High temperature deviations from Rice-Ramsberger-Kassel-Marcus unimolecular rate coefficients expressions

• Published in: The Journal of chemical physics 1989-07, Vol.91 (1), p.205-210
• Main Authors: OREF, I, TARDY, D. C
• Format: Article
• Language: English
• Subjects: Chemistry; Exact sciences and technology; General and physical chemistry; Theory of reactions, general kinetics. Catalysis. Nomenclature, chemical documentation, computer chemistry
• ISSN: 0021-9606; 1089-7690
• DOI: 10.1063/1.457506

The closed form of the solution to the master equation for unimolecular decomposition is investigated. Exact numerical values of the Rice–Ramsberger–Kassel–Marcus unimolecular rate coefficient are calculated for the decomposition of cyclobutene and the isomerization of quadricyclane. A comparison between the approximate strong collision low temperature expression now in general use and the high temperature strong collision exact expression shows that in some cases there are large deviations (∼40%) between the values calculated from the two expressions. The usage of the exact expression causes a shift of the falloff curves toward the high pressure limit. The larger the molecule, the larger the deviation; the higher the value of E0 the smaller the deviation. For E0/RT>20 the approximate expression is sufficient. These deviations may be quite important in practical systems such as combustion and atmospheric chemistry and shock tubes.