Collisional Energy Transfer in Thermal Unimolecular Systems. Dilution Effects and Falloff Region

Abstract

To assess further the operational meaning of efficiency, collisional activation‐deactivation efficiencies β for thermal unimolecular reactions in the falloff region have been computed on a stochastic model with use of an iteration procedure. Stepladder and exponential collisional transition probability models were used; detailed balance and completeness were obeyed. Computations were made for the methyl isocyanide, nitryl chloride, and cyclopropane systems for which accurate values of the microscopic specific rate probabilities kE were also obtained. For a particular collisional model, the efficiency was calculated as a function of the average step size, temperature, the order of reaction, and the dilution of substrate by heat bath molecules. For the conventional definition of efficiency, β decreases as the pressure increases. The quasiuniversal efficiency‐step‐size relations found earlier in the second‐order region have been extended. Correlations with several systems studied in the falloff are made. The nature of the dilution effect on β noted previously for the low‐pressure regime [D. C. Tardy and B. S. Rabinovitch, J. Chem. Phys. 45, 3720(1966)] is further amplified, and examined in the falloff region. The resolution of β into population and transition probability factors is also extended. The comparative behavior of the Poisson and exponential factors relative to the stepladder values, when various amounts of energy are transferred on the average, is illustrated. Experimental β values in the low‐pressure region for the nitryl chloride and methyl isocyanide systems are correlated with conclusions derived from the calculations.