Place exchange reactions of alkyl thiols on gold nanoparticles

The kinetics and equilibrium position of place exchange (alkylthiol-for-alkylthiol) reactions of gold nanoparticles are reported. These reactions were monitored via a gas chromatography analysis of structurally similar incoming and outgoing alkylthiols, as a function of time. The place exchange reactions described here proceed to an equilibrium position, where Keq approximately 1. The product-time data follow Langmuir diffusion kinetics.     

Discovery and mechanistic study of Al(III)-catalyzed transamidation of tertiary amides

Cleavage of the C-N bond of carboxamides generally requires harsh conditions. This study reveals that tris(amido)Al(III) catalysts, such as Al2(NMe2)6, promote facile equilibrium-controlled transamidation of tertiary carboxamides with secondary amines. The mechanism of these reactions was investigated by kinetic, spectroscopic, and density functional theory (DFT) computational methods. The catalyst resting state consists of an equilibrium mixture of a tris(amido)Al(III) dimer and a monomeric tris(amido)Al(III)-carboxamide adduct, and the turnover-limiting step involves intramolecular nucleophilic attack of an amido ligand on the coordinated carboxamide or subsequent rearrangement (intramolecular ligand substitution) of the tetrahedral intermediate. Fundamental mechanistic differences between these tertiary transamidation reactions and previously characterized transamidations involving secondary amides and primary amines suggest that tertiary amide/secondary amine systems are particularly promising for future development of metal-catalyzed amide metathesis reactions that proceed via transamidation.     

Note Concerning the Concepts of Reaction Numbers,Independent Reactions and Resistant Groups in Chemical Thermodynamics and Kinetics

A linear algebraic characterization for sets of independent reactions, independent reaction numbers, conservation laws and resistant groups for both equilibrium and kinetic systems is suggested. Basing on the ranks of two stochiometric matrices N and M, these concepts are discussed and formulas given for setting up of independent sets of equilibrium and kinetic equations. The difference in the definition of independent reaction numbers for these two types of systems is pointed out. A brief consideration is devoted to the transformations of equilibrium and kinetic equations induced by transformation from one set of linearly independent reactions to another. Finally the conservation laws are formulated as chemical invariants within the N, M framework.     

A kinetic study of the intercalation of lithium salts into Al(OH)3

The intercalation of five lithium salts into the gibbsite and bayerite polymorphs of Al(OH)3 has been studied using in situ energy-dispersive X-ray diffraction. The kinetics and mechanisms of the reactions have been modeled using the Avrami-Erofe'ev model. The kinetic data suggest that the reaction mechanisms are predominantly nucleation controlled, although the intercalation of LiNO3 into bayerite and of Li2SO4 into gibbsite proceed via two-stage mechanisms, one part of which is diffusion controlled. All the reactions proceed directly from the host to the product, except for the intercalation of Li2SO4 into gibbsite where a more hydrated intermediate form of [LiAl2(OH)6]2SO4 x yH2O is generated prior to the final product.     

Thermal reactions of selected solids including reactants that melt during chemical change

Selected kinetic and mechanistic studies of thermal reactions of initially solid substances are reviewed with emphasis on the evidence that some of these chemical changes proceed with the essential participation of melting. The reactions considered are classified on the extent and the role of such melting and the various types of behaviour observed are discussed with reference to solid state rate processes in crystals. It is stressed that melting is an important feature in theoretical considerations of crystal reactivity because chemical changes often proceed more rapidly in a melt than in the solid state. However, literature reports concerned with reactions of solids do not always explicitly mention the possibility of melting during discussions of reactions mechanisms. The present paper comments on methods capable of detecting liquefaction during reaction, a feature of behaviour that is not always easily identified experimentally. Also considered here is the recognition of reaction intermediates, which provide important evidence concerning the course of the chemical changes through which the reactant is transformed into product. This short review draws attention to the considerable value of chemical evidence in elucidating mechanisms of reactions of solids including the necessity for identifying intermediates and the role of any melt or liquid participating.     

Multiple rate-determining steps for nonideal and fractal kinetics

We show that the kinetic model of a single rate-determining step in a reaction mechanism can be extended to systems with multiple overall reactions for which the elementary reactions obey nonideal or fractal kinetics. The following assumptions are necessary: (1) The system studied is either closed or open, but no constraints exist preventing the evolution toward equilibrium. (2) Elementary reactions occur in pairs of forward and backward steps. (3) The kinetics of the elementary steps are either nonideal or fractal and are compatible with equilibrium thermodynamics. (4) The number of reaction routes is identical with the number of rate-determining steps. If these hypotheses are valid, then the overall reaction rates can be explicitly evaluated: they have a form similar to the kinetic equations for the elementary reactions and the apparent reaction orders and fractal coefficients can be expressed analytically in terms of the kinetic parameters of the elementary reactions. We derive a set of relationships which connect the equilibrium constants of the reaction routes, the corresponding overall rate coefficients, and the stoichiometric numbers of the rate-determining steps. We also derive a set of generalized Boreskov relations among the apparent activation energies of the forward and backward overall processes, the corresponding reaction enthalpies, and the stoichiometric coefficients of the rate-determining steps. If the elementary reactions obey fractal kinetics, the same is true for the rate-determining steps. The fractal exponents of the forward and backward overall reactions are linear combinations of the fractal exponents of the fractal elementary reactions. Similar to the theory of single rate-determining steps, our approach can be used for selecting suitable reaction mechanisms from experimental data.     

