Photoelectron spectroscopy, gas phase acidity, and thermochemistry of <i>tert</i>-butyl hydroperoxide: Mechanisms for the rearrangement of peroxyl radicals

Abstract

The 3.531 eV negative ion photoelectron spectra of the hydroperoxide ion and the tert-butylperoxide ion have been studied. We find HO2−+ℏω351.1 nm→HO2+e− EA[HO2,X̃ 2A″]=1.089±0.006 eV, (CH3)3COO−+ℏω351.1 nm→(CH3)3COO+e− EA[(CH3)3COO,X̃ 2A″]=1.196±0.011 eV. The photoelectron spectra show detachment to the ground state of the peroxyl radicals and to a low lying electronic state. The intercombination gaps are measured to be ΔE(X̃ 2A″–Ã 2A′)[HO2]=0.871±0.007 eV and ΔE(X̃ 2A″–2A′)[(CH3)3COO]=0.967±0.011 eV. The gas phase acidity of (CH3)3COOH was measured in a tandem flowing afterglow-selected ion flow tube (FA-SIFT) to be ΔacidG298=363.2±2.0 kcal mol−1 and we find ΔacidH298[(CH3)3COO–H]=370.9±2.0 kcal mol−1. Use of ΔacidH298[(CH3)3COO–H] and EA[(CH3)3COO] leads to the bond energies DH298[(CH3)3COO–H]=85±2 kcal mol−1 and D0[(CH3)3COO–H]=83±2 kcal mol−1. The thermochemistry of the alkylperoxyl radicals, RO2, is reviewed. A mechanism for the rearrangement of chemically activated peroxyl radicals is proposed [RO2]X̃ 2A″→[RO2]*Ã 2A′→aldehydes/ketones+HO(2Π), [RO2]X̃ 2A″→[RO2]*Ã 2A′ →alkenes+HO2(X̃ 2A″).