The energetics of electron transfer from an extensive series of alkyl-substituted benzenes are measured both in solution and in the gas phase. The standard oxidation potentials stem from the reversible cyclic voltammograms (CV) in trifluoroacetic acid using the recently developed microvoltammetric electrodes. These values show an excellent correlation with the vertical ionization potentials 7p of the same aromatic hydrocarbons in the gas phase. Thermochemical analysis indicates that the slope of less than unity for the correlation arises mainly from solvation differences, particularly in the highly substituted polyalkylbenzenes. The values of £Ar°a lso correlate extremely well with the anodic peak potentials £p of the irreversible cyclic voltammograms, which can be readily measured in acetonitrile at a standard sweep rate. A slope of unity for the linear correlation is discussed in terms of the maximum error in the CV peak shift (£_ -£°) resulting from the electrochemical kinetics of the EC mechanism for arene oxidation. The correlation of E^0 and £p affords a prediction of the standard oxidation potential of benzene, which otherwise could not be measured directly. The same series of alkylbenzenes form stable x-complexes with chromium tricarbonyl that show reversible electrochemical behavior. The standard oxidation potentials £ArCr°o f the complexes also correlate very well with E^f of the free arene. The diminished slope of the correlation arises from the nonbonding character of the HOMO.The predicted value of E^0 for benzene from this relationship is in reasonable agreement with that obtained from the irreversible peak potentials.

The mean (N-O) bond dissociation enthalpies were derived for three 2-methyl-3-(R)-quinoxaline 1,4-dioxide (1) derivatives, with R ) methyl (1a), ethoxycarbonyl (1b), and benzyl (1c). The standard molar enthalpies of formation in the gaseous state at T ) 298.15 K for the three 1 derivatives were determined from the enthalpies of combustion of the crystalline solids and their enthalpies of sublimation. In parallel, accurate density functional theory-based calculations were carried out in order to estimate the gas-phase enthalpies of formation for the corresponding quinoxaline derivatives. Also, theoretical calculations were used to obtain the first and second N-O dissociation enthalpies. These dissociation enthalpies are in excellent agreement with the experimental results herewith reported.

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The standard enthalpy of formation and the enthalpy of sublimation of crystalline 2-hydroxyphenazine-di-N-oxide, at T ) 298.15 K, were determined from isoperibol static bomb combustion calorimetry and from Knudsen effusion experiments, as -76.7 ( 4.2 kJ‚mol -1 and 197 ( 5 kJ‚mol -1 , respectively. The sum of these two quantities gives the standard enthalpy of formation in the gas-phase for this compound, ∆ f H°m(g) ) 120 ( 6 kJ‚mol -1 . This value was combined with the gas-phase standard enthalpy of formation for 2-hydroxyphenazine retrieved from a group estimative method yielding the mean (N-O) bond dissociation enthalpy, in the gas-phase, for 2-hydroxyphenazine-di-N-oxide. The result obtained with this strategy is 〈DH°m (N -O)〉 ) 263 ( 4 kJ‚mol -1 , which is in excellent agreement with the B3LYP/6-311+G(2d,2p)// B3LYP/6-31G(d) computed value, 265 kJ‚mol -1 .