KINETICS OF THE UNIMOLECULAR DECOMPOSITION OF ISO-C3H7 - WEAK COLLISION EFFECTS IN HELIUM, ARGON, AND NITROGEN

Rate constants for the unimolecular decomposition Of iso-C3H7 have bee n determined by laser flash photolysis coupled with photoionization ma ss spectrometry, over the temperature range 720-910 K. The reaction wa s studied in He, at densities of (3-30) x 10(16) atoms cm-3. More limi ted measurements were made for Ar and N2. The reaction is in the fallo ff region under all conditions studied. Three methods of data analysis were employed: (i) A transition state model was constructed by refere nce to literature values of dissociation and association limiting high -pressure rate constants over the temperature range 177-910 K. The mod el gives k1infinity = 6.51 X 10(7)T1.83 exp(-17793/T) s-1 and k1infini ty = 9.47 X 10(-15)T1.16 exp(-440/T) cm3 molecule-1 s-1 for dissociati on and association, respectively. The model was incorporated into a mo dified strong collision model and the data fitted using [DELTAE]down a s 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 det ermine both k1infinity with S(T) and employing k1(0) as the variable p arameter, is consistent with [DELTAE]down = 200 cm-1 for He. (iii) Fin ally, the microcanonical rate constants for dissociation were calculat ed by inverse Laplace transformation of the association rate constants of Harris and Pitts and incorporated in a master equation analysis wi th [DELTAE]down and DELTAH0(0) as the variable parameters. The analysi s gives [DELTAE]down = 210 cm-1 for He and DELTAH(f,298)0 (iso-C3H7) = 21.0 kcal mol-1.