) catalyzed by the ZSM-5 zeolite has been investigated by both the quantum cluster and embedded cluster approaches at the B3LYP level of theory using the 6-31G(d,p) basis set. Single-point calculations were carried out at the MP2/6-311G(d,p) level of theory to improve energetic properties. The embedded cluster model suggests that the initial step of the Beckmann rearrangement is not the O-protonated oxime but the N-protonated oxime. The energy barriers derived from the proton shuttle of the N-bound to the O-bound isomer are determined to be approximately 99 and approximately 40 kJ/mol for the embedded cluster and quantum cluster approaches, respectively. The difference in the activation energy is due mainly to the effect of the Madelung potential from the zeolite framework. The next step is the rearrangement step, which is the transformation of the O-protonated oxime to be an enol-formed amide compound, formimidic acid. The activation energy, at the rearrangement step, is calculated to be approximately 125 and approximately 270 kJ/mol for the embedded cluster and quantum cluster approaches, respectively. The final step is the tautomerization step which transforms the enol-form to the keto-form, formamide compound. The energy barrier for tautomerization is calculated to be 123 and 151 kJ/mol for the embedded cluster and quantum cluster approaches, respectively. These calculated results suggest that the rate-determining step of the vapor phase of the Beckmann rearrangement on H-ZSM-5 is the rearrangement or tautomerization step.     

A quantum chemical study on the mechanism of the consecutive addition of HCN to propionitrile

MINDO/3 MO method has been used to study the mechanism of the consecutive addition of HCN to propionitrile. The results obtained for the first five steps show that the reaction is exothermic, and step 1 is the rate determining step.     

Accelerating Effect of Tetramethylthiourea on the Kinetics of Zn2+ Electroreduction

The results of kinetic measurements of Zn²⁺ electroreduction on mercury electrode in the presence of tetramethylthiourea showed the two‐step character of the electrode process. The acquisition of nonlinear wide potential range Tafel plots was found to offer a good opportunity to examine the Zn²⁺ electroreduction mechanism. Three mechanisms including the ion transfer step followed by the electron exchange step (IE mechanism), the adsorption step (IA mechanism) and the chemical step followed by electron transfer (CE) were applied. The accelerating influence of tetramethylthiourea on Zn²⁺ electroreduction was found to result from possible complexes formation in the diffuse layer. The ion transport step is probably combined with the loss or exchange of a ligand. The second step probably consists in the electron transfer or adsorption process connected with partial charge transfer.     

Theoretical investigations on the thiol–thioester exchange steps of different thioesters

As the rate-determining step in native chemical ligation reactions, the thiol–thioester exchange step is important in determining the efficiency of the ligations of peptides. In the present study, systematic theoretical calculations were carried out on the relationships between the structure of different thioesters and the free energy barriers of the thiol–thioester exchange step. According to the calculation results, the thiol–thioester exchange step is disfavored by the steric hindrance around the carbonyl center, while the electronic effect(i.e. conjugation and hyper-conjugation effects) becomes important when the steric hindrance is insignificant.     

Effects of alkyl groups in the rate determining step of the Baeyer-Villiger reaction of phenyl alkyl ketones: a quantum chemistry study

In this work, we have studied the substituent effect of several alkyl groups in the rate-determining step of the catalyzed Baeyer-Villiger (BV) reaction of phenyl alkyl ketones with performic (PFA) and trifluoroperacetic (TFPAA) acids, using quantum chemistry methods. Our results reveal that the substituent effect is more pronounced in the migration step barriers than in the corresponding addition step; that could change the rate-determining step (RDS) of the reaction, as observed in the oxidation of phenyl tert-butyl ketone with both peracids. In addition, the effect of the acid/peracid pairs used is also analyzed. We have demonstrated that the addition step is less susceptible to the acid/peracid nature since the acid strength and the nucleophilicity of the peracid have opposite effects. The effect of the acid/peracid pair is much more pronounced in the migration step because it only depends on the leaving ability of the acid, which in turn depends on its strength. These observations are relevant for understanding the effects of the substrate, the peracid, and the catalyst on the switching of the RDS in the BV reaction.     

