Thermodynamic Properties of Quinoxaline-1,4-Dioxide Derivatives:  A Combined Experimental and Computational Study

The mean (N−O) bond dissociation enthalpies were derived for three 2-methyl-3-(<i>R</i>)-quinoxaline 1,4-dioxide (<b>1</b>) derivatives, with R = methyl (<b>1a</b>), ethoxycarbonyl (<b>1b</b>), and benzyl (<b>1c</b>). The standard molar enthalpies of formation in the gaseous state at <i>T</i> = 298.15 K for the three <b>1</b> 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.