BrSSCl和SSBrCl相对稳定性的理论研究

采用量子化学中的密度泛函理论，在B3LYP／6-311 G(3df)水平上全优化得到了S2BrCl分子线型和分叉型2种异构体的平衡结构，同时对可能发生的分子内原子迁移过程的过渡态进行了考察。计算结果表明，从能量角度看，线型的BrSSCl为稳定构型。采用统计热力学及过渡态理论，研究了Z种平衡结构之间相互转化的热力学和动力学性质。根据计算结果，无论是Cl迁移还是Br迁移，分子内的原子迁移都需要较高的活化能，并且迁移速度较慢。     

Proton-coupled hole transfer in X-irradiated doped crystalline cytosine.H2O

Following exposure to X-irradiation at low temperatures, the main reactions taking place in single crystals of cytosine monohydrate doped with minute amounts of 2-thiocytosine are hole transfer (HT) from the electron-loss centers to the dopant and recombination of oxidation and reduction products, assumedly by electron transfer. A huge deuterium kinetic isotope effect (KIE; >102-103) at 100 K, together with the kinetic curves obtained and density functional theory (DFT) calculations of equilibrium energy changes, indicates that these reactions proceed through a concerted proton-coupled electron/hole transfer where the proton transfer occurs between hydrogen-bonded cytosine molecules. The temperature dependence of these reaction rates between 10 and 150 K in normal and partially deuterated samples was investigated by monitoring the growth and decay of the various radical species over time using electron paramagnetic resonance (EPR) spectroscopy. By assuming a random distribution of the hole donors and acceptors in the crystals, the data are consistent with an exponential distance-dependent rate, giving a distance decay constant (beta) around 1 A-1 for the HT, which indicates that a long-range single-step superexchange mechanism mediates the charge transfer. The reactions undergo a transition from a slow, weakly temperature-dependent rate to an Arrhenius-type rate at 40-50 K, presumably being activated by excitation of low-frequency intermolecular vibrations that couple to the process. Below this transition temperature, the transfer probability might be dominated by temperature-independent nuclear tunneling. A similar beta value in both temperature regions suggests that hopping is not activated.     

甲酰氯异构化和光解的理论研究——从头算水平上的反应热力学、动力学分析

由于对化学反应体系解核运动Schrdinger方程困难,化学界常常求解参加反应粒子的电子运动Schrdinger方程,得到其电荷密度分布、自由价和键级等静态物理量,作为反应性指标;这虽有简明的优点,但往往缺乏传递性和统一性。如用电荷密度分布分析亲电反应,仅在同系物中有定性比较其反应性相对大小的意义,无定量价值。此外,用反应的势垒大小判断反应的难易,忽略了鞍点处曲率的影响,就动力学的Arrhenius速度常数公式看,     

The reactivity of substrate functionalized surfactant vesicles

The hydrolysis and thiolysis of active ester-functionalized cationic surfactant vesicles proceed without kinetic resolution of distinct exovesicular and endovesicular reactions.     

Analysis of ketones by selected ion flow tube mass spectrometry

A selected ion flow tube mass spectrometry (SIFT-MS) study of the reactions of H3O+, NO+ and O2+* ions with the ketones (M) 2-heptanone, 2-octanone, 2-nonanone, 2-undecanone and 2-aminoacetophenone has been conducted in preparation for studies of volatile emissions from bacteria. The H3O+ reactions all proceed rapidly via exothermic proton transfer, producing only MH+ ions that form their monohydrates when water vapour is present in the helium carrier gas. The O2+* reactions proceed rapidly via dissociative charge transfer producing parent cations M+* and some fragment ions. The NO+ reactions form the NO+M adduct ions at rates which are dependent on the pressure of the helium carrier gas. Combining the present NO+ kinetic data with those available from previous SIFT studies, the phenomenon of charge transfer complexing is clearly demonstrated. This results in adduct formation in these NO+/ketone reactions at or near the collisional rate. SIFT-MS spectra are presented to illustrate the simplicity of SIFT-MS analysis of ketones using both H3O+ and NO+ precursor ions.     

