On the question of negative activation energies: absolute rate constants by RRKM and G1 theory for methyl + hydrogen halide .fwdarw. methane + halogen (Cl, Br) reactions

Chen, Yonghua; Rauk, Arvi; Tschuikow‐Roux, E.

doi:10.1021/j100177a053

Cited by 71 publications

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“…This constrain is fully compatible with the experimental conditions employed by Zhao et al 17 to measure the rate constant for the Ca + +CH 3 F → CaF + +CH 3 reaction ͑P = 0.35 Torr͒ Concentrations of C1 and C2 structures are determined by steady state rather than equilibrium conditions. 30,35 The systems considered in this work exhibit rotational constants fulfilling A i Ͼ B i Ϸ C i and therefore the usual approximation that the molecules be treated as a prolate symmetry top rotor should work well. The rotational energy levels depend on two quantum numbers, J and K, as follows,…”

Section: B Kinetics Calculationsmentioning

confidence: 99%

Gas-phase reaction between calcium monocation and fluoromethane: Analysis of the potential energy hypersurface and kinetics calculations

Varela‐Álvarez

Rayón

Barrientos

et al. 2009

The Journal of Chemical Physics

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A quantum reaction dynamics study of the translational, vibrational, and rotational motion effects on the HD + H3+ reaction J. Chem. Phys. 135, 114307 (2011) First-principles simulation of molecular dissociation-recombination equilibrium J. Chem. Phys. 135, 104310 (2011) Direct simulation of electron transfer using ring polymer molecular dynamics: Comparison with semiclassical instanton theory and exact quantum methods J. Chem. Phys. 135, 074106 (2011) Charge-transfer-induced suppression of galvanic replacement and synthesis of (Au@Ag)@Au double shell nanoparticles for highly uniform, robust and sensitive bioprobes Appl. Phys. Lett. 99, 073107 (2011) Additional information on J. Chem. Phys. The gas-phase reaction between calcium monocation and fluoromethane: Ca + +CH 3 F → CaF + +CH 3 was theoretically analyzed. The potential energy hypersurface was explored by using density functional theory methodology with different functionals and Pople's, Dunning's, Ahlrichs', and Stuttgart-Dresden basis sets. Kinetics calculations ͑energy and total angular momentum resolved microcanonical variational/conventional theory͒ were accomplished. The theoretically predicted range for the global kinetic rate constant values at 295 K ͑7.2ϫ 10 −11 -5.9 ϫ 10 −10 cm 3 molecule −1 s −1 ͒ agrees reasonably well with the experimental value at the same temperature ͓͑2.6Ϯ 0.8͒ ϫ 10 −10 cm 3 molecule −1 s −1 ͔. Explicit consideration of a two transition state model, where the formation of a weakly bounded prereactive complex is preceded by an outer transition state ͑entrance channel͒ and followed by an inner transition state connecting with a second intermediate that finally leads to products, is mandatory. Experimental observations on the correlation, or lack of correlation, between reaction rate constants and second ionization energies of the metal might well be rationalized in terms of this two transition state model.

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Section: B Kinetics Calculationsmentioning

confidence: 99%

Gas-phase reaction between calcium monocation and fluoromethane: Analysis of the potential energy hypersurface and kinetics calculations

Varela‐Álvarez

Rayón

Barrientos

et al. 2009

The Journal of Chemical Physics

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“…21 As noted above, they computed a positive barrier to reaction which had to be adjusted to a negative value to match experiment, and they included a simple tunneling correction. Both corrections were criticized by Benson and Dobis, 22 who argued in favor of smaller CH 3 + HBr rate constants with a positive activation energy.…”

Section: Introductionmentioning

confidence: 99%

Modified Transition State Theory and Negative Apparent Activation Energies of Simple Metathesis Reactions: Application to the Reaction CH₃ + HBr → CH₄ + Br

2005

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A modified transition state theory (MTST) has been developed for gas-phase reactions with "negative barriers". The theory was applied to the reactions CH 3 + HBr(DBr) f CH 4 (CH 3 D) + Br (1a, 1b), which exhibit negative temperature dependences. Accurate ab initio calculations performed with coupled cluster theory extrapolated to the complete basis set limit revealed a transition state located at -2.3 kJ mol -1 relative to the ground state of the reactants (in reaction 1a), as well as a shallow bound complex. The negative temperature dependence, the absolute values of the rate constant, and the isotope substitution effect are reproduced with good accuracy (10%), without any adjustment or fitting parameters. Analytical expressions are presented for MTST including angular momentum conservation, centrifugal barriers and tunneling. This analysis uses information about the possibly loose entrance barrier and the transition state but does not invoke a statistical intermediate complex.

