Capture Rate Constants Research Articles

Rate constant of capture of electron charge carriers on a screened repulsive center

The rate constant of capture of electron charge carriers on a screened repulsive center are performed. Approximate expressions for the potential barrier width, the capture cross section, and rate constant are derived. It is shown that the increase in the concentration of free charge carriers in silver azide from 1016 to 1020 cm−3 results in an increase in the capture rate constant by four orders of magnitude. It is also shown that, with increasing concentration of free carriers, the temperature dependence of the rate constant weakens and the effective activation energy of capture in silver azide decreases from 0.18 to 0.01 eV.

Reactivity of First-Row Transition Metal Monocations (Sc+, Ti+, V+, Zn+) with Methyl Fluoride: A Computational Study

The gas-phase reactivity of methyl fluoride with selected first-row transition metal monocations (Sc(+), Ti(+), V(+), and Zn(+)) has been theoretically investigated. Our thermochemical and kinetics study shows that early transition-metal cations exhibit a much more active chemistry than the latest transition metal monocation Zn(+). The strong C-F bond in methyl fluorine can be activated by scandium, titanium, and vanadium monocations yielding the metal fluorine cation, MF(+). However, the rate efficiencies vary dramatically along the period 0.73 (Sc), 0.91 (Ti), and 0.028 (V) in agreement with the experimental observation. The kinetics results show the relative importance of the entrance and exit channels, apart from the "inner" bottleneck, to control the global rate constant of these reactions. At the mPW1K/QZVPP level our computed kglobal (at 295 K), 1.99 × 10(-9) cm(3) molecule(-1) s(-1) (Sc(+)), 1.29 × 10(-9) cm(3) molecule(-1) s(-1) (Ti(+)), and 3.46 × 10(-10) cm(3) molecule(-1) s(-1) (V(+)) are in good agreement with the experimental data at the same temperature. For the reaction of Zn(+) and CH3F our predicted value for kouter, at 295 K, 3.79 × 10(-9) cm(3) molecule(-1) s(-1), is in accordance with the capture rate constant. Our study suggests that consideration of the lowest excited states for Ti(+) and V(+) is mandatory to reach agreement between calculations and experimental measurements.

An Adiabatic Capture Theory and Quasiclassical Trajectory Study of C + NO and O + CN on the 2A′, 2A″, and 4A″ Potential Energy Surfaces

The adiabatic capture centrifugal sudden approximation (ACCSA) has been applied to the C + NO and O + CN reactions, along with quasiclassical trajectory simulations. Existing global analytic fits to the potential energy surfaces of the CNO system in the (2)A', (2)A", and (4)A" electronic states have been used. Thermal rate constants for reaction in each of the electronic states have been calculated. In all cases a strong temperature dependence is evident in the calculated rate constants. The agreement between the calculated adiabatic capture and quasiclassical trajectory rate constants is excellent in some cases, but these rate constants differ considerably in other cases. This behavior is analyzed in terms of the anisotropy of the potential energy surfaces. On the basis of this analysis, we propose a new diagnostic for the reliability of ACCSA capture calculations.

Electron capture by finite-size polarizable molecules and clusters

The effects of finite molecular target size are investigated for partial-wave selected capture of electrons by isotropically polarizable molecules and clusters within a generalized Vogt-Wannier model. It is shown how expressions for partial-wave selected capture probabilities of zero-size targets from Dashevskaya et al. (Phys. Chem. Chem. Phys., 2009, 11, 9364) can be modified to account for finite target sizes of the molecules and clusters. The transition from quantum to classical, from single- to multiple- and all-wave, behaviour of capture probabilities, cross sections, and rate constants is illustrated.

