Collisional Transition Probabilities for Deactivation of Highly Vibrationally Excited Cyclopropane and Dimethylcyclopropane

Abstract

Collisional deactivation of vibrationally excited cyclopropane and dimethylcyclopropane molecules, in their ground electronic states, has been studied with ethylene and cis-butene as the respective deactivators. Activation is to an energy region just above 100 kcal mole—1, which is reached by chemical activation. The activated molecules may isomerize, and the measurable cascade is to a reaction threshold at 60–65 kcal; below the threshold the excited molecules are stable with respect to reaction. The principle and characteristics of this steady-state method has been described in detail in Ref. 5. The two substrate molecules offer an interesting difference in internal rotational degrees of freedom. Cyclopropane was investigated at 298°, 598°, and 723°K, and dimethylcyclopropane at 573° and 673°K. The observed behavior was compared with model calculations for various assumed functional forms of collisional transition probabilities pij which include stepladder, Gaussian, and skewed distributions. The data, unfortunately, are too inaccurate to distinguish between these models clearly; but they do show that ethylene removes energy from cyclopropane in average steps which are at least 7 kcal (and in one system, perhaps, as high as 30 kcal); butene removes energy from dimethylcyclopropane in steps of at least 10–14 kcal; there is an indication that a step or Gaussian distribution fits the data better than a skewed exponential model. There is some evidence that the average step size decreases with increase of ambient temperature.