THERMOCHEMICAL AND THEORETICAL-STUDIES OF SOME BIPYRIDINES

The following standard molar enthalpies of formation in the gaseous state at 298.15 K were determined from the enthalpies of combustion of the crystalline solids and the respective enthalpies of sublimation: 2,2'-bipyridine, 267.9 +/- 3.0 kJ mol(-1); 2,4'-bipyridine, 284.2 +/- 2.7 kJ mol(-1); 4,4'-bipyridine, 293.1 +/- 3.6 kJ mol(-1). Ab initio geometry optimizations of these molecules at the 3-21G level suggest that those with at least one ortho nitrogen atom assume near planar conformations preferentially, while the others have nonplanar most stable forms.