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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 (Delta(hyd)H degrees (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 (Delta(f)H degrees (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 (Delta(f)H degrees (g) = -73.09 +/- 0.87 kcal mol(-)(1)) was transformed to Delta(f)H degrees (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 Delta(f)H degrees (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.

The standard (p°) 0.1 MPa) molar enthalpies of formation for 2-, 3-, and 4-methoxyphenol and 2,3-, 2,6-, and 3,5-dimethoxyphenol in the gaseous phase were derived from the standard molar enthalpies of combustion, in oxygen, at 298.15 K, measured by static bomb combustion calorimetry, and the standard molar enthalpies of evaporation at 298.15 K, measured by Calvet microcalorimetry: 2-methoxyphenol, -( 246.1 ( 1.9) kJ mol -1 ; 3-methoxyphenol, -(240.4 ( 2.1) kJ mol -1 ; 4-methoxyphenol, -(229.7 ( 1.8) kJ mol -1 ; 2,3-dimethoxyphenol, -(386.0 ( 2.2) kJ mol -1 ; 2,6-dimethoxyphenol, -(381.7 ( 1.9) kJ mol -1 ; 3,5-dimethoxyphenol, -(399.4 ( 3.0) kJ mol -1 . Density functional theory calculations for all the methoxyand dimethoxyphenols and respective phenoxyl radicals and phenoxide anions were performed using extended basis sets, which allowed the estimation of the gas-phase enthalpies of formation for all compounds. The good agreement of the calculated and experimental gas-phase enthalpies of formation for the closed-shell systems gives confidence to the estimates concerning the isomers which were not experimentally studied and to the estimates concerning the radicals and the anions. Substituent effects on the homolytic and heterolytic O-H bond dissociation energies have been analyzed, the results being in good agreement with available experimental data. Detailed analysis of these effects suggests that electronic exchange phenomena between the substituents dominate the effect the substituents have on these systems.