Secondary deuterium kinetic isotope effect for aquation, solvolysis, and isomerization reactions of trans-[Co(en)2(OSMe2)N3]2+, and the resolution of a mechanistic anomaly

The two closely spaced NH signals in the (1)H NMR spectrum of trans-[Co(en)(2)(OSMe(2))(N(3))](2+) have been reassigned using 2D NMR and other techniques. Thus, the unusual syn to anti (to Co-N(3)) NH rearrangement on base catalyzed substitution of the selectively deuterated complex in ND(3)(l) has been reinterpreted as "normal", with inversion of the effective deprotonation site accompanying the act of substitution. The re-examination of this system required a repeat study of the secondary isotope effect for the acid hydrolysis reaction, previously used to assign syn and anti amine sites, and this has been extended to other solvents (Me(2)SO, MeCN). The relative NH proton exchange rates are also reconsidered. A systematic rate reduction for Me(2)SO substitution is observed for deuterium incorporation into the cis-NH centers, irrespective of whether these are syn or anti, and the effect is much greater in Me(2)SO than in water. The results are interpreted in terms of zero point energy effects and coupled vibrations.     

On the exotherm of polyacrylonitrile. II. Examination of deuterated polymers

A series of α-, β,β-, and α,β,β-deuterium-labelled acrylonitrile monomers were prepared and polymerized. Secondary deuterium isotope effects on the polymerization and on the pyrolysis reactions that precede carbon fiber formation were observed. When deuterium is in the α-position, the polymerization rate is greater and the molecular weight is higher. It is proposed that either the propagation rate constant or both that constant and the termination rate constant are increased on deuterium substitution. In differential scanning calorimetry, the polyacrylonitrile exotherm occurs at higher temperatures and is narrower when deuterium is substituted at the α-position. On the other hand, the thermal gravimetric analysis activation energy for weight loss of polymer at temperatures below the acrylic exotherm is lower when deuterium is in the α-position, relative to the α-hydrogen polymers. As there is no correlation between the weight loss energy of activation and the various exotherm parameters, the weight loss energy of activation and the various exotherm parameters, the weight loss and exotherm are considered to be independent events. Examination of the distribution of deuterium substituted ammonia species evolved during 100–240°C thermal treatment of the α-and β, β-deuterated polyacrylonitriles provides a clear indication that both the α- and β-positions are directly involved in hydrogen migration to nitrogen, but the mechanism of ammonia generation remains unclear. © 1995 John Wiley & Sons, Inc.     

MNDO study of reaction paths: Hydroboration of methyl cyanide

The hydroboration reaction of methyl cyanide has been investigated by the MNDO method. It has been shown that the reaction requires an activation energy of 25.3 kcal/mol and involves a four-center-like transition state in the rate-determining step. This reaction has been compared with the corresponding reaction of hydrogen cyanide, and the effect of methyl substitution on the reaction has been discussed. The charge-transfer effects accompanying the reaction have also been studied.     

Substituent effects on SN2 reaction between substituted benzyl chloride and chloride ion in gas phase

Nucleophilic substitution reaction between some substituted benzyl chlorides and chloride ion has been investigated by ab initio and DFT methods. New calculated energy data are in better agreement with experimental data. The electron‐withdrawing groups increase the energy barriers and the electron‐donating groups decrease them. The changes of geometrical parameters and energy data are in good agreement with the results of atoms in molecules and natural bond orbital analyses. The relationship between Hammett coefficients and energy data (and geometry parameters) has been established and the ρ constant has been calculated for this reaction. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Thermokinetics on the reaction of formation of Dy[(C_5H_8NS_2)_3(C_(12)H_8N_2)]

The enthalpy change of formation of the reaction of hydrous dysprosium chloride with ammonium pyrrolidinedithiocarbamate (APDC) and 1,10-phenanthroline (o-phen·H2O) in absolute ethanol at 298.15 K has been determined as (-16.12±0.05) kJ·mol-1 by a microcalor-meter. Thermodynamic parameters (the activation enthalpy, the activation entropy and the activation free energy), rate constant and kinetics parameters (the apparent activation energy, the pre-exponential constant and the reaction order) of the reaction have also been calculated. The enthalpy change of the solid-phase reaction at 298.15 K has been obtained as (53.59±0.29) kJ·mol-1 by a thermochemistry cycle. The values of the enthalpy change of formation both in liquid-phase and solid-phase reaction indicated that the complex could only be synthesized in liquid-phase reaction.     

