Accurate ab initio predictions of ionization energies, electron affinities, and heats of formation for the peroxyl radicals XOO (X = H, CH₃ and CH₃CH₂)

The ionization energies (IEs), electron affinities (EAs), and heats of formation (HOFs) for the peroxyl radicals HOO, CH₃OO, and CH₃CH₂OO have been calculated by the wave-function-based ab initio CCSD(T)/CBS approach together with various energy corrections, including the zero-point vibrational energy (ZPVE), core-valence (CV), scalar relativistic (SR), diagonal Born-Oppenheimer corrections (DBOC), spin-orbitials corrections (SOC) and high-order correction (HOC) effects. The comparison between the theoretical predictions and the highly precise experiment EA values for the peroxyl radicals show that the CCSD(T)/CBS method is capable of providing an accurate EA prediction for the peroxyl radicals with a uncertainty of +/-10 meV. The HOC effects make significant contribution for improving the accuracy of the predicted EAs. The CCSD(T)/CBS IE(HOO) value of 11.353 eV obtained in the present study is in very good agreement with the experimental result of 11.352 +/- 0.007 eV. However, the present predictions of IE(CH₃OO) = 11.271 eV is 0.079 eV lower than the experimental value of 11.35 +/- 0.05 eV. For the CH₃CH₂OO radical, our recommendation is IE(CH₃CH₂OO) = 9.599 eV. Furthermore, our work provides reliable prediction for the 0 K and 298 K HOFs of the three radicals and their cations and anions. The CCSD(T)/CBS estimations are found to be in excellent good agreement with available experiment determination and previous theoretical calculations. Our best theoretical predictions for the 298 K HOF of CH₃OO and C2H₅OO are 3.84 and -4.70 kcal mol^-1, respectively. The present study shows that the CCSD(T)/CBS approach with high-level energy corrections can be extended to provide reliable IE, EA, and HOF predictions for the peroxyl radicals.