Capture of asymmetric top dipolar molecules by ions: Rate constants for capture of H 2O, HDO, and D 2O by arbitrary ions

The capture of rotationally state-selected and unselected asymmetric top polar molecules by ions is investigated. Analytical expressions (for all rotational states up to j = 2) of capture rate constants in the perturbed-rotor second-order limit are derived for application to low temperature conditions. Approximate analytical representations over wider temperature ranges are also given for rotationally unselected molecules. The capture of H 2O, D 2O, and HDO by arbitrary ions is chosen for demonstration of the approach. Capture rate constants for the about 60 reactions of H 2O with ions listed in the UMIST 2006 data base for astrochemistry are calculated, compared with experimental data, and represented in the format k cap( T) ≈ c 1 + c 2( T/300 K) −1/2. The parameters c 1 and c 2 can be predicted in a very simple way. The approach allows one to identify capture-controlled mechanisms and/or to trace experimental artifacts. The approach applies equally well to the capture of symmetric top and linear dipole molecules by arbitrary ions.

A new apparatus for measuring rate constants and activation energies of thermal electron capture processes in the gas phase

The new apparatus for investigation of kinetics of a thermal electron capture process and its temperature dependence with pulsed Townsend technique has been built. This method gives straightforward results on electron drift velocities in any mixture and allows calculating the rate constants for electron capture as well as activation energies of this process. The thermal electron capture rate constants and activation energies for CH 3CCl 3, CHCl 2CH 2Cl and CH 3CHClCH 2CH 3 have been determined.

On the Chaperon Mechanism: Application to ClO + ClO (+N2) → ClOOCl (+N2)

The dynamics of the ClO + ClO (+N(2)) radical complex (or chaperon) mechanism is studied by electronic structure methods and quasi-classical trajectory calculations. The geometries and frequencies of the stationary points on the potential energy surface (PES) are optimized at the B3LYP/6-311+G(3df) level of theory, and the energies are refined at the CCSD(T)/6-311+G(3df) (single-point) level of theory. Basis set superposition error (BSSE) corrections are applied to obtain 1.5 kcal mol(-1) for the binding energy of the ClO.N(2) van der Waals (VDW) complex. A model PES is developed and used in quasi-classical trajectory calculations to obtain the capture rate constant and nascent energy distributions of ClOOCl* produced via the chaperon mechanism. A range of VDW binding energies from 1.5 to 9.0 kcal mol(-1) are investigated. The anisotropic PES for the ClO.N(2) complex and a separable anharmonic oscillator approximation are used to estimate the equilibrium constant for formation of the VDW complex. Rate constants, branching ratios to produce ClOOCl, and nascent energy distributions of excited ClOOCl* are discussed with respect to the VDW binding energy and temperature. Interestingly, even for weak VDW binding energies, the N(2) usually carries away enough energy to stabilize the nascent ClOOCl*, although the VDW equilibrium constant is small. For stronger binding energies, the stabilization efficiency is reduced, but the capture rate constant is increased commensurately. The resulting rate constants for forming ClOOCl* from the title reaction are only weakly dependent on the VDW binding energy and temperature. As a result, the relative importance of the chaperon mechanism is mostly dependent on the VDW equilibrium constant. For the calculated ClO.N(2) binding energy of 1.5 kcal mol(-1), the VDW equilibrium constant is small, and the chaperon mechanism is only important at very high pressures.

A Simple Method Relating Specific Rate Constants k(E,J) and Thermally Averaged Rate Constants k∞(T) of Unimolecular Bond Fission and the Reverse Barrierless Association Reactions

This work describes a simple method linking specific rate constants k(E,J) of bond fission reactions AB --> A + B with thermally averaged capture rate constants k(cap)(T) of the reverse barrierless combination reactions A + B --> AB (or the corresponding high-pressure dissociation or recombination rate constants k(infinity)(T)). Practical applications are given for ionic and neutral reaction systems. The method, in the first stage, requires a phase-space theoretical treatment with the most realistic minimum energy path potential available, either from reduced dimensionality ab initio or from model calculations of the potential, providing the centrifugal barriers E(0)(J). The effects of the anisotropy of the potential afterward are expressed in terms of specific and thermal rigidity factors f(rigid)(E,J) and f(rigid)(T), respectively. Simple relationships provide a link between f(rigid)(E,J) and f(rigid)(T) where J is an average value of J related to J(max)(E), i.e., the maximum J value compatible with E > or = E0(J), and f(rigid)(E,J) applies to the transitional modes. Methods for constructing f(rigid)(E,J) from f(rigid)(E,J) are also described. The derived relationships are adaptable and can be used on that level of information which is available either from more detailed theoretical calculations or from limited experimental information on specific or thermally averaged rate constants. The examples used for illustration are the systems C6H6+ <==> C6H5+ + H, C8H10+ --> C7H7+ + CH3, n-C9H12+ <==> C7H7+ + C2H5, n-C10H14+ <==> C7H7+ + C3H7, HO2 <==> H + O2, HO2 <==> HO + O, and H2O2 <==> 2HO.

