Negative-Ion Photoelectron Spectroscopy, Gas-Phase Acidity, and Thermochemistry of the Peroxyl Radicals CH3OO and CH3CH2OO

Methyl, methyl-d₃, and ethyl hydroperoxide anions (CH₃OO^-, CD₃OO^-, and CH₃CH₂OO^-) have been prepared by deprotonation of their respective hydroperoxides in a stream of helium buffer gas. Photodetachment with 364 nm (3.408 eV) radiation was used to measure the adiabatic electron affinities: EA[CH₃OO, X̃ ²A″]=1.161±0.005 eV, EA[CD₃OO, X̃ ²A″]=1.154±0.004 eV, and EA[CH₃CH₂OO, X̃ ²A″]=1.186±0.004 eV. The photoelectron spectra yield values for the term energies: ΔE(X̃ ²A″-Ã ²A′)-[CH₃OO]=0.914±0.005 eV, ΔE(X̃ ²A″-Ã ²A′)[CD₃OO]=0.913±0.004 eV, and ΔE(X̃ ²A″-Ã ²A′)-[CH₃CH₂OO]=0.938±0.004 eV. A localized RO-O stretching mode was observed near 1100 cm^-1for the ground state of all three radicals, and low-frequency R-O-O bending modes are also reported. Proton-transfer kinetics of the hydroperoxides have been measured in a tandem flowing afterglow-selected ion flow tube (FA-SIFT) to determine the gas-phase acidity of the parent hydroperoxides: Δ_acidG₂₉₈(CH₃OOH)=367.6±0.7 kcal mol^-1, Δ_acidG₂₉₈(CD₃OOH)=367.9±0.9 kcal mol^-1, and Δ_acidG₂₉₈(CH₃CH ₂OOH)=363.9±2.0 kcal mol^-1. From these acidities we have derived the enthalpies of deprotonation: Δ_acidH₂₉₈(CH₃OOH)=374.6±1.0 kcal mol^-1, Δ_acidH₂₉₈(CD₃OOH)=374.9 1.1 kcal mol^-1, and Δ_acidH₂₉₈(CH₃CH ₂OOH)=371.0±2.2 kcal mol^-1. Use of the negative-ion acidity/EA cycle provides the ROO-H bond enthalpies: DH₂₉₈(CH₃OO-H)=87.8±1.0 kcal mol^-1, DH₂₉₈(CD₃OO-H)=87.9±1.1 kcal mol^-1, and DH₂₉₈(CH₃-CH₂OO-H)=84.8±2.2 kcal mol^-1. We review the thermochemistry of the peroxyl radicals, CH₃OO and CH₃CH₂OO. Using experimental bond enthalpies, DH₂₉₈(ROO-H), and CBS/APNO ab initio electronic structure calculations for the energies of the corresponding hydroperoxides, we derive the heats of formation of the peroxyl radicals. The "electron affinity/acidity/CBS" cycle yields Δ_fH₂₉₈[CH₃OO]=4.8±1.2 kcal mol^-1 and Δ_fH₂₉₈[CH₃CH ₂OO]=-6.8±2.3 kcal mol^-1.