The propyl + O2 reaction is an important model of chain branching reactions in larger combustion systems. Highly-accurate energetics of the n- and i-propyl + O2 systems were obtained by focal point analyses (FPA) extrapolating to the ab initio limit based on explicit quantum chemical computations with electron correlation treatments through CCSDT(Q) and basis sets up to cc-pV5Z. A mixed Hessian methodology was implemented and benchmarked which makes computations of CCSD(T)/cc-pVTZ vibrational frequencies feasible for large systems and thus provides necessary improvements to the zero-point vibrational energies (ZPVE) for the propyl + O2 systems. The first systematic conformational search of four QOOH intermediates of the propyl + O2 systems were also completed, uncovering a total of 35 rotamers lying within 1.6 kcal mol mol1 of their respective lowest-energy minima. The definitive energetics for stationary points on the propyl + O2 potential energy surfaces provide key benchmarks for future studies of hydrocarbon oxidation. We also present a comprehensive study of the enigmatic hydridotrioxygen (HO3) radical. This species has been probed in numerous gas-phase spectroscopy experiments, in part because it was thought to have a role in the tropospheric HOx cycle. Moreover, HO3 has been the subject of a vast amount of computational research over the past 50 years, which has served to highlight the difficult and unusual molecular structure and energetics of this molecule. For example, the central OO bond length in HO3 has been highly vexing for quantum chemical methods, with reported values ranging all the way from 1.34 to 1.75 ! We have solved the prominent riddles of HO3 using convergent coupled-cluster methods extended all the way to CCSDTQ(P).