The study of elementary steps of the photolysis of silver halides by the method of microwave photoconductivity

The major part of inconsistencies in the description of the mechanism of formation of nonhalide silver clusters under exposure to light is due to the lack of quantitative data on electronic-ionic processes in microdispersed silver halide systems. The use of the microwave photoconductivity technique partially fills the gap. The drift mobility of holes and electrons in powdered AgBr (0.8 and 60 cm2 V−1 s−1, respectively) and of electrons in powdered AgCl (40 cm2 V−1 s−1) and the rate constants of the recombination of localized electrons and free holes in powder AgBr (1.4 × 10−7 cm3 s−1) and of free electrons and holes in AgBr and AgCl (1.5 ± 0.5) × 10−11 and 2 × 10−12 cm3 s− 1, respectively) were determined. The range of the depth of traps formed by treatment with aqueous solutions of sodium thiosulfate in AgCl (0.45–0.63 eV) and by preliminary exposure of AgCl and AgBr grains (0.4 and 0.25–0.36 eV, respectively) was estimated. The rate constant of the hole capture by the centers formed by introducing AgI (7 × 10−9 cm3 s−1) was determined, and the lifetime of the Ag atom (10−4 s) was estimated. The paper summarizes the results obtained.

Classical Trajectory and Statistical Adiabatic Channel Study of the Dynamics of Capture and Unimolecular Bond Fission. VII. Thermal Capture and Specific Rate Constants k(E,J) for the Dissociation of Molecular Ions

Abstract Specific rate constants, k(E,J), and thermal capture rate constants, k cap(T), are determined by statistical adiabatic channel model/classical trajectory (SACM/CT) calculations for unimolecular dissociation and the reverse association reactions of representative polyatomic molecular ions. Simple short-range valence/long-range ion-induced dipole model potentials without reverse barriers have been employed, using the reactions C8H10 + ⇔ C7H7 + + CH3 and C9H12 + ⇔ C7H7 + + C2H5 as illustrative examples. Simplified representations of k(E) and k cap(T) from rigid activated complex Rice–Ramsperger–Kassel–Marcus (RRKM) theory are compared with the SACM/CT treatment and with experimental results. The Massey parameters of the transitional mode dynamics, for the systems considered, are smaller than unity such that their dynamics is nonadiabatic while the dynamics of the conserved modes is adiabatic. Because of the long-range/short-range switching character of the potential, simple rigid activated complex RRKM theory cannot be used without modifications. The effects of a shifting of the effective bottle-neck of the dynamics with increasing energy towards smaller interfragment distances in the present cases are amplified by a shift into a range of increasing anisotropy of the potential. As a consequence, the thermal capture rate constants markedly decrease with increasing temperature.

Low energy electron attachment by bromoalkanes

Thermal electron attachment processes in the mixtures of CH3CH2CH2Br, CH3CHBrCH3, CH2BrCH2Br, CH2FCH2Br and CF3CH2Br with carbon dioxide have been investigated using an electron swarm method. It has been found that all the investigated compounds attach electrons only in a two-body process. Corresponding rate constants have been determined to be equal to 1.1×10−11, 1.4×10−12, 1.8×10−8, 5×10−11 and 1.4×10−9cm3molecule−1s−1, respectively. The dependence of the electron capture rate constants on electronic polarizability of the accepting center of the molecule has been demonstrated. The average rate constant for CH3Br, CH3CH2Br and CH3CH2CH2Br equal to (6±2)×10−12cm3molecule−1s−1 is recommended.

