The enthalpies of formation of o-, m-, and p-benzoquinone: Gas-phase ion energetics, combustion calorimetry, and quantum chemical computations combined

Radical anions of o-, m-, and p-benzoquinone were produced in a Fourier transform mass spectrometer by low energy electron attachment or collision-induced dissociation and were differentiated. Classical derivatization experiments also were carried out to authenticate the ortho and meta anions. Gas-phase techniques were used to measure the proton affinities of all three radical anions and the electron affinities of o- and m-benzoquinone. By combining these results in thermodynamic cycles, we derived heats of hydrogenation of o-, m-, and p-benzoquinone (Δ_hydH°(1o, 1m, and 1p) = 42.8 ± 4.1, 74.8 ± 4.1, and 38.5 ± 3.0 kcal mol^-1, respectively) and their heats of formation (Δ_fH°(1o, 1m, and 1p) = -23.1 ± 4.1, 6.8 ± 4.1, and -27.7 ± 3.0 kcal mol^-1, respectively). Good accord with the literature value for the para derivative was obtained. Combustion calorimetry and heats of sublimation also were measured for benzil and 3,5-di-tert-butyl-o-benzoquinone. The former heat of formation agreed with previous determinations, while the latter result (Δ_fH°(g) = -73.09 ± 0.87 kcal mol^-1) was transformed to Δ_fH°(1o) = -18.9 ± 2.2 kcal mol^-1 by removing the effect of the tert-butyl groups via isodesmic reactions. This led to a final value of Δ_fH°(1o) = -21.0 ± 3.1 kcal mol^-1. Additivity was found to work well for m-benzoquinone, but BDE1 and BDE2 for 1,2- and 1,4-dihydroxybenzene differed by a remarkably small 14.1 ± 4.2 and 23.5 ± 3.7 kcal mol^-1, respectively, indicating that o- and p-benzoquinone should be excellent radical traps.