Dimorphism and structural modulation in quinoxaline

The Z' = 1 and Z' = 5 structures of quinoxaline are compared. The nature of the intermolecular interactions in the Z' = 5 structure is studied by means of variable-temperature single-crystal X-ray diffraction. The C-H center dot center dot center dot N and pi ... pi it interactions in these structures are of a stabilizing nature. The high Z' structure has the better interactions, whereas the low Z' structure has the better stability. This trade-off is a recurrent theme in molecular crystals and is a manifestation of the distinction between thermodynamically and kinetically favoured crystal forms.