Computational study on vapor phase coupling reaction between diiso(thio)cyanates with diamines,diols, and dithiols

Coupling between iso(thio)cyanates and amines, alcohols, and thiols to yield (thio)urea/urethane in the gas phase is important for the vacuum deposition processes of functional organic thin films such as molecular layer deposition or chemical vapor deposition. In this study, the kinetics and thermodynamics of 12 reactions between bifunctional reactants containing ? NCO/? NCS and ? NH₂/? OH/? SH moieties were calculated using double‐hybrid density functional theory to find systematic structure–reactivity relationships. The activation energy for the proton‐transfer step was correlated with the basicity of the nucleophile/Brønsted acid reactants, while the exothermicity of the coupling reaction depends on whether the other functionality is ? NCO or ? NCS. Analysis of the transition states revealed that the location of the transition state is affected by the basicity of the reactants. Vibrational and electronic spectra of the product were obtained to help future experimental investigations.     

Kinetics of reactions between bromoplatinum(II) complexes and silver nitrate

The kinetics of the reactions of bromide complexes [[PtenBr₂], [Pt(NH₃)₂Br₂], and [enPtBr₂Pten](NO₃)₂ with AgNO₃ are studied potentiometrically. The reactions occur in two stages with dramatically different rates. Rate constants are determined for the second stage. The kinetics of the reaction between the dimeric complex and AgNO₃ are studied at 5, 15, 25, and 35°C. The activation energy is determined.     

Apparent Kinetics of Co-Gasification of Biomass and Vacuum Gas Oil (VGO)

This communication reports the beneficial effects of co-gasification of biomass and residual oil to produce syngas. In this regard, various blends of glucose (a biomass surrogate) to vacuum gas oil (VGO) have been employed to investigate the synergic effects on the gasification process. The non-isothermal co-gasification experiments were conducted in a thermogravimetric analyzer at different heating rates and gasifying agents. The analysis showed that the co-gasification rate increased with the increase of glucose content in the feedstock. The presence of the oxygen in the biomass molecules helped the overall gasification process. The maximum gasification rate of 42.70 wt/min (DTG_max) was observed with 25 wt% glucose containing sample. The use of gasifying agents appeared to have some influence, especially during high temperature gasification of the glucose-VGO blends. At a same gasification temperature, the co-gasification rate of glucose-VGO blends were found to be 125.7 wt/min and 98.59 wt%/min for N₂ and CO₂, respectively. The kinetics of the co-gasification of glucose-VGO blends was conducted based on modified random pore model using TGA experimental data and implemented in MATLAB. The estimated activation energy and rate constants were found to be consistent to the observed co-gasification rates. The apparent activation energies of co-gasification of VGO/biomass blends with different weight percentages shows values ranging 60.56–48.25 kJ/mol. The kinetics analysis suggested that the addition of biomass helped to increase the reaction rate by lowering the activation energy required for accomplishing the reactions compared with petroleum carbonaceous feedstocks. The reaction rate constants isotherms are plotted to show that the k-values are exhibiting similar trends at moderate heating rates between 20 and 60 °C/min. This remark arises due to the nature of the reactions involved which are considered to be inherently similar in this range of heating rate.     

Substitution Reactions of cis- and/or trans-Bromomesitylbis(triethylphosphine) Platinum (II) with Thiourea in Ethanol,Dimethylsulfoxide, and Acetone

The pressure and/or temperature dependencies of the rates of substitution of thiourea in cis- and/or trans-[PtBr(2, 4, 6-Me₃Ph)(Et₃P)₂] were studied in ethanol, DMSO, and acetone. The rate constants for both the nucleophile independent and nucleophile dependent reactions were measured. The rate and activation parameters are discussed in reference to data reported earlier for these reactions in methanol.     

Resolving the Heat Generated from ZrO2 Atomic Layer Deposition Surface Reactions

In situ pyroelectric calorimetry and spectroscopic ellipsometry were used to investigate surface reactions in atomic layer deposition (ALD) of zirconium oxide (ZrO₂). Calibrated and time-resolved in situ ALD calorimetry provides new insights into the thermodynamics and kinetics of saturating surface reactions for tetrakis(dimethylamino)zirconium(IV) (TDMAZr) and water. The net ALD reaction heat ranged from 0.197 mJ cm⁻² at 76 °C to 0.155 mJ cm⁻² at 158 °C, corresponding to an average of 4.0 eV/Zr at all temperatures. A temperature dependence for reaction kinetics was not resolved over the range investigated. The temperature dependence of net reaction heat and distribution among metalorganic and oxygen source exposure is attributed to factors including growth rate, equilibrium surface hydroxylation, and the extent of the reaction. ZrO₂-forming surface reactions were investigated computationally using DFT methods to better understand the influence of surface hydration on reaction thermodynamics.     

