Quinoxaline derivatives play an important role in the development of new therapeutic agents, due to their relevant antibacterial, antiviral or antifungal activities. The oxidation of one or both nitrogen atoms of the heterocyclic ring increases the diversity of their biological properties, since their potential to act as oxidizing agents leads to a large variety of therapeutic applications, as for example the hypoxia-selective activity evidenced by some quinoxaline di-N-oxides.
Over the last decade, experimental and theoretical energetic studies have been expanded to several quinoxaline 1,4-di-N-oxides, in order to evaluate the influence of the chemical vicinity on the dissociation enthalpies of the N-O bonds. However, experimental studies for quinoxaline 1-N-oxide derivatives have not been described so far, mainly due to the difficulty in obtaining convenient amounts of highly pure samples of those compounds.
The present work reports the first experimental thermochemical study of a mono-N-oxide quinoxaline, the methyl 2-methylquinoxaline-3-carboxylate 1-N-oxide. The values of the enthalpy of formation, in the condensed state, and of the enthalpy of sublimation, measured by static bomb calorimetry and Calvet microcalorimetry, respectively, are combined to derive the standard molar enthalpy of dissociation of the N-O bond, in the gaseous state, for this compound. The experimental results for the compounds in the gas-phase are also compared with computational results obtained at the DFT level of theory.

Thanks are due to FCT and to FEDER for financial support to the research project (POCTI / 44471 / QUI / 2002). J.R.B.G. and A.R.M. thank FCT for the award of their research scholarships. Thanks are also due to the University of Porto for funding through the programme "Investigação Científica na Pré-graduação" and supported by Caixa Geral de Depósitos.

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1. Computational Study on the Thermochemistry of β-Diketones and Their Radicals: O-H vs C-H Homolytic Bond Cleavage