Fall-off behavior of a thermal unimolecular system in the presence of a weak collider inert bath gas

Abstract

The behavior of the thermal unimolecular methyl isocyanide isomerization system has been examined at 245°over the whole range of fall-off from the low to high pressure regions at infinite dilution by the weak collider, helium, and by the strong collider, butane. The pressure displacement factor for the helium fall-off curve relative to the behavior of the pure substrate increases with decreasing order of reaction, as predicted. A comparison with theoretical calculations indicates that the collisional behavior of helium is intermediate between exponential and step ladder models for the collisional transition probability distribution function although the behavior seems closer to the latter, in disagreement with earlier work. The average amount of energy transferred per collision is (AE)sl = 350 cm-1 for helium in this system.

Unimolecular Reaction of Methyl Isocyanide to Acetonitrile: A High-Level Theoretical Study

“…Earlier, trajectory calculations done by Bunker and Hase using collisional energies of 293 to 837 kJ mol –1 showed non-RRKM unimolecular kinetics for CH 3 NC. This theoretical finding is potentially not relevant to thermal experiments − since the collisional energies are so far above that at the reaction threshold.…”

“…The isomerization of methyl isocyanide (CH 3 NC) to acetonitrile (CH 3 CN) is a prototypical homogeneous unimolecular reaction − that has long been studied with various experimental techniques. − A half century ago, Rabinovitch and co-workers − measured thermal rate constants over a wide range of temperatures and pressures in the falloff region. These authors reported an activation energy of 160.7 kJ mol –1 at the high-pressure limit and an Arrhenius pre-exponential factor of 10 13.5 s –1 , results that were verified by subsequent experimental studies. − In addition, on the basis of models and statistical Rice–Ramsperger–Kassel–Marcus (RRKM) theory, , Rabinovitch and co-workers were able to satisfactorily reproduce the measured rate constants in the falloff region (within a factor of 2) .…”