Regioselectivity and equilibrium thermodynamics for addition of Rh-OH to olefins in water

Rhodium(III) tetra(p-sulfonato phenyl) porphyrin ((TSPP)Rh) aquo and hydroxo complexes react with a series of olefins in water to form beta-hydroxyalkyl complexes. Addition reactions of (TSPP)Rh-OH to unactivated terminal alkenes invariably occur with both kinetic and thermodynamic preferences to place rhodium on the terminal carbon to form (TSPP)Rh-CH(2)CH(OH)R complexes. Acrylic and styrenic olefins initially react to place rhodium on the terminal carbon to form Rh-CH(2)CH(OH)X as the kinetically preferred isomer but subsequently proceed to an equilibrium distribution of regioisomers where Rh-CH(CH(2)OH)X is the predominant thermodynamic product. Equilibrium constants for reactions of the diaquo rhodium(III) compound ([(TSPP)Rh(III)(H(2)O)(2)](-3)) in water with a series of terminal olefins that form beta-hydroxyalkyl complexes were directly evaluated and used in deriving thermodynamic values for addition of the Rh-OH unit to olefins. The DeltaG degrees for reactions of the Rh-OH unit with olefins in water is approximately 3 kcal mol(-1) less favorable than the comparable Rh-H reactions in water. Comparisons of the regioisomers and thermodynamics for addition reactions of olefins with Rh-H and Rh-OH units in water are presented and discussed.     

Kinetic resolution and double stereodifferentiation in catalytic asymmetric C-H activation of 2-substituted pyrrolidines

Dirhodium tetrakis(S-(N-dodecylbenzenesulfonyl)prolinate) (Rh(2)(S-DOSP)(4)) catalyzed decomposition of methyl aryldiazoacetates in the presence of 2-substituted pyrrolidines results in highly diastereoselective and enantioselective C-H insertions. These reactions can proceed with impressive levels of double stereodifferentiation and kinetic resolution, which allows for three stereocenters to be controlled during the C-H insertion step.     

Chemical Oscillations

Under constant external conditions, chemical reactions may also proceed in a rhythmic manner. The kinetic mechanisms responsible for such oscillations prove to be unexpectedly complicated. It can nevertheless be demonstrated that chemical periodicity is caused by certain kinds of coupling between simultaneous reactions or transport processes. A general survey of the chemistry and phenomenology of the principal chemical oscillations is followed by a discussion of the situations leading to periodic reactions on the basis of the multivariable kinetics of feedback systems. Autocatalysis and autoinhibition play an important role, as also do kinetic instability and spatial propagation.     

Rate and equilibrium constants for the epimerization of the endothelin receptor antagonist J-104,132 in aqueous solution

The degradation of [5S-[5alpha,6beta,7alpha(R*)]]-2-butyl-5-(1,3-benzodioxol-5-yl)-7-[(2-carboxypropyl)-4-methoxyphenyl]-6-dihydro-5H-cyclopenta[b]pyridine-6-carboxylic acid (J-104,132) was studied in aqueous solution as a function of temperature and pH. The degradation reaction does not proceed to completion; rather, a stable equilibrium is attained in which approximately 2% of the degradate is produced. Kinetic data for the formation of the degradate are analyzed using an integrated form of the rate law for a reversible first-order reaction, and the forward and reverse rate constants and overall equilibrium constants are presented. Isolation and spectroscopic structural determination indicate that the degradate is the C7 beta-epimer of the drug. A mechanism for the epimerization reaction involving a novel enamine-like intermediate is proposed and shown to be consistent with the kinetic data. The rate and equilibrium constants are used to predict the room temperature stability of an injectable formulation of J-104,132, and these predictions are compared to actual data from long-term stability studies. It is concluded that the preformulation kinetic studies provide essential data needed for optimum drug product development.     

Temperature-dependent reaction kinetics of NH (a1Δ)

The gas-phase reactions of NH(a¹Δ) with H₂ and selected saturated and unsaturated hydrocarbons have been studied over the 250-600 K temperature range. Olefin reactions proceed at near the gas kinetic collision rate and show no temperature dependence. H₂ and saturated hydrocarbons show temperature-dependent reactions rates, with activation energies of = 0.8-2 kcai/mole. No evidence of electronic quenching of NH(a¹Δ) to the ground state was observed with any of the hydrocarbons studied. First-order reactions rates, Arrhenius A factors and activation energies for the reactions are reported. We discuss a mechanistic interpretation of the kinetics in view of earlier kinetic and reaction-product studies and ab initio SCF Cl calculations.     

