Solvent effects on the spectrokinetic studies on the molecular complexes of dextromethorphan and atenolol drugs with DDQ

The kinetics and mechanism of the interaction between 2,3‐dichloro‐5,6‐dicyano‐1,4‐benzoquinone (DDQ) and dextromethorphan and atenolol drugs has been investigated spectroscopically. In the presence of large excess of donor drug, the 1:1 charge transfer (CT) complex is transformed into a final product, which has been isolated and characterized by using FT‐IR and GC‐MS techniques. The rate of formation of product has been measured as a function of time in different solvents at three temperatures. The thermodynamic parameters, viz. activation energy, enthalpy, entropy, and free energy of activation were computed from temperature dependence of rate constants. On the basis of the spectrokinetic results, a plausible mechanism for the formation of the CT complex and its transformation into final product is presented and discussed. Cyclic voltammetric study supports the observed solvent effect on the extent of CT complexation and the rate with which it is converted into the product. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 40: 559–568, 2008     

Interaction of 2,3-dichloro-1,4-naphthoquinone with n-butylamine in halocarbon solvents

The rapid interaction between 2,3-dichloro-1,4-naphthoquinone (DClNQ) and n-butylamine results in the formation of 2N(n-butylamino)-3-chloro-1,4-naphthoquinone as the final product. The reaction is found to proceed through the initial formation of charge-transfer (CT) complex as an intermediate. The final product of the reaction has been isolated and characterized using FTIR, H1 and C13 NMR spectroscopy, mass spectrometry, and elemental analysis. The rate of formation of product has been measured as a function of time in different halocarbon solvents, viz., chloroform, dichloromethane and 1:1 (v/v) mixture of two solvents. The pseudo first order and second order rate constants at various temperatures for the transformation process were evaluated from the absorbance time data. The activation parameters (E(a), DeltaS#, DeltaH#, and DeltaG#) were obtained from temperature dependence of rate constants. The influence of dielectric constant on the properties of reaction was discussed and the probable course of reaction is presented.     

Kinetic and thermodynamic studies on molecular interaction of antipyrine donor and 2,3‐dichloro‐5,6‐dicyano‐1,4‐benzoquinone as an electron acceptor in different solvents

The charge–transfer (CT) complex of donor antipyrine with Π‐acceptor 2,3‐dichloro‐5,6‐dicyano‐1,4‐benzoquinone (DDQ) has been investigated spectrophotometrically in different halocarbon and acetonitrile solvents. The results indicated immediate formation of an electron donor–acceptor complex (DA), which is followed by two relatively slow consecutive reactions. The pseudo–first‐order rate constants for the formation of the ionic intermediate and the final product at various temperatures were evaluated from the absorbance–time data. The activation parameters, viz. activation energy, enthalpy, entropy, and free energy of activation, were computed from temperature dependence of rate constants. The stoichiometry of the complex was found to be 1:1 by Job's method of continuous variation. The formation constants of the resulting DA complexes were determined by the Benesi–Hildebrand equation at four different temperatures. The enthalpies and entropies of the complex formation reactions have been obtained by temperature dependence of the formation constants using Van't Hoff equation. The results indicate that DDQ complexes of antipyrine in all solvents are enthalpy stabilized but entropy destabilized. Both the kinetics of the interaction and the formation constants of the complexes are dependent upon the polarity of the solvents. © 2012 Wiley Periodicals, Inc. Int J Chem Kinet 45: 81–91, 2013     

Oxidation of nucleic acid bases by hexavalent manganese in aqueous alkaline medium: a kinetic study

Oxidation of uracil or thymine by Mn^VI in aqueous alkali shows first order dependence on [MnO ₄ ²⁻ ] and fractional order dependence on [uracil] or [thymine]. The effect of ionic strength is negligible on the rate of oxidation. The product of oxidation is the 5,6-dihydroxy compound. A mechanism is proposed involving the formation of a 5-membered cyclic complex between the substrate and manganate in a fast step, which subsequently disproportionates to give the final products. The large negative values of the entropy of activation suggest the formation of an intermediate complex. The formation constants of the latter and the rate of its disproportionation have been calculated from the derived rate law. The rate of oxidation of thymine is faster than that of uracil.     

Time-resolved infrared dynamics of C-F bond activation by a tungsten metal-carbonyl

Chemical reactions that break, or activate, C-H and C-F are of tremendous synthetic interest. The intramolecular C-F bond activation of a tungsten carbonyl system has been studied by time-resolved infrared spectroscopy. The formation of solvent complexes and the final product are monitored using time-resolved infrared spectroscopy of the CO stretches. The rate of the reaction is shown to be limited by the formation of an intermolecular complex between the tungsten metal center and a solvent molecule. Comparison with DFT calculations shows that in the absence of solvent molecules the intramolecular complex with the tethered perfluorobenzene ring is energetically favorable, but is not the primary kinetic product because of the initial geometry of the complex.     

