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

Abstract

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.

High-temperature kinetics of the reaction between chlorine atoms and hydrogen sulfide

“…The two combined data sets can be described well by the non-Arrhenius , is reminiscent of that observed for alkyl radical + HBr reactions, which have been rationalized in terms of formation of an initial adduct with no energy barrier, followed by dissociation to final products over a tighter but lower energy barrier, in competition with redissociation back to the original reactants [27,28]. Such a mechanism appears plausible here, given the published potential energy profiles [11,12], and a theoretical analysis of reaction 1 is underway in our group.…”

Kinetic studies of the reaction of atomic chlorine with chlorobenzene

“…Quantitative kinetic analysis of this kind of PES requires RRKM or modified transition state theory approaches [15,16], but three qualitative conclusions may be drawn…”

Modification of Aquifer Pore-Water by Static Diffusion Using Nano-Zero-Valent Metals

“…Negative activation energies are also associated with the interaction of methylene (e.g., ZVM-CH 3 ) and halogen species (e.g., HCl, HBr, etc.) to produce methane + halogen [36] and the breakdown of n-alkanes [37]. [38] in the virtually instantaneous breakdown of alkenes [38,39].…”