Kinetics and mechanisms of oxidation of 1-octene and heptanal by crown ether-solubilized potassium permanganate in non-aqueous solvents

The kinetics of oxidation of 1-octene and heptanal by 18-crown-6-ether-solubilized KMnO₄ in benzene and CH₂Cl₂ have been investigated. In benzene, the oxidation of 1-octene is first order with respect to the oxidant and zero order with respect to the substrate, whereas in CH₂Cl₂ the reaction is first order with respect to both substrate and oxidant. The reaction of heptanal followed different kinetics being first order with respect to both substrate and oxidant, regardless of whether benzene or CH₂Cl₂ was employed as the solvent. The values of activation energy E _a, standard enthalpy H ^*, standard entropy change S ^*, and standard free energy G ^*, for the reaction, are reported. Mechanistic pathways for the studied reactions are also proposed.     

The Power of Accurate Energetics (or Thermochemistry: What is it Good for?)

The utility of measuring the energetics of ion-molecule reactions is discussed. After distinguishing between the terms of thermodynamics (macroscopic, equilibrium quantities) and energetics (microscopic and kinetically relevant quantities), the potential energy surfaces for ion-molecule reactions are reviewed and their implications discussed. Equations describing the kinetic energy dependence of ion-molecule reactions are introduced and the effects of entropy on reaction rates and branching ratios are discussed. Several case histories allow an exploration of the utility of accurate thermochemical information and probe how accurate such energetic information must be to be predictive. These case studies include decomposition of hydrated metal dications, the reaction of FeO⁺ with H₂, and fragmentation of a small protonated peptide (GG). These illustrate a range of interesting systems for which accurate energetic information has been influential in understanding the observed reactivity. Comparisons with theory demonstrate that experimental information is still required for truly predictive capability.      

Ab initio study of catalyzed and uncatalyzed amide bond formation as a model for peptide bond formation: Ammonia-Glycine reactions

As a model reaction for peptide and bond formation, the S_N2 reactions between glycine and ammonia have been studied with and without amine catalysis: using ab initio molecular-orbital methods. For each of the catalyzed and uncatalyzed reactions, two reaction mechanisms have been examined: a two-step and a concerted mechanism. The stationary points of each reaction, including intermediate and transition states, have been identified and free energies calculated for all geometry-optimized reaction species to determine the thermodynamics and kinetics of the reaction. The calculations demonstrate that a second ammonia molecule catalyzes amide bond formation, and that the two-step mechanism is more favorable than the concerted one for the catalyzed reaction, while for the uncatalyzed reaction both mechanisms are competitive.     

Reactivity of haloalkanes in their reactions with the chlorine atom

Experimental kinetic data on reactions of the chlorine atom with halogenated derivatives of methane and ethane (37 reactions) have been analyzed by the intersecting-parabolas method. The following five factors have an effect on the activation energy of these reactions: the enthalpy of reaction, triplet repulsion, the electronegativities of the reaction center atoms, the dipole–dipole and multidipole interactions between the reaction center and polar groups, and the effect of π electrons in the vicinity of the reaction center. The increments characterizing the contribution from each factor to the activation energy of the reaction have been calculated. The contribution from the polar interaction, ΔE _μ, to the activation energy depends on the dipole moment of the polar group and obeys the following empirical equation: ln(Δ_{E μ}/Σμ) = ?0.74 + 0.87(ΔE _μ/Σμ) ? 0.084(ΔE _μ/Σμ)².     

Theoretical study of Diels-Alder reaction: Role of substituent in regioselectivity and aromaticity

A comparative study on the influence of the substituents on the Diels-Alder reaction was performed. The energy profiles for 11 sets of Diels-Alder reaction between monosubstituted derivatives of butadiene and ethylene have been studied and the structures of all transition states were located at B3LYP/6-31+G* level. Four pathways were independently investigated; the reaction between substituted ethylene and 1-substituted butadiene leading to ortho (a ₁) and meta (a ₂) adducts, and in the same manner, the reaction between substituted ethylene and 2-substituted butadiene yields para (b ₁) and meta (b ₂) adducts. Inspection of both the activation barriers and the reaction energies for 44 reactions revealed that the pathway b ₁ is both thermodynamically and kinetically more favorable in all types of Diels-Alder reactions; while the pathway a ₁ can be labeled only as kinetic pathway. The aromaticity of all 44 transition state structures was measured using para delocalization index to study the effect of aromaticity on the reaction path. The calculations suggest that in normal and neutral DA reactions there is a gain in aromatic stabilization of the transition state which reduces slightly the activation barrier of the kinetic pathway a ₁.     

