Unimolecular reactions at high temperatures and pressures. Experimental and computational studies

Single-pulse shock tube measurements for four unimolecular reactions at elevated temperatures are reported at several pressures. Contrary to expectations based on extrapolation from the lower temperature falloff curves, the rate law is not first order. A simple theoretical approach based on a statistical (prior) approximation for inelastic collisions is used to interpret the experimental results. It is shown that at elevated temperatures most of the molecules which have enough energy to dissociate will not be deactivated in a single collision. Indeed, a substantial minority will be further activated. The border line between the lower and higher temperatures is determined by the ratio E₀/(s - 1)RT where E₀ is the dissociation energy and s the number of vibrational modes.