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“…It is essential that the normal mode frequencies yielded by our ab initio calculations do not require further multiplication by a scaled factor, by contrast to the previous studies [6,10,11] which neglect intramolecular electron correlations. Table 5.…”

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confidence: 99%

Molecular, thermodynamic, and kinetic parameters of iodomethanes and iodomethyl radicals: ab initio calculations

2004

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The wave functions and enthalpies of formation of the ground states of iodomethanes CH 4 3x I x and iodomethyl radicals CH 3 3x I x . (x = 133) were calculated ab initio with regard to electron correlation. The geometries of the molecules of these compounds were determined, as well as the normal mode frequencies and other parameters, which were used for calculating the thermodynamic functions in the 031500 K range.These functions were used for calculating the constants of the CH 4 3x I x 76 47 CH 4 3x I x 31 + I and CH 4 3x I x + I 76 47 CH 4 3x I x 31 + I 2 equilibria, which, in turn, were used for calculating the corresponding rate constants in the high concentration limit.Studies of gas-phase pyrolysis of alkyl halides have been successfully used in developing fundamentals of chemical kinetics for almost a century [1,2]. The key reactions for the alkyl halides being pyrolyzed are those between (halo)alkyl radical and atomic hydrogen: R . + H . 76 RH, halogen: R . + Hlg . 76 RHlg, or hydrogen halide molecules: R . + HHlg 76 RH + Hlg . , Hlg = Cl, Br, I.Hence, these pyrolytic experiments furnish kinetic and thermochemical information about free radicals [336]. For the last-named reactions (CH 3 . + HI, CH 3 . + HBr, C 2 H 5 . + HBr, CH 2 Br . + HI...), peculiar phenomenon of a negative activation energy was recently discovered [739], whose nature still remains to be elucidated [10313]. Accurate thermodynamic and kinetic parameters of chloro-, bromo-, and iodoalkanes and their corresponding radicals are also essential for solving less fundamental but practically vital problems, such as, e.g., simulation of the Earth ozone layer depletion or modeling of incineration of halogencontaining waste.

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On the question of negative activation energies: absolute rate constants by RRKM and G1 theory for methyl + hydrogen halide .fwdarw. methane + halogen (Cl, Br) reactions

Cited by 71 publications

References 0 publications

Gas-phase reaction between calcium monocation and fluoromethane: Analysis of the potential energy hypersurface and kinetics calculations

Gas-phase reaction between calcium monocation and fluoromethane: Analysis of the potential energy hypersurface and kinetics calculations

Modified Transition State Theory and Negative Apparent Activation Energies of Simple Metathesis Reactions: Application to the Reaction CH₃ + HBr → CH₄ + Br

Molecular, thermodynamic, and kinetic parameters of iodomethanes and iodomethyl radicals: ab initio calculations

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On the question of negative activation energies: absolute rate constants by RRKM and G1 theory for methyl + hydrogen halide .fwdarw. methane + halogen (Cl, Br) reactions

Cited by 71 publications

References 0 publications

Gas-phase reaction between calcium monocation and fluoromethane: Analysis of the potential energy hypersurface and kinetics calculations

Gas-phase reaction between calcium monocation and fluoromethane: Analysis of the potential energy hypersurface and kinetics calculations

Modified Transition State Theory and Negative Apparent Activation Energies of Simple Metathesis Reactions: Application to the Reaction CH3 + HBr → CH4 + Br

Molecular, thermodynamic, and kinetic parameters of iodomethanes and iodomethyl radicals: ab initio calculations

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Modified Transition State Theory and Negative Apparent Activation Energies of Simple Metathesis Reactions: Application to the Reaction CH₃ + HBr → CH₄ + Br