Temperature dependence of vibrational energy transfer in the “low” pressure competitive two-channel 1,1-cyclopropane- d2 system

Abstract

Intermolecular energy transfer has been studied in the two-channel competitive isomerization of 1,1-cyclopropane- d 2, both in the neat system and in the presence of helium bath gas at values of k/k − centered around 0.02. The competing path ways differ in threshold energy by ≈ 0.6 kcal. The temperature range 773 K to 973 K was covered. Several methods of treating the data, whether by isotopic ratios of rate constants or by temperature dependence of fall off, are each independent of a knowledge of collision cross sections. Used in conjunction, they provide measurements of these quantities. Cyclopropane is an operationally strong collider (β ω = 1) for itself at 773 K with an average down step size, <Δ E/s> >/ 10 kcal mole −1 (>/ 3500 cm −1). At 973 K the substrate is no longer a strong collider; β ω declines to ≈ 0.55 with <Δ E/s> ≈ 5.2 kcal mole −1. For helium the corresponding quantities are β ω ≈ 0.078 <Δ E/s> ≈ 1.1 kcal mole −1 declining to β ω ≈ 0.010 and <Δ E/s> ≈ 0.53 kcal mole −1. The several methods of measuring these quantities give excellent independent agreement. Comparison with the earlier theoretical formulation of Tardy and Rabinovitch gives good agreement, the temperature dependence of β ω for the weak collider, helium, follows the relation β ω ∝ T − m , where m /s> 2.