Kinetics of the Unimolecular Decomposition of iso-C3H7: Weak Collision Effects in Helium, Argon, and Nitrogen

Article abstract of DOI:10.1021/j100119a032

Rate constants for the unimolecular decomposition of iso-C3H7 have been determined by laser flash photolysis coupled with photoionization mass spectrometry, over the temperature range 720-910 K.The reaction was studied in He, at densities of (3-30) x 10¹⁶ atoms cm^-3.More limited measurements were made for Ar and N2.The reaction is in the falloff region under all conditions studied.Three methods of data analysis were employed: (i) A transition state model was constructed by reference to literature values of dissociation and association limiting high-pressure rate constants over the temperature range 177-910 K.The model gives k₁^infinite = 6.51 x 10⁷T^1.83 exp(-17793/T) s^-1 and k₁^infinite = 9.47 x 10^-15T^1.16 exp(-440/T) cm³ molecule^-1 s^-1 for dissociation and association, respectively.The model was incorporated into a modified strong collision model and the data fitted using <ΔE>_down as a variable parameter, giving 136 cm^-1 (He), 130 cm^-1 (Ar) and 129 cm^-1 (N2). (ii) A Troe analysis, using the transition-state model to determine both k₁^infinite with S_T and employing k₁⁰ as the variable parameter, is consistent with <ΔE>_down = 200 cm^-1 for He. (iii) Finally, the microcanonical rate constants for dissociation were calculated by inverse Laplace transformation of the association rate constants of Harris and Pitts and incorporated in a master equation analysis with <ΔE>_down and ΔH₀⁰ as the variable parameters.The analysis gives <ΔE>_down = 210 cm^-1 for He and ΔH_f,298⁰ (iso-C3H7) = 21.0 kcal mol^-1.