Mechanisms of hydrogen-, oxygen-, and electron-transfer reactions of cumylperoxyl radical

Rates of hydrogen-transfer reactions from a series of para-substituted N,N-dimethylanilines to cumylperoxyl radical and oxygen-transfer reactions from cumylperoxyl radical to a series of sulfides and phosphines have been determined in propionitrile (EtCN) and pentane at low temperatures by use of ESR. The observed rate constants exhibit first-order and second-order dependence with respect to concentrations of N,N-dimethylanilines. This indicates that the hydrogen- and oxygen-transfer reactions proceed via 1:1 charge-transfer (CT) complexes formed between the substrates and cumylperoxyl radical. The primary kinetic isotope effects are determined by comparing the rates of N,N-dimethylanilines and the corresponding N,N-bis(trideuteriomethyl)anilines. The isotope effect profiles are quite different from those reported for the P-450 model oxidation of the same series of substrates. Rates of electron-transfer reactions from ferrocene derivatives to cumylperoxyl radical have also been determined by use of ESR. The catalytic effects of Sc(OTf)(3) (OTf = triflate) on the electron-transfer reactions are compared with those of Sc(OTf)(3) on the hydrogen- and oxygen-transfer reactions. Such comparison provides strong evidence that the hydrogen- and oxygen- transfer reactions of cumylperoxyl radical proceed via a one-step hydrogen atom and oxygen atom transfer rather than via an electron transfer from substrates to cumylperoxyl radical.     

Reactivity of aquacobalamin and reduced cobalamin toward S-nitrosoglutathione and S-nitroso-N-acetylpenicillamine

The reactions of aquacobalamin (Cbl(III)H2O, vitamin B12a) and reduced cobalamin (Cbl(II), vitamin B12r) with the nitrosothiols S-nitrosoglutathione (GSNO) and S-nitroso-N-acetylpenicillamine (SNAP) were studied in aqueous solution at pH 7.4. UV-vis and NMR spectroscopic studies and semiquantitative kinetic investigations indicated complex reactivity patterns for the studied reactions. The detailed reaction routes depend on the oxidation state of the cobalt center in cobalamin, as well as on the structure of the nitrosothiol. Reactions of aquacobalamin with GSNO and SNAP involve initial formation of Cbl(III)-RSNO adducts followed by nitrosothiol decomposition via heterolytic S-NO bond cleavage. Formation of Cbl(III)(NO-) as the main cobalamin product indicates that the latter step leads to efficient transfer of the NO- group to the Co(III) center with concomitant oxidation of the nitrosothiol. Considerably faster reactions with Cbl(II) proceed through initial Cbl(II)-RSNO intermediates, which undergo subsequent electron-transfer processes leading to oxidation of the cobalt center and reduction of the nitrosothiol. In the case of GSNO, the overall reaction is fast (k approximately 1.2 x 10(6) M(-1) s(-1)) and leads to formation of glutathionylcobalamin (Cbl(III)SG) and nitrosylcobalamin (Cbl(III)(NO-)) as the final cobalamin products. A mechanism involving the reversible equilibrium Cbl(II) + RSNO <==> Cbl(III)SR + NO is suggested for the reaction on the basis of the obtained kinetic and mechanistic information. The corresponding reaction with SNAP is considerably slower and occurs in two distinct reaction steps, which result in the formation of Cbl(III)(NO-) as the ultimate cobalamin product. The significantly different kinetic and mechanistic features observed for the reaction of GSNO and SNAP illustrate the important influence of the nitrosothiol structure on its reactivity toward metal centers of biomolecules. The potential biological implications of the results are briefly discussed.     

Study on ketalization reaction of poly(vinyl alcohol) by ketones. IX. Kinetic study on acetalization and ketalization reaction of 1,3-butanediol as a model compound for poly(vinyl alcohol)

A kinetic study was carried out on the acetalization reaction of 1,3-butanediol, as a model compound for poly(vinyl alcohol) (PVA), in water, under acidic conditions. Since these equilibrium constants of ketalization reaction of 1,3-butanediol and ethylene glycol are so small, the kinetic parameters were estimated from the hydrolysis reactions of the corresponding ketals. It was made clear that these reactions proceed in the reversible bimolecular reaction, and the heat of reaction and activation energy are nearly equal to that of PVA. The rate constants of hydrolysis reaction (k′s) of model compounds were calculated on the basis of value of acetone ketal, Hammett-Taft's equation log k′s/k′so – 0.54(n – 6) = ρ^*σ^* was established, and the value of ρ^* was obtained (3.60), which coincided with the value of PVA. Therefore, it was made clear that the hydrolysis reactions of acetals and ketals are electrophilic reaction (SE II reaction) and the step of rate determination is the formation of hemiacetal and hemiketal. The rate constants of hydrolysis reaction of 1,3-butanediol acetals and ketals were approximately 10–20 larger, and those of ethylene glycol were approximetly 50–80 larger except for ketals, and those of ethanol were roughly 2000–10,000 larger compared with that of high-molecular weight compound (PVA). It can be well explained that these differences in the rate constant depend on their entropy and the mobility of molecules. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35 : 1719–1931, 1997