Decomposition of Chemically Activated Ethyl-d3 Radicals. Primary Intramolecular Kinetic Isotope Effect in a Nonequilibrium System

Abstract

The rate of decomposition of ethyl-d3 radicals formed by chemical activation was studied at 195° and 300°K over the pressure range from 0.05 to 3.0 mm. The excited radicals decomposed by hydrogen or deuterium atom rupture or were collisionally stabilized. The rate of decomposition was determined relative to the rate of stabilization by complete product analysis. Detailed description is given. The limiting high-pressure rate for hydrogen rupture from energized ethyl-d3 radicals was determined to be 6.5×107 sec—1 at 300°K and 2.5×107 sec—1 at 195°K. The limiting low-pressure rate for hydrogen rupture was found to be 2.0×107 sec—1 at 300°K and 1.0×107 sec—1 at 195°K. The intramolecular hydrogen to deuterium rupture ratio was determined to be ∼2.0 at 300°K and ∼3.0 at 195°K for this nonequilibrium reaction system, and appreciably less than similar ratios determined earlier for the ethyl-d2 system. The disproportionation to recombination ratio for ethyl radicals was found to be 0.14 at 300°K and to be 0.17 at 195°K. The apparent isotopic H/D disproportionation ratio in ethyl-d3, measured at both temperatures, was 1.6 in reasonable agreement with the value of 1.4 of Boddy and Steacie. Theoretical rates were calculated from a quantum statistical harmonic oscillator formulation of the rate constant. Good qualitative and semiquantitative agreement has been found between the various experimental quantities of this study (as well as a previous one dealing with ethyl-d2 radicals) and magnitudes calculated from a model which has quite a loose activated complex (150 cm—1, C–C–H bends) with the figure axis rotation active, and in which the energized radical has both an active free internal rotation and figure axis rotation.