Vaporization, Sublimation Enthalpy, and Crystal Structures of Imidazo[1,2-a]pyrazine and Phthalazine

The vaporization enthalpy of imidazo[1,2- a ]pyrazine is evaluated by correlation gas chromatography using two sets of standards and by transpiration. Vaporization enthalpies were evaluated using heterocyclic standards with dipole moments that vary from 0 to 3.0 D and using standards with dipole moments > 3.0 D. A value of (70.7 ± 1.0) kJ·mol –1  measured by transpiration compares to (67.9 ± 1.4) kJ·mol –1  measured by correlation gas chromatography using standards characterized by dipole moments > 3.0 D. The results suggest that imidazo[1,2- a ]pyrazine is a member of this series of compounds whose vaporization enthalpies exceed those less polar heterocycles by a fairly constant amount, (6.9 ± 0.3) kJ·mol –1 , in this case by (5.9 ± 3.0) kJ·mol –1  using the less polar heterocycles. The crystal structures of phthalazine, another member of the polar series, and of imidazo[1,2- a ]pyrazine were determined to examine whether π–π stacking, present in the solid state of two other members of the polar series, was a characteristic of this series. If present in the liquid, π–π stacking could offer a possible explanation for the vaporization enthalpy differences observed. The sublimation enthalpy of imidazo[1,2- a ]pyrazine was also evaluated.