THERMOCHEMICAL AND THEORETICAL-STUDY OF SOME QUINOXALINE 1,4-DIOXIDESAND OF PYRAZINE 1,4-DIOXIDE

The following standard molar enthalpies of formation in the gaseous st ate at 298.15 K were determined from the enthalpies of combustion of t he crystalline solids, and their enthalpies of sublimation and the mea n (N-O) bond dissociation enthalpies were derived. Delta(f)H(m)(0)(g); [D(N-O)] (kJ mol(-1)): quinoxaline 1,4-dioxide, 227.1 +/- 2.4; 255.8 +/- 2.0; 2-methylquinoxaline 1,4-dioxide, 169.9 +/- 7.2; 268.3 +/- 4.9 ; 2-methyl-3-acetylquinoxaline 1,4-dioxide, 33.1 +/- 5.0; 251.6 +/- 4. 2; 2-phenyl-3-benzoylquinoxaline 1,4-dioxide, 355.2 +/- 7.1; 227.3 +/- 5.4; 2-methyl-3-carbomethoxyquinoxaline 1,4-dioxide, -148.7 +/- 3.2; 242.3 +/- 3.9; pyrazine 1,4-dioxide, 186.5 +/- 1.9; 254.0 +/- 2.3; 2-m ethyl-5-pyrazinecarboxylic acid, -213.6 +/- 1.7. Unconstrained geometr y optimizations by ab initio calculations showed the effect of steric hindrance on changes in extended delocalizations and were in accord wi th the trends in the mean bond dissociation enthalpies.