羟基负离子与苯分子的反应机理

在G3MP2B3理论水平下研究了羟基负离子和苯的反应机理, 系统地分析了该反应体系中可能存在的主要热力学产物通道. 计算结果证实了前人的实验观测结果, 其主要产物是[C6H6…OH]-络合物, 质子转移和置换氢的产物通道为吸热过程, 在较低实验碰撞能量的情况下难以发生, 而生成氢气的反应通道虽然是强放热过程(-119.5 kJ·mol-1), 但其相应的反应能垒较高而无法发生. 计算对比了羟基负离子和氧负离子、氟负离子抽取苯分子中质子的机理所存在的差异, 并结合Mulliken电荷布居分析研究了其中涉及的电子交换过程. 此外, 还对比分析了羟基负离子、羟基自由基与苯反应不同的机理.     

Effect of the reactants molar ratio on the kinetics of cycloaddition of 2,3-dimethylbuta-1,3-diene to allyl methacrylate

The cycloaddition reaction between 2,3-dimethylbuta-1,3-diene and allyl methacrylate proceeds by the second order kinetics. The rate constants increase with the increase in the excess of one of the reactants. The change in the effective rate constants is described by the Michaelis-Menten equation indicating that the reaction proceeds through the initial equilibrium stage of formation of an intermediate complex which then transforms into the product. The effective rate constants, the equilibrium constants of formation of the intermediate complex, and the rate constant of its transformation into the reaction product were determined, as well as the thermodynamic parameters of the formation of the intermediate complex and the activation parameters of the transformation of the intermediate complex into the product. The limiting stage of the reaction is established and its mechanism is suggested.     

Electron donor-acceptor interaction of 3,4-dimethylaniline with 2,3-dicyano-1,4-naphthoquinone

The electron donor–acceptor (EDA) interaction between 2,3-dicyano-1,4-naphthoquinone (DCNQ) and 3,4-dimethylaniline (3,4-DMA) is studied in chloroform, dichloromethane and 1:1 (v/v) mixture of chloroform and dichloromethane. The rate of formation of the product was measured as a function of time using UV–vis spectrophotometer. The formation constant (K) and molar extinction coefficient (?) values for the formation of EDA complex were evaluated in the temperature range of 20–35 °C. The pseudo-first-order rate constant (k₁) and the second-order rate constant (k₂) for the disappearance of EDA complex and for the formation of product were evaluated. The activation parameters (ΔH^#, ΔS^# and ΔG^#) of the reaction were determined by temperature dependence of rate constants using the Arrhenius plots. The effect of relative permittivity of the medium on the reaction is discussed. The observed results indicate that formation of final product proceeds through initial formation of EDA complex as an intermediate. The product of the reaction was purified by column chromatography method and identified as 3-(N-3,4-dimethyl-phenylamino)-2-cyano-1,4-naphthoquinone by elemental analysis, IR and NMR spectroscopy. On the basis of kinetic, analytical and spectroscopic results, a plausible mechanism for the formation of EDA complex and its transformation into product is proposed.     

Spectroscopic investigation of the donor-acceptor interaction between o-phenylenediamine and tetracyanoethylene in dichloromethane

The interaction between the electron donor o-phenylenediamine (o-phendn) and the pi-acceptor tetracyanoethylene (TCNE) has been investigated spectrophotometrically in dichloromethane at 25 degrees C. The results indicate that the reaction proceeds via two major steps. The formation of the 1:2 charge-transfer complex [(o-phendn)(2), (TCNE)(4)] in which two o-phenylenediamine molecules undergoes slow N-substitution by one TCNE forming the final reaction product 2,2-di-o-aminoaniline-1,1-dicyanoethylene and 2HCN molecules. The reaction was studied using electronic, infrared, photometric titration and cyclic voltametric measurements. A general mechanism for the reactions is proposed.     

