Don L. Bunker

On non‐RRKM unimolecular kinetics: Molecules in general, and CH3NC in particular

Monte Carlo rate constants for model CH3NC isomerization, determined at 200, 100, and 70 kcal/mole, disagree with theoretical predictions. Also, three different approximate methods of generating initial conditions at 200 kcal lead to divergent results. The molecule does not appear to us to obey the random lifetime assumption of conventional unimolecular rate theory at any of these energies. A discussion is given of the systematics of this kind of effect, and comments are made on the relationship between our results and those obtained in the laboratory.

Monte Carlo Calculations. VI. A Re‐evaluation of the RRKM Theory of Unimolecular Reaction Rates

Previously obtained Monte Carlo rate constants for unimolecular decomposition of model molecules [J. Chem. Phys. 40, 1946 (1964)] are compared with the predictions of a modified version of the Rice‐Ramsperger‐Kassel‐Marcus theory. The principal modification is an unambiguous method of specification of the critical value of the reaction coordinate. Anharmonicity corrections are accurately calculated, and an improved way of treating rotational state densities, closely related to that of Marcus [J. Chem. Phys. 43, 2658 (1965)], is used. The agreement between theory and Monte Carlo results is drastically improved; remaining deviations are about  ± 50% for bent molecules and undetectable (within 20%) for linear ones.

An exploratory study of reactant vibrational effects in CH3 + H2 and its isotopic variants

An empirically calibrated potential energy surface, obtained previously [J. Chem. Phys. 61, 21 (1974)], was used in a trajectory study of the effect of reactant energy partitioning on cross section for CH3 + H2 → CH4 + H. Artificial variations in the barrier position, and artificial and naturally occuring changes in the H masses, were also made. For natural CH3 + H2, H2 vibration enhances and CH3 out‐of‐plane bending depresses the cross section, at constant (25 kcal) translational energy of approach. The isotope substitution and barrier position effects are more complex and not predictable from triatomic A + BC generalizations. The relationship of these results to experimental ones is discussed.

Trajectory Studies of Hot‐Atom Reactions. I. Tritium and Methane

We have studied the reactions of T+CH4 and T+CD4, treating these as six distinct particles, using a variety of potential energy surfaces subject to the restriction that only one methane hydrogen at a time is reactive. Our principal findings are: (1) This trial assumption about the potential is unjustified. Substitution (products CH3T+H and CD3T+D) involves strong interactions between at least four atoms. (2) There were no inertial isotope effects of any kind when CH4 was replaced by CD4. (3) From (2) and the details of the trajectories, there is suggestive but not conclusive evidence that substitution in CH4 proceeds by Walden inversion. (4) Abstraction (products CH3+HT and CD3+DT) is direct and concerted and occurs at relatively low energy. In our calculations it had a maximum cross section of 3.5 A2 for a reactant translation energy of 65 kcal. At sufficiently high energy it is a stripping reaction. (5) About half the abstraction product energy is translational; the remainder appears as internal energy ...

Discrete simulation methods in combustion kinetics

Abstract A hybrid method, intermediate between diffential equation solution and Monte Carlo, is described for calculating the consequences of an assumed mechanism and set of rate constants. It is compared with standard methods as to simplicity, efficiency, stability, and other features. Situations in which it can replace or supplement other procedures are discussed. An example calculation is presented. It consists of a routine simulation of C 3 H 8 oxidation with a 37-reaction mechanism and variable temperature, along with a comparison with results obtained with a collapsed mechanism, the latter simplication being one that is particularly facile when discrete stimulation is used.

Trajectory studies of hot atom reactions. II. An unrestricted potential for CH5

An approximate empirical potential energy hypersurface has been fitted to a combination of experimental results and molecular structural information, by means of a trajectory analysis, without the restrictions imposed in Part I. On the basis of this, predictions are made for the incident energy dependence of the reactive cross sections when the reactants are T+CH4, T+CD4, D+CH4, H+CD4, and H+CH4. The scattering properties and some aspects of the energy disposal in these reactions are also examined, and the most characteristic molecular dynamic features of the reactions are described.

Dynamical effects in unimolecular decomposition: A classical trajectory study of the dissociation of C2H6.

The unimolecular dissociation of ethane via C–C and C–H scission has been simulated by trajectory calculation on a realistic and well coupled potential energy surface. Detailed lifetime distributions were obtained from this calculation for both C–C and C–H fragmentation channels at several energies (180, 210, and 240 kcal/mole) and for several energization patterns. The potential surface was also used to determine vibrational frequencies for the activated molecular model and various critical configurations. These frequencies were applied in the accurate census of phase space volumes for precisely tailored RRKM calculations. The trajectory results for the different energization patterns do not agree with corresponding statistical predictions nor with each other. The details of these deviations are discussed with regard to the importance of dynamical effects in intramolecular relaxation and unimolecular dissociation.

Revised General A + BC Potential for Trajectory Studies

A previously proposed adjustable potential‐energy expression [D. L. Bunker and N. C. Blais, J. Chem. Phys. 41, 2377 (1964)] has been modified and extended. Besides its former properties, it now has variable cross section and provision for potential barriers and bound intermediates.

Trajectory Studies of Halogen Atom—Molecule Exchange Reactions

We studied the reactions X+Y2 →XY+Y, usually with X=Br, Y=I but with a few calculations for other choices (X=Cl or Y=Br). We used empirical potential surfaces having minima 0–10 kcal deep. In addition we could control all aspects of the shape and position of this potential well. Comparison with recent experimental results reveals that no reasonable potential well for collinear reactant approach will explain the observed scattering. The calculated behavior of the reaction is relatively insensitive to well properties, though it continues to depend on other potential shape parameters in the usual way. We suggest that the characteristic features of X+Y2 reactive scattering may arise from the predominant effect of some feature of the interaction potential at fairly large reactant separation. Comparison of our results with those of trajectory studies for D++H2 indicates that formation of a long‐lived intermediate complex in the neighborhood of a potential well will in practice depend sensitively on the particle...

Methyl isocyanide is probably a non-RRKM molecule

Abstract Classical trajectory calculations strongly indicate that CH 3 NC isomerization does not obey the RRKM theory, even under thermal conditions. Recent experimental work reinforces this conclusion.