Unimolecular Decomposition and Intramolecular Energy Relaxation in the Suprahigh-Pressure Region

The unimolecular rate constant ka for decomposition of chemically activated sec-butyl radicals was measured at pressures 55–203 atm of hydrogen. The reaction enters its high-pressure region above 0.01 atm. This study extends the range of earlier work. Butyl radicals were produced by mercury photosensitization of hydrogen: cis-2-butene mixtures. The value of ka was obtained from the ratio of stabilization (S) to decomposition (D) according to the equation ka = βωD / S, where β represents the relative collisional efficiency of hydrogen. The average value of ka was found to be 1.78(± 0.2) × 107 sec−1. No systematic change occurred in the value of ka with increase of pressure; this is taken as support for the continued validity of the random lifetime assumption at average intervals between collision of 2 × 10−13 sec, at least in this case. The disporportionation-recombination ratio of sec-butyl radicals was measured as 0.57 in agreement with earlier values from this laboratory.