Non-isothermal degradation-based solid state kinetics study of copper (II) Schiff base complex,at different heating rates

The coordination complex of Cu (II) with the Schiff base derived from 4-chloroaniline with salicylaldehyde have been synthesized and characterized by micro analytical data; FT–IR, UV–Vis, FAB-mass and thermal analysis studies. Thermal data show degradation of complexes. We carried out thermal analysis at three different heating rates viz. 5, 10 and 20 °C per min. The activation thermodynamic parameters, such as activation energy (E*), entropy of activation (ΔS*), enthalpy of activation (ΔH*) and Gibbs free energy (ΔG*) have been calculated with the help of TG, DTA and DTG curves using Coats–Redfern method. The stoichiometry of the complexes are in 1:2 (M:L) molar ratio. Synthesized complex has been tested for their reactivity and substitution behaviour.     

Thermokinetics on the reaction of formation of Dy[(C5H8NS2)3(C12H8N2)]

The enthalpy change of formation of the reaction of hydrous dysprosium chloride with ammonium pyrrolidinedithiocarbamate (APDC) and 1,10-phenanthroline (o-phen•H₂O) in absolute ethanol at 298.15 K has been determined as (-16.12 ± 0.05) kJ•mol^-1 by a microcalormeter. Thermodynamic parameters (the activation enthalpy, the activation entropy and the activation free energy), rate constant and kinetics parameters (the apparent activation energy, the pre-exponential constant and the reaction order) of the reaction have also been calculated. The enthalpy change of the solid-phase reaction at 298.15 K has been obtained as (53.59 ± 0.29) kJ•mol^t-1 by a thermochemistry cycle. The values of the enthalpy change of formation both in liquid-phase and solid-phase reaction indicated that the complex could only be synthesized in liquid-phase reaction.

     

Substituent Effects on Rates and Stereoselectivities of Conrotatory Electrocyclic Reactions of Cyclobutenes. A Theoretical Study

Substituent effects on the geometries and conrotatory electrocyclic ring openings of cyclobutenes were studied. This work extends the original investigations to many more substituents and provides a comprehensive theory of substituent effects on geometries and reaction rates. The effects of substitution at the 1 position are minimal; donor substituents raise the activation energy slightly, and powerful acceptor substituents slightly lower the activation energy. Substituents on C(3) cause small distortions of the cyclobutene geometry, in the same direction as the favored stereochemistry of reaction. Donors prefer outward rotation, while strong acceptors prefer inward rotation. The activation energy changes and cyclobutene geometrical perturbations were found to correlate with Taft sigma(R)(0) parameters. Amino, hydroxy, fluoro, chloro, methyl, cyano, formyl, and vinyl substituents were studied in the 1 position. Boryl, dimethylboryl, nitroso, formyl, nitro, carboxyl (neutral, protonated, and deprotonated), cyano, trifluoromethyl, sulfoxyl, sulfonyl, sulfinic acid, imino, N-protonated imino, ammonio, ethynyl, methyl, mercapto, chloro, fluoro, hydroxyl, amino, lithium oxy, vinyl, and acetyl were calculated as substituents in the 3 position. Comparisons with experimental results are given when available, and predictions are made in other cases.     