A kinetic study of the interaction of DNA with gold nanoparticles: mechanistic aspects of the interaction

A kinetic study of the interaction of gold nanoparticles capped with N-(2-mercaptopropionyl)glycine with double stranded DNA was carried out in water and in salt (NaCl) solutions. The kinetic curves are biexponential and reveal the presence of three kinetic steps. The dependence of the reciprocal fast and slow relaxation time, on the DNA concentration, is a curve and tends to a plateau at high DNA concentrations. The simplest mechanism consistent with the kinetic results involves a simple three-step series mechanism reaction scheme. The first step corresponds to a very fast step that is related to a diffusion controlled formation of an external precursor complex (DNA, AuNPs); the second step involves the formation of a (DNA/AuNPs)(I) complex, as a result of the binding affinity between hydrophilic groups of the tiopronin and the DNA grooves. Finally, the third step has been interpreted as a consequence of a conformational change of the (DNA/AuNPs)(I) complex formed in the second step, to a more compacted form (DNA/AuNPs)(II). The values of the rate constants of each step decrease as NaCl concentration increases. The results have been discussed in terms of solvation of the species and changes in the viscosity of the solution.     

Theoretical Analysis of the Detailed Mechanism of Native Chemical Ligation Reactions

The method of native chemical ligation between an unprotected peptide α‐thioester and an N‐terminal cysteine–peptide to give a native peptide in aqueous solution is one of the most effective peptide ligation methods. In this work, a systematic theoretical study was carried out to fully understand the detailed mechanism of ligation. It was found that for the conventional native chemical ligation reaction between a peptide thioalkyl ester and a cysteine in combination with an added aryl thiol as catalyst, both the thiol‐thioester exchange step and the transthioesterification step proceed by an anionic concerted S_N2 displacement mechanism, whereas the intramolecular rearrangement proceeds by an addition–elimination mechanism, and the rate‐limiting step is the thiol‐thioester exchange step. The theoretical method was then extended to study the detailed mechanism of the auxiliary‐mediated peptide ligation between a peptide thiophenyl ester and an N‐2‐mercaptobenzyl peptide in which both the thiol‐thioester exchange step and intramolecular acyl‐transfer step proceed by a concerted S_N2‐type displacement mechanism. The energy barrier of the thiol‐thioester exchange step depends on the side‐chain steric hindrance of the C‐terminal amino acid, whereas that of the acyl‐transfer step depends on the side‐chain steric hindrance of the N‐terminal amino acid.     

Ligand exchange as the first irreversible step in the nickel-catalyzed cross-coupling reaction of Grignard reagents

Mechanistic studies of the Ni-catalyzed cross-coupling reaction of Grignard reagents through analysis of kinetic isotope effects and theoretical calculations indicated that the product-to-substrate ligand exchange process is the first irreversible step and affects the turnover efficiency and selectivity of the reaction. On the other hand, the oxidative addition step is the first irreversible step in Pd catalysis. This finding has useful implications for the development of efficient Ni catalysis and also illustrates the importance of the catalyst turnover step that has so far received less attention than subsequent catalytic steps.     

Ni(510)台阶面对氢分子解离吸附的影响

应用对势方法获得Ｈ２／Ｎｉ（１００）和Ｈ２／Ｎｉ（５１０）相互作用体系的ＬＥＰＳ势能面，考察了氢分子在Ｎｉ（１００）面上的解离吸附行为。理论计算结果和实验结果符合很好，对Ｈ２／Ｎｉ（５１０）体系的研究表明，台阶的作用为：（１）在台阶下部，氢分子直接解离的最低能量反应途径密度增大，且平动活化垒降低，（２）在台阶上部及台阶边棱处，前驱态的最低能量反应途径密度增大。因此，台阶处成为氢分子解离和复合的活性     

Investigation of the Curing Mechanism in the GPTS/APTS Binder System

The acid catalyzed sol-gel reaction in the mixed binder system, 3-glycidoxypropyltrimethoxysilane (GPTS)/3-aminopropyltriethoxysilane (APTS) was investigated and one step and two step synthesis process were compared. Hydrolysis product was observed using the ¹H, ¹³C NMR and Raman spectra. Especially, based on the Raman spectra, epoxy ring opening was observed, varying the ratio of GPTS to APTS. The two step process made clear sol, while the one step process resulted in a milky suspension. According to the Raman spectra, the epoxy ring opening reaction kinetics proceeded slower in the two step process than one step process. Throughout the two step process, it was possible to apply the binder for the coating of substrate.     

