Master equation simulations of the vibrational overtone activation of methylcyclopropene

Abstract

The complete kinetic data for the vibrational overtone activation of methylcyclopropene have been simulated using master equation calculations. The simulation included photoactivation, collisional energy transfer, and reaction into three unimolecular channels. A good fit to the Stern–Volmer plots for all the products at six different photolysis energies was obtained. The fit required an adjustment of the thermal activation barriers input into the RRKM calculation. The best fit barriers were 12 833 cm−1 for 2-butyne, 14 547 cm−1 for 1,3-butadiene, and 14 685 cm−1 for 1,2-butadiene. The collisional deactivation was fit with a single exponential energy transfer distribution function with an average amount of energy transferred down per collision of 1000 cm−1. This average value fit all of the Stern–Volmer plots. The product yield ratios were examined for local mode specific effects, but none were found. Previously obtained thermal data can be fit if log A is changed from 12.72 to 12.30. Stern–Volmer plots were constructed for methylcyclopropene diluted in helium, argon, and sulfur hexafluoride for the Δv=6 olefinic CH stretch transition. These plots were simulated using the same calculation parameters as mentioned earlier except for those having to do with the collider gas. For these simulations the average amounts of energy transferred down per collision were 150, 200, and 500 cm−1 for helium, argon, and sulfur hexafluoride, respectively.