Thermokinetics on the reaction of formation of Dy[(C<Subscript>5</Subscript>H<Subscript>8</Subscript>NS<Subscript>2</Subscript>)<Subscript>3</Subscript>(C<Subscript>12</Subscript>H<Subscript>8</Subscript>N<Subscript>2</Subscript>)]

The enthalpy change of formation of the reaction of hydrous dysprosium chloride with ammonium pyrrolidinedithiocarbamate (APDC) and 1,10-phenanthroline (o-phen?H₂O) in absolute ethanol at 298.15 K has been determined as (-16.12 ± 0.05) kJ?mol^-1 by a microcalormeter. Thermodynamic parameters (the activation enthalpy, the activation entropy and the activation free energy), rate constant and kinetics parameters (the apparent activation energy, the pre-exponential constant and the reaction order) of the reaction have also been calculated. The enthalpy change of the solid-phase reaction at 298.15 K has been obtained as (53.59 ± 0.29) kJ?mol^t-1 by a thermochemistry cycle. The values of the enthalpy change of formation both in liquid-phase and solid-phase reaction indicated that the complex could only be synthesized in liquid-phase reaction.     

Kinetic energy release and position of transition state during the intramolecular substitution of ionized 2-benzoyl pyridines

The loss of substituents X from molecular ions of ortho substituted 2-benzoyl pyridines has been investigated as a function of the dissociation energy of the C? X bond. Comparison of unimolecular and collisional induced decompositions of the resulting [M ? X]⁺ ions and reference ions arising from 3-hydroxypyrido[1,2-α]indole shows that cyclic fragment ions are formed in every case by an intramolecular substitution reaction with the exception of the parent compound (X = H), which gives rise to a mixture of [M ? H]⁺ ions with different structures. The heat of formation of the cyclic ion has been estimated experimentally and by calculation using thermochemical data, and from this value and the appearance energies, the activation energies of the reverse reactions have been evaluated for the different reaction systems. Measurement of the kinetic energy release during the substitution reactions shows that only part of the reverse activation energy is released as kinetic energy. The energy partitioning quotient varies from 0.37 to 0.08 depending on the dissociation energy of the C? X bond or the reaction enthalpy. A sudden change in the energy partitioning quotient is observed with increasing exothermicity of the reaction, paralleling the behaviour of similar reaction systems. These results are interpreted as a demonstration of the influence of the variation of transition state position on the energy partitioning quotient.     

Structure-reactivity correlations in the reaction of ethyl bromoacetate with aliphatic carboxylate ions

The second-order rate constants at three different temperatures have been determined for nucleophilic substitution reactions of ethyl bromoacetate with various substituted aliphatic carboxylate ions (XCH₂COO⁻) in 90% acetone-10% water (v/v) mixture and the activation parameters have been evaluated. The rate data have been analysed in terms of electronic and steric effects by using various single and multi-parameterequations. Analysis of the results shows that localized and steric effects are more important than the delocalized effect. The regression coefficient of the inductive component is negative indicating that inductively electron-releasing substituents accelerate the reaction and electron-withdrawing substituents retard it. The steric component corresponds to the reaction being subject to steric acceleration by the substituents. The per cent steric effect has been calculated. The Br?nsted type correlation is fair, and theβ _N value has been determined.     

Kinetics and mechanism of myristic acid and isopropyl alcohol esterification reaction with homogeneous and heterogeneous catalysts

The reaction of myristic acid (MA) and isopropyl alcohol (IPA) was carried out by using both homogeneous and heterogeneous catalysts. For a homogeneously catalyzed system, the experimental data have been interpreted with a second order, using the power‐law kinetic model, and a good agreement between the experimental data and the model has been obtained. In this approach, it was assumed that a protonated carboxylic acid is a possible reaction intermediate. After a mathematical model was proposed, reaction rate constants were computed by the Polymath* program. For a heterogeneously catalyzed system, interestingly, no pore diffusion limitation was detected. The influences of initial molar ratios, catalyst loading and type, temperature, and water amount in the feed have been examined, as well as the effects of catalyst size for heterogeneous catalyst systems. Among used catalysts, p‐toluene sulfonic acid (p‐TSA) gave highest reaction rates. Kinetic parameters such as activation energy and frequency factor were determined from model fitting. Experimental K values were found to be 0.54 and 1.49 at 60°C and 80°C, respectively. Furthermore, activation energy and frequency factor at forward were calculated as 54.2 kJ mol^?1 and 1828 L mol^?1 s^?1, respectively. © 2008 Wiley Periodicals, Inc. 40: 136–144, 2008     

