Chain initiation of neopentane pyrolysis and a suggested reconciliation of the thermochemically calculated and measured rate constants for the recombination of t-butyl radicals

Abstract

The rates of initiation of neopentane pyrolysis over the temperature range 756–845 K have been measured at reactant pressures in the range 100–200 Torr, by processing of data relating to the formation of the termination product, ethane, using an exact algebraic procedure. Thus, the data are free of the ambiguities introduced by the empirical extrapolation procedures used in previous direct determinations of the initial rates and the derived Arrhenius parameters of neo–C₅H₁₂→ CH₃+ t–C₄H₉. (1) The result obtained is log (k₁/s^–1)= 16.1 ± 1.0 –(330 ± 15 kJ mol^–1)/2.303 RT. Values of k₁ are in agreement with previously measured values but the Arrhenius parameters are inconsistent with the presently accepted thermochemistry of reaction (1). A modified thermochemistry for the t-butyl radical is proposed leaving the heat capacity unchanged but using the new values for 298 K, ΔH_f/kJ mol^–1= 39, S(1 mol cm^–3)/J mol^–1 K^–1= 210.8. These, and the presently derived k₁ values allow the calculation for 800 K of log (k_–1/cm³ mol^–1S^–1)= 12.9 with essentially zero activation energy and hence, via the geometric mean rule, lead to log (k₉/cm³ mol^–1 s^–1)= 11.9 for the mutual recombination reaction, 2t–C₄H₉→ 2,2,3,3-tetramethylbutane. (9)

The new thermochemistry suggested for t-butyl reconciles most thermochemically calculated values of k₉ with directly measured values over a wide temperature range. However, further experimental and theoretical justification is necessary before the new thermochemistry can be unequivocally accepted.

Concurrently evaluated with k₁ is the result log (k₃k₆/k₅/cm³ mol^–1 s^–1)= 13.2 ± 1.5 –(127 ± 12 kJ mol^–1)/2.303 RT CH₃+ neo–C₅H₁₂→ CH₄+ C₅H₁₁(3), CH₃+ i–C₄H₈→ CH₄+ C₄H₇(6), 2CH₃→ C₂H₆(5) which is shown to be in excellent agreement with that calculated from the independent results of other workers.