Gas-Phase Properties and Reactivity of the Acetate Radical Anion. Determination of the C-H Bond Strengths in Acetic Acid and Acetate Ion

Abstract

The acetate radical anion, CH[sub 2]CO[sub 2] [sup [center dot]-], has been generated in the gas phase at room temperature and its thermochemical properties and reactivity have been examined with use of a flowing afterglow-triple quadrupole instrument. This ion is formed in high yield from the reaction between F[sub 2] and the enolate ions of either acetic acid or trimethylsilyl acetate. Collision-induced dissociation (CID) of CH[sub 2]CO[sub 2][sup [center dot]-] occurs by loss of CO[sub 2] forming CH[sub 2][sup [center dot]-] with a measured threshold energy of 60.9 [+-] 2.7 kcal/mol. The (oxygen) proton affinity of CH[sub 2]CO[sub 2][sup [center dot]-] ([Delta]H[sub acid[sub -]] [[sub [center dot]]CH[sub 2]CO[sub 2]-H]) has been determined to be 347.0 [+-] 1.1 kcal/mol from measurements of the relative yields of the carboxylate ion fragments resulting from CID of proton-bound dimer ions formed by termolecular association of CH[sub 2]CO[sub 2][sup [center dot]-] with carboxylic acids with known gas-phase acidities (i.e., by the Cooks kinetic method). This result indicates that removal of a hydrogen atom from the [alpha]-carbon of acetic acid ([Delta]H[sub acid](CH[sub 3]CO[sub 2]H) = 348.6 [+-] 2.9 kcal/mol) increases the acidity by 1.6 kcal/mol. These data are used to derive the 298 K heat ofmore » formation for acetate radical anion and the C-H bond dissociation energies. 38 refs., 5 figs., 2 tabs.« less