Thermochemical and theoretical study of some methyldiazines

Abstract

The standard (p0 = 0.1 MPa) molar enthalpies of formation for the liquid 2,3-dimethylpyrazine and trimethylpyrazine and the crystalline 2,3-dimethylquinoxaline and tetramethylpyrazine were derived from the standard molar enthalpies of combustion, in oxygen, at T = 298.15 K, measured by static-bomb combustion calorimetry. The standard molar enthalpies of vaporization or of sublimation for the same compounds were determined by Calvet microcalorimetry. Ab initio full geometry optimization at the 3-21G and 6-31G* levels were also performed for all the methylpyrazine isomers. MP2/RHF/3-21G//3-21G and DFT energies were also calculated for all the methylpyrazine isomers, thus allowing us to estimate their isodesmic resonance energies.

Thermochemical and Theoretical Studies of Dimethylpyridine-2,6-dicarboxylate and Pyridine-2,3-, Pyridine-2,5-, and Pyridine-2,6-dicarboxylic Acids

“…The study of the thermochemical parameters of heterocyclic molecules, namely, pyridines, 1,2 bipyridines, 3 quinolines, 4 diazines, 5 and benzodiazines, 6 has been a continuing interest in our laboratory. Besides, we have been looking for the energetic effect on the dissociation enthalpy of the (N-O) bond due to substituents on the ring of the nitrogen heterocycle N-oxides.…”

Thermochemical study of chloropyrazines and chloroquinoxalines

“…In previous work, we have studied the thermochemistry of benzodiazines [1], chloropyridines [2], methyldiazines [3], hydroxyquinoxalines [4], ethylpyridines and ethylpyrazines [5]. In the present work, we continue with the study of pyrazine and quinoxaline derivatives as the thermochemical database for aromatic heterocyclic compounds is still too scarce.…”

“…These are compared with the parent compound, pyrazine or quinoxaline, thus trying to show the effects of chlorination on the ring geometry. This effect is also studied on the energetic ÀD c H m (cr, l)/(kJ Á mol À1 ) D g cr;l H m /(kJ Á mol À1 ) 2-Chloropyrazine, 1 2149.4 AE 1.0 45.1 AE 1.5 2,6-Dichloropyrazine, cr (2, 1981.6 AE 1.7 69.9 AE 2.0 2,3-Dichloroquinoxaline, cr (2, 3878.1 AE 1.3 91.8 AE 1.1 2,3,6,7-Tetrachloroquinoxaline, cr (2,3,6, 3564.1 AE 1.3 105.9 AE 2.2 point of view. For that we have calculated the energy of two different types of isodesmic reactions involving the choropyrazines and quinoxalines and, with the known experimental values of the enthalpy of formation for auxiliary compounds, estimates of the standard molar enthalpies of formation in the gas phase were obtained.…”

Thermochemical study of cyanopyrazines: Experimental and theoretical approaches

“…We have performed the experimental study of cyanopyrazine [7] and in the present work we report the standard molar enthalpy of formation of 2,3-dicyanopyrazine (2, in the gaseous phase, obtained from measurements of the combustion energy using a static bomb calorimeter and from the enthalpy of sublimation of the compound measured by microcalorimetry Calvet. Experimental determinations for tri-and tetracyanopyrazine were precluded due to the impossibility to obtain pure samples of the compounds in the needed quantity.…”

“…The thermochemical study of pyrazine derivatives has been a continuing interest of researchers in our laboratory, in order to establish relationships among the structural and energetical properties in this kind of compounds [1][2][3][4][5][6][7]. It is our aim with this paper to enlarge the thermochemical database as well as to predict theoretically thermochemical parameters for other compounds whose experimental study is impossible due to difficulties in their synthesis and/or in obtaining them pure in the needed quantity.…”