硅苯与HX (X=F,OH, NH2)的1,2-及1,4-加成反应

采用密度泛函理论方法在B3LYP/6-311++G(d,p)水平上, 研究了硅苯与HX (X=F, OH, NH₂)的1,2-及1,4-加成反应的微观机理和势能剖面, 考察了Si 原子上的取代基及四氢呋喃溶剂对反应势能剖面的影响. 研究结果表明, 标题反应有两种可能的机理: (1) 硅苯与一个HX (X=F, OH, NH₂)分子先形成中间复合物, 然后经过四元环过渡态(机理1)生成最终产物; (2) 硅苯与两个HX分子先形成中间复合物, 然后经过六元环过渡态(机理2)生成另一中间复合物, 该中间复合物脱去一个HX分子形成最终产物. 机理2 在动力学上远较机理1 有利. 1,2-及1,4-加成产物哪种优先形成由动力学控制且与X基团的种类有关. HX在气相中参与加成反应从易到难的次序为: HF>H₂O>NH₃. Si 原子上具有较强供电子和吸电子性质的取代基, 在热力学和动力学上均有利于反应的进行, 但具有较大体积的2,4,6-三甲基苯基取代基对反应反而不利. 四氢呋喃溶剂在热力学上不利于硅苯与HX的1,2-及1,4-加成反应, 在动力学上对HF或H₂O作为加成试剂的反应也不利, 但对NH3作为加成试剂的反应反而有利.     

Crossed-beam dynamics studies of the radical-radical combustion reaction O((3)P) + CH3 (methyl)

The dynamics of the radical-radical reaction O((3)P) + CH(3), a prototypical case for the reactions of atomic oxygen with alkyl radicals of great relevance in combustion chemistry, has been investigated by means of the crossed molecular beam technique with mass spectrometric detection at a collision energy of 55.9 kJ mol(-1). The results have been examined in the light of previous kinetic and theoretical work. From product angular and velocity distribution measurements, the dynamics of the predominant H-displacement channel leading to formaldehyde formation has been characterized. This channel has been found to proceed via the formation of an osculating complex; a significant coupling between the product centre-of-mass angular and translational energy distributions has been noted. Experimental attempts to characterize the dynamics of the channel leading to HCO + H(2) have failed and it remains unclear whether HCO is formed by the reaction and/or, if formed, a part of HCO does not dissociate quickly into CO + H.     

Catalytic mechanism and product specificity of the histone lysine methyltransferase SET7/9: an ab initio QM/MM-FE study with multiple initial structures

Histone lysine methylation is emerging as an important mechanism to regulate chromatin structure and gene activity. To provide theoretical understanding of its reaction mechanism and product specificity, ab initio quantum mechanical/molecular mechanical free energy (QM/MM-FE) calculations and molecular dynamics simulations have been carried out to investigate the histone lysine methyltransferase SET7/9. It is found that the methyl-transfer reaction catalyzed by SET7/9 is a typical in-line S(N)2 nucleophilic substitution reaction with a transition state of 70% dissociative character. The calculated average free energy barrier at the MP2(6-31+G) QM/MM level is 20.4 +/- 1.1 kcal/mol, consistent with the activation barrier of 20.9 kcal/mol estimated from the experimental reaction rate. The barrier fluctuation has a strong correlation with the nucleophilic attack distance and angle in the reactant complex. The calculation results show that the product specificity of SET7/9 as a monomethyltransferase is achieved by disrupting the formation of near-attack conformations for the dimethylation reaction.     

异戊烯与甲醇成醚反应机理的量子化学研究

对2-甲基-1-丁烯、2-甲基-2-丁烯与甲醇反应生成甲基叔戊基醚的反应历程进行了量子化学研究, 结果表明, 反应过程包括两个基元步骤: 2-甲基-1-丁烯和2-甲基-2-丁烯与氢离子作用生成碳正离子, 活化能分别为E1=2.26 kJ/mol, E2=7.72 kJ/mol; 甲醇与叔碳正离子反应成醚, 活化能为E3=1.29 kJ/mol, 碳正离子的生成是反应的速控步骤. 2-甲基-1-丁烯与2-甲基-2-丁烯相互转化的异构化活化能分别为E'1=4.40 kJ/mol, E'2=63.11 kJ/mol, 高于成醚的活化能, 反应体系不发生烯烃相互转化的异构化反应.     

Spectrophotometric Investigation of the Charge–Transfer Interaction of 2,3-Dicyano-1,4-naphthoquinone with N-Ethylaniline in Chlorinated Solvents

The charge?Ctransfer interaction between 2,3-dicyano-1,4-naphthoquinone (DCNQ) and N-ethylaniline was studied in the halocarbons chloroform and dichloromethane and a 1:1 (v/v) mixture of these two solvents. A rapid interaction between DCNQ and N-ethylaniline results in the formation of the monosubstituted final product. The reaction is found to proceed through an initial charge?Ctransfer (CT) complex intermediate. The equilibrium constants of the ??-complex, in all the three media, were evaluated along with first-order and second-order rate constants for the appearance and disappearance of the ??- and ??-complexes. The final product was isolated and characterized by IR, NMR and elemental analysis. Based on the kinetic results a mechanism was proposed for the reaction.     

