Kramers problem for nonequilibrium current-induced chemical reactions

We discuss the use of tunneling electron current to control and catalyze chemical reactions. Assuming the separation of time scales for electronic and nuclear dynamics we employ Langevin equation for a reaction coordinate. The Langevin equation contains nonconservative current-induced forces and gives nonequilibrium, effective potential energy surface for current-carrying molecular systems. The current-induced forces are computed via Keldysh nonequilibrium Green's functions. Once a nonequilibrium, current-depended potential energy surface is defined, the chemical reaction is modeled as an escape of a Brownian particle from the potential well. We demonstrate that the barrier between the reactant and the product states can be controlled by the bias voltage. When the molecule is asymmetrically coupled to the electrodes, the reaction can be catalyzed or stopped depending on the polarity of the tunneling current.     

Hamiltonian dynamics of chemical reactions. Consecutive first-order reactions and reactions possessing one autocatalytic intermediate

A recently proposed Hamiltonian approach to phenomenological chemical kinetics [T. Georgian and G.L. Findley, Int. J. Quantum Chem., Quantum Biol. Symp. 10 , 331 (1983); T. Georgian, J.M. Halpin, and G.L. Findley, Int. J. Quantum Chem., Quantum Biol. Symp. 11 , 347 (1984)] is applied to all consecutive first-order, single-step reactions, and to all reactions possessing one autocatalytic intermediate. The reaction Hamiltonians presented are shown to be consistent with the phenomenological rate equations and the relationship between reaction form and the form of the reaction potential is discussed. In particular, we show: (1) that the interaction between consecutive reactions manifests itself as a coupling term in the reaction potential, a term which may be eliminated via transition to “normal reaction coordinates” for the chemical system; and (2) that coupled sets of autocatalytic reactions give rise to coupling terms in the reaction Hamiltonian which are characteristic of the reaction mechanism.     

Understanding the kinetics of spin-forbidden chemical reactions

Many chemical reactions involve a change in spin-state and are formally forbidden. This article summarises a number of previously published applications showing that a form of Transition State Theory (TST) can account for the kinetics of these reactions. New calculations for the emblematic spin-forbidden reaction HC + N(2) are also reported. The observed reactivity is determined by two factors. The first is the critical energy required for reaction to occur, which in spin-forbidden reactions is often defined by the relative energy of the Minimum Energy Crossing Point (MECP) between potential energy surfaces corresponding to the different spin states. The second factor is the probability of hopping from one surface to the other in the vicinity of the crossing region, which is largely defined by the spin-orbit coupling matrix element between the two electronic wavefunctions. The spin-forbidden transition state theory takes both factors into account and gives good results. The shortcomings of the theory, which are largely analogous to those of standard TST, are discussed. Finally, it is shown that in cases where the surface-hopping probability is low, the kinetics of spin-forbidden reactions will be characterised by unusually unfavourable entropies of activation. As a consequence, reactions involving a spin-state change can be expected to compete poorly with spin-allowed reactions at high temperatures (or energies).     

Ab initio chemical kinetics for the unimolecular decomposition of Si2H5 radical and related reverse bimolecular reactions

For plasma enhanced and catalytic chemical vapor deposition (PECVD and Cat‐CVD) processes using small silanes as precursors, disilanyl radical (Si₂H₅) is a potential reactive intermediate involved in various chemical reactions. For modeling and optimization of homogeneous a‐Si:H film growth on large‐area substrates, we have investigated the kinetics and mechanisms for the thermal decomposition of Si₂H₅ producing smaller silicon hydrides including SiH, SiH₂, SiH_3, and Si₂H₄, and the related reverse reactions involving these species by using ab initio molecular‐orbital calculations. The results show that the lowest energy path is the production of SiH + SiH₄ that proceeds via a transition state with a barrier of 33.4 kcal/mol relative to Si₂H₅. Additionally, the dissociation energies for breaking the Si? Si and H? SiH₂ bonds were predicted to be 53.4 and 61.4 kcal/mol, respectively. To validate the predicted enthalpies of reaction, we have evaluated the enthalpies of formation for SiH, SiH₂, HSiSiH₂, and Si₂H₄(C_2h) at 0 K by using the isodesmic reactions, such as ²HSiSiH₂ + ¹C₂H₆→¹Si₂H₆ + ²HCCH₂ and ¹Si₂H₄(C_2h) + ¹C₂H₆ → ¹Si₂H₆ + ¹C₂H₄. The results of SiH (87.2 kcal/mol), SiH₂ (64.9 kcal/mol), HSiSiH₂ (98.0 kcal/mol), and Si₂H₄ (68.9 kcal/mol) agree reasonably well previous published data. Furthermore, the rate constants for the decomposition of Si₂H₅ and the related bimolecular reverse reactions have been predicted and tabulated for different T, P‐conditions with variational Rice–Ramsperger–Kassel–Marcus (RRKM) theory by solving the master equation. The result indicates that the formation of SiH + SiH₄ product pair is most favored in the decomposition as well as in the bimolecular reactions of SiH₂ + SiH₃, HSiSiH₂ + H₂, and Si₂H₄(C_2h) + H under T, P‐conditions typically used in PECVD and Cat‐CVD. © 2013 Wiley Periodicals, Inc.     

Application of simplified complementary tristimulus colorimetry to chemical kinetics in solution-II Determination of rate constants of consecutive irreversible first-order reactions

A simple but highly reproducible method based on simplified complementary tristimulus colorimetry is proposed for determination of the rate constants of two consecutive irreversible first-order reactions. For the binary system the complementary colour points can be calculated in the same manner as in conventional tristimulus colorimetry. Hence the mole fractions of the three reacting species can be calculated as a function of time. Application of the method is illustrated by the assay of xanthine oxidase with hypoxanthine.     