On the Mechanism of Hydrogen Activation by Frustrated Lewis Pairs

We report herein a comprehensive theoretical study of the thermodynamics and kinetics of molecular hydrogen activation by frustrated Lewis pairs (FLPs). A series of intermolecularly combined boranes (Lewis acids) and phosphines (Lewis bases), with experimentally established different reactivities towards H₂, have been subjected to DFT and (SCS‐)MP2 calculations, and analyzed in terms of their structural properties, the energetics of association of the FLPs, and the kinetics of their interactions with H₂ and hydrogenation to the ion‐pair products. The analysis included the following steps: 1) assessment of the ability/inability of the Lewis species to preorganize into FLPs with an optimum arrangement of the acid and base sites for preconditioning the reaction with H₂, 2) comprehension of the different thermodynamics of hydrogenation of the selected FLPs by comparing the Gibbs energies of the overall reactions, and 3) estimation of the mechanism of the activation of H₂ by identifying the reaction steps and the associated kinetic barriers. The results of our studies correlate well with experimental findings and have clarified the reasons for the observed different reactivities of the investigated systems, ranging from reversible or nonreversible activation to no reaction with H₂. The derived predictions could assist the future design of Lewis acid–base systems with desired properties and applicability as metal‐free hydrogenation catalysts.     

Hydrolyse und Alkoholyse von Reaktivfarbstoffen: III. Sorbitolyse von Monochlortriazin-Farbstoffen. 3. Mitteilung über reaktionsmechanistische Untersuchungen an Reaktivfarbstoffen

• 1 The kinetics of the competitive reactions of three monochlorotriazine reactive dyes with water and with sorbitol have been investigated.
• 2 The reactions of the monochlorotriazine dyes with sorbitol anions and hydroxyl ions can be divided as follows: (a) With dyes without a NH-group between the triazine nucleus and the other part of the molecule, the reaction follows the simple addition-elimination mechanism (A_N2E), in which the addition of the nucleophile is ratelimiting. (b) The reactions of dyes containing a NH-group can be explained by assuming that more than one of the tautomeric isomers are reactive. With one isomer the addition of the nucleophile is the slowest step; with another the base-catalysed decomposition of the addition complex is rate-limiting.
• 3 The first acid dissociation constant of sorbitol is evaluated (pK_a = 13.14;60°).

     

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.     

A mechanistic MEDT study of the competitive catalysed [4+2] and [2+2] cycloaddition reactions between 1-methyl-1-phenylallene and methyl acrylate: the role of Lewis acid on the mechanism and selectivity

The selectivity and the nature of the mechanism of the competitive Lewis acid catalysed [4+2]/[2+2] cycloaddition reactions of 1-methyl-1-phenylallene (MPA) with methylacrylate (MA) have been theoretically studied within the Molecular Electron Density Theory using DFT methods at the B3LYP/6-31G(d) theoretical level. DFT reactivity indices indicate that MPA is a strong nucleophile and the LA-MA complex is a strong electrophile. The coordination of LA to MA enhances the reaction rate and increases the asynchronicity of the [4+2] CA reaction, changes the nature of the mechanism from one step to stepwise for the [2+2] CA reaction and increases the polar character of these cycloaddition reactions, which become demands a relatively low activation energy. Analysis of different energy profiles indicates that these competitive LA-catalysed CA reactions favour the formation of a mixture of meta regioisomers in both types of cycloaddition, in which the [4+2] cycloadducts were obtained in majority amount, in agreement with the experiment. Analysis based on Electron Localisation Function topological shows that the favoured [4+2] CA reaction takes place through a non-concerted two-stage one-step mechanism.     

Rational design of FLP catalysts for reversible H2 activation: A DFT study of the geometric and electronic effects

The thermodynamics of reversible H₂ activation could be controlled by adjusting substituents of LA group and using different polar solvents, which forges a guide to design potential FLPs catalysts for reversible H₂ activation.     

Hydroxylamine derivatives in the Purex Process Part VII. The redox reactive kinetics and mechanism of dimethylhydroxylamine and vanadium(V) in nitric medium

The kinetics of the redox reaction between dimethylhydroxylamine (DMH) and vanadium(V) in nitric acid has been studied by spectrophotometry at 23.1 °C. The rate equation of the reaction is determined as -d[V(V)]/dt=k[V(V)][DMH] by investigating the influence of the concentrations of V(V) and DMH, acidity, ionic strength and the ratio of the initial concentrations of reactants on the redox reaction. The rate constant of the reaction k = 9.95±0.52 (mol/l)^-1.s^-1 when the ionic strength is 1.00 mol/l. The activation energy of the reaction is 22.1 kJ/mol. A possible mechanism of the redox reaction has been suggested on the basis of an electron spin resonance(ESR) spectrum of dimethyl nitroxyl radical, (CH₃)₂O.     