Kinetics of aquation of chloropentamminecobalt(III) perchlorate in ethylene glycol-water media

Summary The aquation of chloropentamminecobalt(III) ion has been studied in ethylene glycol-water mixtures up to 80% by weight at five temperatures from 35 to 55°C. Specific solvent effects on the reaction rate and mechanism have been investigated and discussed. The validity of our previously proposed equations, correlating the specific reaction rate with the dielectric constant of the medium has been examined. The thermodynamic parameters of activation have been calculated and presented as evidence for the solvation effects and in support of the proposed reaction mechanism.     

Isotope effects on miscibility of 1-butyl-3-methylimidazolium tetrafluoroborate with butanols

The liquid-liquid miscibility temperatures as a function of composition and deuterium substitution have been experimentally determined for the binary mixtures of 1-butyl-3-methylimidazolium tetrafluroborate with 1-butanol, isobutyl alcohol, 2-butanol, and tert-butyl alcohol and their deuterated forms (OH/OD substitution). All systems exhibit upper critical solution temperatures (UCSTs) with a visible effect of branching in alcohols. Deuteration of alcohols in the hydroxyl group results in a decrease of the UCST of the given system and the largest shift is observed for tert-butyl alcohol. These solvent isotope effects nicely correlate with the polarity expressed by dielectric constants or E(T)(30) parameters of alcohols. The effect of the isotope substitution on the miscibility of ILs with butanols can be rationalized by using the statistical-mechanical theory of the isotope effects coupled with a phenomenological g(E) model. Following this procedure one finds that the isotope shift of UCST is associated mainly with the zero-point energy contribution.     

Polar and solvation effects in reactions of oxygen atoms and hydroxyl and alkoxyl radicals with oxygen-containing compounds

Experimental data on the activation energies of reactions of H-abstraction from oxygencontaining compounds by oxygen atoms and hydroxyl and alkoxyl radicals in the gas and liquid phases have been analyzed by means of the parabolic model of the transition state. The contribution of polar interaction to the activation energies of the reactions has been calculated. The contribution of solvation to the activation energy has been calculated by comparison of the reaction parameters of the respective reaction in the liquid and gas phases.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 38–42, January, 1994.     

Effect of a complex formation on the calculated low-pressure rate constant of a bimolecular gas-phase reaction governed by tunneling

Many important bimolecular hydrogen-transfer processes that take place in the atmosphere proceed via a potential energy minimum (hydrogen-bonded complex) that precedes along the minimum energy path the unique saddle point of the reaction, the one corresponding to the hydrogen transfer. It is clear that the one-step low-pressure rate constant of such a reaction does not depend on the existence of any complex along the minimum energy path below the reactant if the reaction takes place by thermal activation over a transition state that lies quite above the reactants (for instance 10 kcal/mol). However, we have quantitatively shown in this article that the scenario notoriously changes if the reaction involves significant tunneling. In this work, we have theoretically calculated the rate constants and their temperature dependence for the reaction HO+HOH→HOH+OH by means of a canonical variational transition state theory and a canonical unified statistical theory (when necessary). Multidimensional tunneling effects have been included with a semiclassical transmission coefficient. Two kinds of modified potential energy surfaces (PESs), obtained from an original ab initio potential energy surface, previously calculated by us, have been used. The Eckart-modified PESs serve to model the hydrogen-abstraction profiles with no complexes along the path, while the Gaussian-modified PESs model the energy profiles with two complexes along the path symmetrically distributed at each side of the abstraction saddle point. Our results show that the existence of those complexes reduces the thickness of the classically forbidden region for energies below the adiabatic barrier, and then tunneling is promoted and the reaction is accelerated. The effect of the complex formation in several kinetic magnitudes, as the Arrhenius parameters and the kinetic isotope effect has also been analyzed. © 1999 John Wiley & Sons, Inc. J Comput Chem 20: 1685–1692, 1999     