HNCO热解为CO2和HNCNH的反应机理理论研究

利用从头算ＲＨＦ／３－２１Ｇ方法研究了ＨＮＣＯ二聚后生成ＨＮＣＮＨ和ＣＯ２的反应机理。计算表明,该反应是分步反应,由反应物经第一过渡态生成四元环中间体,再经过第二过渡态分解为产物,与实验得到的结论一致。反应的第一步是速度控制步骤,计算得到的活化位垒为１７２．５５ｋＪ·ｍｏｌ＾－１,与实验上测得的１７６．４０±１６．３０ｋＪ·ｍｏｌ＾－１相吻合。反应的第二位垒为８３．６８ｋＪ·ｍｏｌ＾－１,在实验条     

Mechanism of the axial ligand substitution reactions on the head-to-tail alpha-pyridonato-bridged cis-diammineplatinum(III) dinuclear complex with olefins

Reactions of the head-to-tail alpha-pyridonato-bridged cis-diammineplatinum(III) dinuclear complex having equivalent two platinum atoms, Pt(N(3)O), with p-styrenesulfonate and 4-penten-1-ol were studied kinetically. Under the pseudo first-order reaction conditions in which the concentration of the Pt(III) dinuclear complex is much smaller than that of olefin, a consecutive basically four-step reaction was observed for the reaction with p-styrenesulfonate, but for the reaction with 4-penten-1-ol, the reaction was three step. The olefin pi-coordinates to one of the two equivalent Pt atoms in the first step (step 1), followed by the second pi-coordination of another olefin molecule to the other Pt atom (step 2). In the next step (step 3), the nucleophilic attack of water to the first pi-coordinated olefin initiates its pi-sigma bond conversion on the Pt atom, and the second pi-bonding olefin molecule on the other Pt atom is released. Finally, dissociation of the alkyl group on the Pt(N(3)O) and reduction of the Pt(III) dinuclear complex to the Pt(II) dinuclear complex occur (step 4). The first water substitution with olefin (step 1) consists of two paths, the reaction of the diaqua dimer complex (path a) and the reaction of the aquahydroxo dimer complex (path b), whereas the second substitution (step 2) proceeds through three reaction paths: the normal path of the direct substitution of H(2)O (path c), the path of the coordinated OH(-) substitution (path d), and the path via the coordinatively unsaturated five-coordinate intermediate (path e). The reaction with p-styrenesulfonate proceeds through paths c, d, and e, whereas the reaction with 4-penten-1-ol proceeds through paths c and d. The third step (step 3) for the reaction with p-styrenesulfonate involves the coordinatively unsaturated intermediate, but that for the 4-pentene reaction does not. The reactivities of the HH dimer and HT dimer with olefins are compared and discussed.     

Theoretical study of the orientation rules in photonucleophilic aromatic substitutions

The photonucleophilic aromatic substitution reactions of nitrobenzene derivatives were studied by ab initio and Density Functional Theory methods. The photohydrolysis is shown to proceed via an addition-elimination mechanism with two intermediates, except in the case of a chlorine leaving group. Depending on the substituents, the addition step, the elimination step, or the radiationless transition is the rate-determining process. The solvent effect on the SN2 Ar* reactions was evaluated by a continuum model. Next, the regioselectivity of the addition step is investigated within the framework of the so-called spin-polarized conceptual density functional theory. It is shown that the preference observed for the meta or para (with respect to the NO2 group) pathways in the addition step can be predicted by using the spin-polarized Fukui functions applied for the prereactive pi-complex.     

A detailed TPD study of H2O and pre-adsorbed O on the stepped Pt(553) surface

Water molecules desorbing from the bare Pt(553) surface desorb in a three peak structure, associated with, respectively, desorption from step and terrace sites and the water multilayer. Upon pre-covering the step sites with O(ad) we mainly observe OH formation on step sites. When terrace sites are also pre-covered with O(ad), OH(terrace) formation is favored over OH(step) formation, presumably because OH formed at terrace sites is more easily incorporated in a hydrogen bonded network of OH/H(2)O. This is a gradual process: with increasing θ(O) less OH(step) is formed. Thus, in spite of the fact that OH at step sites has a higher binding energy than OH at terrace sites, the possibility of the formation of OH at terrace sites actually inhibits the formation of OH at step sites, leaving O(step) as the most stable water dissociation product on the step.     

Asymmetric synthesis and conformational analysis of the two enantiomers of the saturated analog of the potent thrombin inhibitor MOL-376

Asymmetric synthesis of the two enantiomers of a small molecule thrombin inhibitor is described. The key step in the synthesis is the glucose-directed chiral induction in the hetero Diels-Alder cycloaddition step. Conformational analysis indicates that the S-enantiomer is a better fit for the idealized β-strand conformation.