Heat of Formation of the Hydroperoxyl Radical HOO Via Negative Ion Studies

Abstract

We present a determination of ΔfH298(HOO) based upon a negative ion thermodynamic cycle. The photoelectron spectra of HOO- and DOO- were used to measure the molecular electron affinities (EAs). In a separate experiment, a tandem flowing afterglow-selected ion flow tube (FA-SIFT) was used to measure the forward and reverse rate constants for HOO- + HC≡CH ⇄ HOOH + HC≡C- at 298 K, which gave a value for ΔacidH298(HOO−H). The experiments yield the following values: EA(HOO) = 1.078 ± 0.006 eV; T0(X̃ HOO − Ã HOO) = 0.872 ± 0.007 eV; EA(DOO) = 1.077 ± 0.005 eV; T0(X̃ DOO − Ã DOO) = 0.874 ± 0.007 eV; ΔacidG298(HOO−H) = 369.5 ± 0.4 kcal mol-1; and ΔacidH298(HOO−H) = 376.5 ± 0.4 kcal mol-1. The acidity/EA thermochemical cycle yields values for the bond enthalpies of DH298(HOO−H) = 87.8 ± 0.5 kcal mol-1 and D0(HOO−H) = 86.6 ± 0.5 kcal mol-1. We recommend the following values for the heats of formation of the hydroperoxyl radical: ΔfH298(HOO) = 3.2 ± 0.5 kcal mol-1 and ΔfH0(HOO) = 3.9 ± 0.5 kcal mol-1; we recommend that these values supersede those listed in the current NIST−JANAF thermochemical tables.