Homolytic substitution in trialkyltin iodides by photochemically generated iodine atoms. I. Kinetics

The kinetics of the homolytic substitution of several trialkyltin iodides by iodine atoms are presented. Rate constants have been determined at three different temperatures and the following activation parameters calculated: A, E_a, and ΔS°^≠. The observation that the activation energy, ΔG^≠, is related to the driving force of the ion-pair formation, leads to the conclusion that the charge-transfer model is a valid approach for substitution in the reaction between R₃SnI compounds and iodine atoms.     

Ester decomposition as a two-center synchronous reaction

Experimental data on the molecular decomposition of esters with various structures into an olefin and the corresponding acid in the gas phase are analyzed in terms of the intersecting parabolas method. Enthalpies and kinetic parameters characterizing this decomposition have been calculated for 33 reactions. Ester decomposition is a concerted two-center reaction characterized by a very high classical potential barrier of thermoneutral reaction (148–206 kJ/mol). The totality of reactions examined is divided into eight classes. Activation energies and rate constants have been calculated for 38 reactions using the kinetic parameters obtained. The activation energies and rate constants of the reverse bimolecular reaction of acid addition to olefins have been calculated by the intersecting parabolas method. Factors in the activation energy of ester decomposition and formation reactions are discussed.     

An investigation of the transmission of electronic effects in a series of 5- and 6-substituted benzazoles by the methods of basic deuterium exchange and NMR spectroscopy

The kinetics of deuterium exchange at the methyl groups in 5- and 6-substituted 2-methylbenzothiazoles, 6-substituted 2-methylbenzoxazoles, and 5-substituted 1,2-dimethylbenzimidazoles have been studied. A quantitative estimate of the influence of the substituents on the free energy of activation of deuterium exchange and on the chemical shifts in the ¹H, ¹³C, and ¹⁹F NMR spectra of the benzazoles investigated and of the substituted quinolines and naphthalenes used as standard systems has been made with the aid of correlation analysis. It has been shown that the decrease in the transmission capacity of the benzazole nucleus in the transition state of the reaction as compared with the initial state is due to the influence of cross-conjugation effects disturbing the additive nature of the electronic interactions. The question of the probable structure of the transition state of the reaction is discussed.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 9, pp. 1235–1245, September, 1984.     

Influence of mass-transfer effect on isoconversional calculations

A theoretical simulation of the influence of mass-transfer effect on the kinetics of solid–gas reactions has been carried out. The influence of mass-transfer phenomena on the shape of the thermoanalytical curves and on the apparent activation energy, calculated by advanced isoconversional methods (Vyazovkin method) is discussed. The Vyazovkin equation has been adapted to CRTA data and, a modification of this equation, to account for pressure correction term in the reaction rate was achieved. To check the equations developed in this paper, the standard isoconversional procedure has been modified, instead of a set of experiments performed under different heating rates (or reaction rates C in the case of CRTA) for a given conversion we use a set of experiments under different pressure of the gas self-generated in the reaction at one heating rate β (or reaction rate C), respectively.The results obtained allow for trustworthy estimates of the activation energy from advanced isoconversional method in reaction systems whose kinetics are affected by the pressure of the gases self-generated by the reaction. Theoretical considerations are verified on simulated non-isothermal TG, and non-isothermal non-linear controlled rate thermal analysis (CRTA) data. Experimental data of calcite have been used.