Low-Energy Electron Attachment by Chloroalkanes

Thermal electron attachment rate constants for 1,1,2-trichloroethane, 1,1,2,2-tetrachloroethane, 1- and 2-chloropropanes, and 1,1-, 1,2-, 2,2-, and 1,3-dichloropropanes have been measured using an electron swarm method. It has been found that all the investigated compounds attach electrons only in a two-body process. Corresponding rate constants are equal to 1.4 × 10-10, 3.2 × 10-8, 2.7 × 10-13, 3.8 × 10-12, 5.7 × 10-11, 8.1 × 10-12, 6.3 × 10-12, and 1.2 × 10-11 cm3 molecule-1 s-1, respectively. The dependence of the electron capture rate constants on the electronic polarizability of the accepting center of the molecule and the vertical attachment energy has been demonstrated.

Low-temperature behavior of capture rate constants for inverse power potentials

The energy dependence of the capture cross section and the temperature dependence of the capture rate constants for inverse power attractive potentials V∝−R−n is considered in the regime where the quantum character of the relative motion of colliding partners is important. For practically interesting cases n=4 and n=6, a simple formula for the cross section is suggested which interpolates between the classical and the quantum Bethe limits. We have shown that the classical approximation for the capture cross section performs well far below the simple estimations of the onset the quantum regime. This seemingly “classical” feature of the cross section and the rate constant is due to the large quantum effects of the waves in transmission through and reflection above the centrifugal potential barriers.

Classical theories of the ion/linear quadrupole capture

The dependence of the ion/linear quadrupole capture rate constant on the resultant orbital-rotational momentum and individual energetic and molecular parameters that characterize a given system are calculated using a microcanonical version of the transition state theory and classical trajectory method. Both methods indicate that the dependencies are similar to those obtained for the ion/linear dipole system. It is shown that, contrary to the ion/polar molecule systems, the capture rate constant for the quadrupoles frequently does not fall on the plateau which corresponds to the assumption that the resultant momentum can be approximated to the orbital momentum. Therefore, the theories that use this approximation are not adequate for description of the ion/quadrupole capture. Both TST and classical trajectory calculations agree very well with experiment and give identical or slightly different results for | b 3|≤10 which covers the whole range of temperatures down to extremely low. At so low temperatures the discrepancies are larger which seems to be of little consequence since both methods do not account for the quantum effects.

An experimental and theoretical study of the product distribution of the reaction CH2 (X 3B1) + NO.

Measurements of the product branching ratios of the reaction CH2 (X 3B1) + NO (1) are presented together with calculations of the thermal rate constant and branching ratios using unimolecular rate theory. The reaction was investigated experimentally at room temperature using FTIR spectroscopy. The yields of the main products HCNO and HCN were found to be gamma HCNO = 0.89 +/- 0.06, gamma HCN = 0.11 +/- 0.06. Other minor products could be rationalized by numerical simulations of the reaction system taking into account possible consecutive reactions. The potential energy surface for the reaction was characterized by quantum chemical calculations using ab initio and density functional methods. The proposed reaction pathways connecting reactants to products were explored by multi-channel unimolecular rate theory calculations to determine the CH2 (X) + NO capture rate constant and the rate constants for the different product channels as a function of temperature. The calculated capture rate constant of k = 2.3 x 10(13) cm3 mol-1 s-1 is in good agreement with experimental values at room temperature. Collisional stabilization of the initial H2CNO recombination complex was predicted to be negligible up to pressures of > 1 bar. For ambient pressures and temperatures up to 2000 K, HCNO + H were calculated as the dominating products, with gamma HCNO approximately 0.94 in agreement with the experiments. The channel to HCN + OH was calculated with 0.015 < or = gamma HCN < or = 0.05, only slightly below the experimental value.