A kinetic study of the thermal elimination of hydrogen fluoride from 1,2-difluoroethane. Determination of the bond dissociation energies D(CH2F?CH2F) and D(CH2F?H).

The kinetics of the thermal elimination of HF from 1,2-difluoroethane have been studied in a static system over the temperature range 734–820°K. The reaction was shown to be first order and homogeneous, with a rate constant of where θ = 2.303RT in kcal/mole. The A-factor falls within the normal range for such reactions and is in line with transition state theory; the activation energy is similarly consistent with an estimate based on data for the analogous reactions of ethyl fluoride and other alkyl halides. The above activation energy has been compared with values of the critical energy calculated from data on the decomposition of chemically activated 1,2-difluoroethane by the RRKM theory and the bond dissociation energy, D(CH₂F? CH₂F) = 88 ± 2 kcal/mole, derived. It follows from thermochemistry that ΔH_f⁰(CH₂F) = -7.8 and D(CH₂F? H) = 101 ± 2 kcal/mole. Bond dissociation energies in fluoromethanes and fluoroethanes are discussed.     

Kinetics of the gas-phase reaction between iodine and monosilane and the bond dissociation energy D(H3Si?H)

The title reaction has been investigated in the temperature range of 494–545 K. During the early stages of reaction the only observed products were silyl iodide and hydrogen iodide. Initial rates were found to obey the rate law over a wide range of initial iodine and monosilane pressures. Secondary reactions, most probably of SiH₃I with I₂, became more important as the reaction progressed. However, provided [SiH₄]₀/[I₂]₀ > 20, these secondary processes had a negligible effect on the kinetics, and an integrated rate expression could be used. These kinetics are consistent with an iodine atom abstraction chain mechanism, and for the step has been deduced. From this the bond dissociation energy D(SiH₃? H) = 378 ± 5 kJ/mol (90 kcal/mol) is obtained. The kinetic and thermochemical implications of this value, especially to the pyrolysis of monosilane, are discussed.     

Study on Coordination Behaviour of Manganese Chloride with L-a-Histidine

The reaction thermodynamic and kinetic equations for the non-reversible reactions are established. The enthalpy change of formation reaction of manganese(II) histidine (His) complex in water has been determined by microcalorimetry, using manganese chloride with L-a-histidine at 298.15-323.15 K. The standard enthalpy of formation of Mn(His)₂ ²⁺(aq) has been calculated. On the basis of experimental and calculated results, three thermodynamics parameters (the activation enthalpy, the activation entropy and the activation free energy), the rate constants, along with three kinetic parameters (the apparent activation energies, the pre-exponential constant and the reaction order) are obtained. The results show that the reaction easily takes place over the studied temperature range. The solid complex Mn(His)₂Cl₂·4H₂O was prepared and characterized by IR and TG-DTG. This revised version was published online in August 2006 with corrections to the Cover Date.     

Rheological study on the cure kinetics of two-component addition cured silicone rubber

The cure kinetics for two-component silicone rubber formed by addition reaction was studied by the rheological method. The influence of reaction temperature (T) on the cure kinetics was explored in detail. It was observed that the data of gel time (t _gel, i.e. the time when the reaction reaches the gel point) or a specific reaction time (t _nc) (defined as the reaction time before which time the influence of confinement of network on the diffusion of reaction components can be neglected) versus T obey certain functional relationship, which was well explained by the cure kinetics model of thermoset network. The cure kinetics for the two-component silicone rubber can be well fitted by the Kamal-Sourour(autocatalyst) reaction model rather than Kissinger model. When the reaction time was before or equal to t _nc, the reaction order obtained by the Kamal-Sourour reaction model was 2, which was consistent with the reaction order inferred from the two components chemical reaction when the diffusion of reaction components was not influenced by the formed cross-linked polymer network. When the reaction time was larger than t _nc, such as to the end of reaction (t _e), the influence of confinement of network on the diffusion of reaction components cannot be neglected, and the reaction order obtained by the Kamal-Sourour reaction model was larger than 2. It was concluded that the confinement effect of network had a greater influence on the cure kinetics of the silicone rubber. The reaction rate constants (k _r) under different temperatures were also determined by Kamal-Sourour reaction model. The activation energy (E) for the two-component silicone rubber was also calculated from the results of lnt _gel, lnt _nc, and lnk _r versus 1/T, respectively. The three values of E were close, which indicated that above analyses were self-consistent.