An absolute measurement of the rate constant for ethyl radical combination

An absolute value of kr of ethyl radicals at 860 ± 17°K of 4.5 × 109M−1·sec−1 was determined under VLPP conditions, where the value of kr/kr∞ should be about 1/2. Thus kr∞(M−1·sec−1) ∼ 1010 at 860°K. An error of as much as a factor of 2 in kr would be surprising, but possible. The value of 1010M−1·sec−1 seems to be a factor of from 2 to 5 too high to be compatible with extensive data on the reverse reaction and the accepted thermochemistry. Changes in the heat of formation and entropy of the ethyl radical can change the situation somewhat, but even these changes when applied to the work of Hiatt and Benson [3] indicate that ethyl combination should be ∼ 109.3M−1·sec−1. More work is necessary if a better value is desired.

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