The mean dissociation enthalpy of the (N-O) bonds in 2-hydroxyphenazine-di-N-oxide

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

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

The study of the influence of the molecular environment in the vicinityof 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 of 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 ofr heteropolycyclic N-oxide compounds as 2-hydroxyphenazine-di-N-oxide. A previous study [1] for this kind of molecules shows difficulties on the understanding of this 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-hydroxyphenazine-di-N-oxide, 2OHPHDNO, derived by either experimental or computational techniques. From the experimental side, teh 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 stattic bomb combustion calorimetry and Knudsen effusion technique, respectively. Then, the mean dissociation enthalpy of the (N-O) bonds, DH(N-O), in 2OHPHDNO has been calculated as one half of the enthalpy of the following reaction:

2OHPHDNO (g) ----> 2OHPH (g) + 2 O (g)

(2OHPH = 2-hydroxyphenazine)

From the theoretical side, corrected energies for T=298.15 K were obtained at the B3LYP/TZVP 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 2OHPHdNO.

Thanks are due to Fundação para a Ciência e a Tecnologia for financial support to Centro de Investigação em Química and for the project POCTI(44471/QUI/2002). E.A.S. and J.R.B.G. thank the research grants awarded (SFRH/BD/5355/2001) and (SFRH/BPD/11582/2002), respectively.

[1] M. L. P. Leitão, G. Pilcher, W. E. Acree, Jr., A. I. Zvaigzne, S. A. Tucker, M. D. M. C. Ribeiro da Silva, J. Chem. Thermodynamics, 22, 923 (1990).