Mathematical basis of the integral formalism of chemical kinetics. Compact representation of the general solution of the first-order linear differential equation

The standard approach in chemical and photochemical kinetics is to proceed from the kinetic scheme to the corresponding system of first-order differential equations, and then to integrate it, analytically or numerically. An equivalent integral formulation circumventing such system was recently developed on the basis of physical arguments. The mathematical basis of this ansatz is discussed here. A compact representation of the general solution of the linear first-order differential equation is also obtained.     

Fitting isoperibolic calorimeter data for reactions with pseudo-first order chemical kinetics

The non-linear least squares fitting of the chemical heat flow and the reactor temperature are compared for reactions with pseudo-first order chemical kinetics carried out in an isoperibolic calorimeter operating quasi-isothermally. Both methods give very similar results for the reaction rate constant and enthalpy of reaction but fitting the reactor temperature appears to have some advantages especially when there is an enthalpy of mixing of the reagents.     

Computer graphics-assisted applications of first-order chemical kinetics in the determination of inorganic complexes complexes

Computer graphics-assisted applications of first-order chemical kinetics are applied to absorbance data from spectrophotometric measurements of the decomposition of di-μ-hydroxobis[dioxalatocobaltate(III)] (at pH 4.0) and trioxalatocobaltate(III) (in 1.0 M perchloric acid) in order to estimate the purity of the complexes. A program called KINFIR is used.     

Chemical kinetics of reactions in the unfrozen solution of ice

Some reactions are accelerated in ice compared to aqueous solution at higher temperatures. Accelerated reactions in ice take place mainly due to the freeze-concentration effect of solutes in an unfrozen solution at temperatures higher than the eutectic point of the solution. Pincock was the first to report an acceleration model for reactions in ice,1 which successfully simulated experimental results. We propose here a modified version of the model for reactions in ice. The new model includes the total molar change involved in reactions in ice. Furthermore, we explain why many reactions are not accelerated in ice. The acceleration of reactions can be observed in the cases of (i) second- or higher-order reactions, (ii) low concentrations, and (iii) reactions with a small activation energy. Reactions with a buffer solution or additives in order to adjust ion strength, zero- or first-order reactions, or reactions containing high reactant concentrations are not accelerated by freezing. We conclude that the acceleration of reactions in the unfrozen solution of ice is not an abnormal phenomenon.     

Consecutive reactions in chemical kinetics: A method for the treatment of experimental data

A method is proposed for the treatment of data from a kinetic system of first order consecutive reactions by using an optimization procedure which converges to the correct solution of the two mathematically possible solutions and which does not need initial estimates of the parameters close to those to be determined.

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Ab initio chemical kinetics for reactions of ClO with Cl2O2 isomers

The mechanisms for the reactions of ClO with ClOClO, ClOOCl, and ClClO(2) have been investigated at the CCSD(T)/6-311+G(3df)//PW91PW91∕6-311+G(3df) level of theory. The rate constants for their low energy channels have been calculated by statistical theory. The results show that the main products for the reaction of ClO with ClOClO are ClOCl + ClOO, which can be produced readily by ClO abstracting the terminal O atom from ClOClO. This process occurs without an intrinsic barrier, with the predicted rate constant: k (ClO + ClOClO) = 7.26 × 10(-10) T(-0.15) × exp (-40/T) cm(3)molecule(-1)s(-1) for 200-1500 K. For the reactions of ClO + ClOOCl and ClClO(2), the lowest abstraction barriers are 7.2 and 7.3 kcal/mol, respectively, suggesting that these two reactions are kinetically unimportant in the Earth's stratosphere as their rate constants are less than 10(-14) cm(3)molecule(-1)s(-1) below 700 K. At T = 200-1500 K, the computed rate constants can be represented by k (ClO+ ClOOCl) = 1.11 × 10 (-14) T (0.87) exp (-3576/T) and k (ClO+ ClClO(2)) = 4.61 × 10(-14) T(0.53) exp (-3588/T) cm(3)molecule(-1)s(-1). For these systems, no experimental or theoretical kinetic data are available for comparison.     

A convolution approach to the kinetics of chemical and photochemical reactions

A new method for writing kinetic equations directly in the integrated form is presented. This method applies to any species that is consumed solely through first order steps, regardless of the complexity of its formation pathways.     

Complementarity methodology as applied for solution of the inverse problem for solid-phase reaction kinetics III

Mathematical analysis has shown that invariant kinetic parameters (IKP) correspond to the real kinetic curve even in the case when the equation prescribing this curve is not used for the calculation of parameters. It has been proved that IKP values coincide with those obtained for isothermal conditions. The theory is verified by calculations using model experimental data. The IKP stability to random experimental errors is studied.     

Studies on thermokinetics of consecutive first-order reactions

The transformation equation for the thermokinetics of consecutive first-order reactions has been deduced, and a thermokinetic research method of irreversible consecutive first-order reactions, which can be used to determine the rate constants of two steps simultaneously, is proposed. The method was validated and its theoretical basis was verified by the experimental results.     

Domain catalyzed chemical reactions: a molecular dynamics simulation of isomerization kinetics

The model for domain catalyzed isomerization kinetics in condensed fluids is applied for a diluted mixture of a chiral solute with a consolute temperature. The solution is quench to phase separation at temperatures below the consolute temperature. The droplet coalescence enhances the isomerization kinetics due to the substantial excess pressure inside the small droplets given by the Laplace equation. The domain catalyzed isomerization kinetics breaks the symmetry, and the droplets end with only one dominating species. We argue that D-glyceraldehyde which is only moderately solvable in water and which has played a crucial role in the evolution is a candidate for the stereo specific ordering in bio-organic matter.