Theoretical study on the kinetics and mechanism for the reaction of FCO with NO

The radical reaction mechanism of FCO + NO on the ground electronic state energy surface has been studied at the G2M level of theory based on the geometric parameters optimized at the B3LYP/6-311+G(d) level of theory. The two kinds of reaction pathways include the direct fluorine abstraction channel producing CO + FNO and the association channel forming the FC(O)NO complex. The former has a distinct barrier of 8.9 kcal mol(-1), while the latter is a barrierless association process. The rate constant of this reaction system in the temperature range 200-3000 K has been calculated by the microcanonical VTST/RRKM theory. The theoretical result shows that the predicted total rate constants exhibit a negative-temperature dependence and positive-pressure effect at lower temperatures. Under the experimental conditions, the predicted values are in good agreement with the experimental results. In addition, the predicted branching ratios clearly indicate that the dominant product channel is the formation of FC(O)NO at low temperatures and FNO + CO at high temperatures (>500 K).     

Kinetic study of charge transfer complexes of ICl3 with DB18C6 and DC18C6 in some nonaqueous solvents

The spectrophotometric kinetic charge–transfer complex formation of iodine trichloride (ICl₃) with Dibenzo-18-crown-6 (DB18C6), Dicyclohexyl-18-crown-6 (DC18C6) has been studied in chloroform; dichloromethane and propylene carbonate solutions at different temperatures. The results indicated immediate formation of an electron donor–electron acceptor complex; which is followed by two relatively slow consecutive reactions. The pseudo-first-order rate constants for the formation of the ionic intermediate and the final product have been evaluated at various temperatures by computer fitting of the absorbance–time data to appropriate equations. The influences of both the crown’s structure and the solvent properties on the formation of donor–electron acceptor complexes and the rates of subsequent reactions are discussed.     

Diaza-crown ethers—VI : A mechanism for metal ion promoted formation of macrocyclic diazaoligoethers

The mechanism of the formation of 1,7,10,16-tetraoxa - 4,13 - diazacylooctadecane has been studied from both a thermodynamic and a kinetic point of view. The cyclisation is favoured by the presence of an alkali metal ion, which forms a complex with the ring precursor. Both this intermediate and the complex react with starting material, but only the complex forms a cyclic product. The rate constants of these reactions have been determined by using simulation methods.     

Thermokinetics on the Reaction of Formation of the Ternary Complex Nd[（C5H8NS2）3（C12H8N2）]

Introduction The chemistry of the complexes containing lantha-nide-sulfur bond has been of substantial interest because of high performance in biological properties1 and fric-tion properties.2 In addition, they have been largely used because of their chemical and physical properties, e.g. as vulcanization accelerator.2,3 The vast investigations have been reported on preparations, characterizations and structures of these compounds,5-17 which are of great importance for illuminating the bonding…     

Kinetic study of Berthelot reaction steps in the absence and presence of coupling reagents

Several reaction steps in the Berthelot reaction for the determination of ammonia have been separately studied. A reaction order of two has been confirmed for the reaction between HOCl and NH(3). The rate constant for this reaction has been determined to be 3.2 x 10(6)l.mole(-1).sec(-1). The first evidence for the formation of benzoquinonechlorimine is presented. Pentacyanoferrate coupling reagents which accelerate the production of indophenol have been found to operate on the reaction between NH(2)Cl and phenol. The rate constant for the final step of the reaction sequence has been determined to be 5.3 x 10(-3)l.mole(-1).sec(-1). A reaction between chlorimine and pentacyanoferrate compounds has been found to be responsible for the formation of a green product in the presence of excess of coupling reagent.     

Transient kinetic and isotopic tracer studies of the myosin adenosine triphosphatase reaction.

From transient kinetic studies of the Mg2+-dependent adenosine triphosphatase of myosin subfragment 1, prepared from rabbit skeletal muscle, a seven-step mechanism has been proposed. Features of this mechanism include two-step processes for ATP and ADP binding in which the binary complex isomerizes in addition to a rapid nucleotide association step. In the case of ATP a large negative standard free energy change is associated with the isomerization. An overall rate-limiting isomerization of the myosin-product complex prior to product release has been identified. Studies on the mechanism of cleavage of ATP bound to the active site indicate the process is readily reversible and can account for the observation that more than one oxygen of the product phosphate arises from water. This proposal has been substantiated by the finding that the oxygen atoms of the gamma-phosphoryl group of bound ATP also undergo extensive exchange with water.