The fate of the tert-butyl radical in low-temperature autoignition reactions

摘要

Alkyl combustion models depend on kinetic parameters derived from reliable experimental or theoretical energetics that are often unavailable for larger species. To this end, we have performed a comprehensive investigation of the tert-butyl radical (R-center dot in this paper) autoignition pathways. CCSD(T)/ANO0 geometries and harmonic vibrational frequencies were obtained for key stationary points for the R-center dot + O-2 and QOOH + O-2 mechanisms. Relative energies were computed to chemical accuracy (+/- 1 kcal mol(-1)) via extrapolation of RCCSD(T) energies to the complete basis-set limit, or usage of RCCSD(T)-F12 methods. At 0 K, the minimum energy R-center dot + O-2 pathway involves direct elimination of HO2 center dot (30.3 kcal mol(-1) barrier) from the tert-butyl peroxy radical (ROO center dot) to give isobutene. This pathway lies well below the competing QOOH-forming intramolecular hydrogen abstraction pathway (36.2 kcal mol(-1) barrier) and ROO center dot dissociation (35.9 kcal mol(-1) barrier). The most favorable decomposition channel for QOOH radicals leads to isobutene oxide (12.0 kcal mol(-1) barrier) over isobutene (18.6 kcal mol(-1) barrier). For the QOOH + O-2 pathways, we studied the transition states and initial products along three pathways: (1) alpha-hydrogen abstraction (42.0 kcal mol(-1) barrier), (2) gamma-hydrogen abstraction (27.0 kcal mol(-1) barrier), and (3) hydrogen transfer to the peroxy moiety (24.4 kcal mol(-1) barrier). The barrier is an extensive modification to the previous 18.7 kcal mol(-1) value and warrants further study. However, it is still likely that the lowest energy QOOH+ O-2 pathway corresponds to pathway (3). We found significant spin contamination and/or multireference character in multiple stationary points, especially for transition states stemming from QOOH. Lastly, we provide evidence for an (A) over tilde-(X) over tilde surface crossing at a C(s-)symmetric, intramolecular hydrogen abstraction structure. Published by AIP Publishing.