Electron capture by haloethanes in a carbon dioxide buffer gas

An electron swarm method for investigation of the electron capture reaction has been modified. It is based on the registration of the temporal evolution of a single electron pulse. The method gives straightforward results on electron drift velocities in any mixture and allows calculating the rate constants for the electron capture. The thermal electron capture kinetics for eight haloethanes has been investigated. Only two-body attachment processes have been found in their mixtures with carbon dioxide. All corresponding rate constants have been determined. A link between the two-body rate constant and an electron polarizability of the attaching center has been demonstrated.

Thermal Electron Capture in the Mixtures of Halocarbons and Environmental Gases

The electron capture processes in the mixtures containing halocarbons (CHF3 or CClF3) and nitrogen have been investigated. Second- and third-order kinetics were observed in both cases. The electron interaction with van der Waals complexes CHF3‚N2 and CClF3‚N2 was invoked to explain this behavior and the corresponding rate constants have been determined. The values for two-body processes (5 10 -14 cm 3 molecule -1 s -1 -CHF3 and 1.3 10 -13 cm 3 molecule -1 s -1 -CClF3) are fully consistent with the literature data. The estimated rate constants for electron capture by van der Waals complexes are equal to 1.2 10 -10 and 2.4 10 -11 cm 3 molecule -1 s -1 for CHF3‚N2 and CClF3‚N2, respectively.

Rethinking fundamentals of enzyme action.

Despite certain limitations, investigators continue to gainfully employ concepts rooted in steady-state kinetics in efforts to draw mechanistically relevant inferences about enzyme catalysis. By reconsidering steady-state enzyme kinetic behavior, this review develops ideas that allow one to arrive at the following new definitions: (a) V/K, the ratio of the maximal initial velocity divided by the Michaelis-Menten constant, is the apparent rate constant for the capture of substrate into enzyme complexes that are destined to yield product(s) at some later point in time; (b) the maximal velocity V is the apparent rate constant for the release of substrate from captured complexes in the form of free product(s); and (c) the Michaelis-Menten constant K is the ratio of the apparent rate constants for release and capture. The physiologic significance of V/K is also explored to illuminate aspects of antibiotic resistance, the concept of "perfection" in enzyme catalysis, and catalytic proficiency. The conceptual basis of congruent thermodynamic cycles is also considered in an attempt to achieve an unambiguous way for comparing an enzyme-catalyzed reaction with its uncatalyzed reference reaction. Such efforts promise a deeper understanding of the origins of catalytic power, as it relates to stabilization of the reactant ground state, stabilization of the transition state, and reciprocal stabilizations of ground and transition states.

Helicity decoupled quantum dynamics and capture model cross sections and rate constants for O(1D)+H2→OH+H

We study, within a helicity decoupled quantum approximation, the total angular momentum J dependence of reaction probabilities for the reaction O(1D)+H2→OH+H. A recently developed real wave packet approach is employed for the quantum calculations. The abinitio based, ground electronic (1A′) potential energy surface of Ho etal. (T-S. Ho, T. Hollebeeck, H. Rabitz, L. B. Harding and G. C. Schatz, J. Chem. Phys., 1996, 105, 10472) is assumed for most of our calculations, although some calculations are also performed with a recent surface due to Dobbyn and Knowles. We find that the low J reaction probabilities tend to be, on average, slightly lower than the high J probabilities. This effect is also found to be reproduced in classical trajectory calculations. A new capture model is proposed that incorporates the available quantum data within an orbital angular momentum or l-shifting approximation to predict total cross sections and rate constants. The results agree well with classical trajectory results and the experimental rate constant at room temperature. However, electronically non-adiabatic effects may become important at higher temperature.

The dependence of electron capture rate constants on some molecular parameters

We have analyzed the dependence of the thermal electron capture rate constant on molecular parameters as dipole moment, electronic and orientational polarizabilities. We have found that there is a linear dependence between the logarithm of the rate constant and the electronic polarizability on the electron-accepting center.