Temperature dependence of vibrational energy transfer in the “low” pressure competitive two-channel 1,1-cyclopropane-d2 system |
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Authors: | IE Klein BS Rabinovitch |
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Institution: | Department of Chemistry, University of Washington, Seattle, Washington 98195, USA |
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Abstract: | Intermolecular energy transfer has been studied in the two-channel competitive isomerization of 1,1-cyclopropane-d2, 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 (βω = 1) for itself at 773 K with an average down step size, <ΔE/s> >/ 10 kcal mole ?1 (>/ 3500 cm?1). At 973 K the substrate is no longer a strong collider; βω declines to ≈ 0.55 with <ΔE/s> ≈ 5.2 kcal mole?1. For helium the corresponding quantities are βω ≈ 0.078 <ΔE/s> ≈ 1.1 kcal mole ?1 declining to βω ≈ 0.010 and <ΔE/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 βω for the weak collider, helium, follows the relation βω ∝ T?m, where m /s> 2. |
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