THERMOCHEMICAL AND THEORETICAL-STUDY OF SOME METHYLDIAZINES

The standard (p(0) = 0.1 MPa) molar enthalpies of formation for the li quid 2,3-dimethylpyrazine and trimethylpyrazine and the crystalline 2, 3-dimethylquinoxaline and tetramethylpyrazine were derived from the st andard molar enthalpies of combustion, in oxygen, at T = 298.15 K, mea sured by static-bomb combustion calorimetry. The standard molar enthal pies of vaporization or of sublimation for the same compounds were det ermined by Calvet microcalorimetry. Ab initio full geometry optimizati on at the 3-21G and 6-31G levels were also performed for all the meth ylpyrazine isomers. MP2/RHF/3-21G//3-21G and DFT energies were also ca lculated for air the methylpyrazine isomers, thus allowing us to estim ate their isodesmic resonance energies.