THE MEAN DISSOCIATION ENTHALPY OF THE (N-O) BONDS IN 2,3-DIMETHYLQUINOXALINE 1,4-DIOXIDE

(a)Maria D. M. C. Ribeiro da Silva, (a)Jorge M. Gonçalves, (a)Emanuel A. Sousa, (a)J. R. B. Gomes, (b)W. E. Acree Jr.

(a)Centro de Investigação em Química, Department of Chemistry, Faculty of Science, University of Porto, R. Campo Alegre, 687,P-4169-007 Porto, Portugal

(b)Department of Chemistry, University of North Texas, Denton, TX 76203-5070, U. S. A.

  The study of the influence of the molecular environment in the vicinity of the N-O bond in different classes of compounds has been one of our main interests during the last ten years. Indeed, the dissociation enthalpy of the N-O bond, DH(N-O), shows large dependency on the immediate vicinity of the bond and we have already reported values for DH(N-O) for several classes of compounds, in order to compile them in a simple reactivity scale. Data for compounds involving some powerful oxidizing agents are scarce, particularly for highly strained N-oxide compounds as quinoxaline 1,4-dioxide derivatives. A previous study 1 for this kind of molecules shows difficulties on the understanding of their behaviour, which led us to undertake further research in order to assure the interpretation of the trends of DH(N-O) for that class of compounds.

  This communication presents new thermochemical data for the 2,3-dimethylquinoxaline 1,4-dioxide compound, DMQXDO, derived by either experimental or computational techniques. From the experimental side, the gas-phase standard enthalpy of formation was obtained by combining the value of the standard molar enthalpy in the crystalline phase and the enthalpy of sublimation measured by static bomb combustion calorimetry [2] and Knudsen effusion technique, respectively. Then, the mean dissociation enthalpy of the (N-O) bonds, (N-O), in DMQXDO has been calculated as one half of the enthalpy of the following reaction:

DMQXDO (g) ---> DMQX (g) + O (g)

  From the theoretical side, corrected energies for T=298.15 K were obtained at the B3LYP/6-31G(d) level of theory and combined with selected isodesmic reactions to estimate both the gas-phase enthalpy of formation and the dissociation enthalpy of N-O bonds in DMQXDO.

Thanks are due to Fundação para a Ciência e Tecnologia for financial support to Centro de Investigação em Química and for the research grants awarded to E.A.S. (SFRH/BD/5355/2001) and to J.R.B.G. (SFRH/BPD/11582/2002).

[1] W.E. Acree Jr., J. R. Powell, S. A. Tucker, M. D. M. C. Ribeiro da Silva, M. A. R. Matos, J. M. Gonçalves, L. M. N. B. F. Santos, V. M. F. Morais, G. Pilcher, J. Organic Chem., 62, 3722 (1997).

[2] Maria D. M. C. Ribeiro da Silva, Emanuel A. Sousa, W. E. Acree, Jr., Book of Abstracts of the 17th IUPAC Conference on Chemical Thermodynamics, Rostock, Germany, 28th July-2nd August (2002).