Answers are grounded in this paper and cite their sources.
Co-crystals of 1-iodo-3,5-dinitrobenzene and 1,4-diazabicyclo[2.2.2]octane manifest either two strong or one strong and one weak intermolecular 2 : 1 halogen bond (XB) motifs in polymorphic structures I and II, respectively, whereas weaker XB-donor 4,4-bipyridine just forms 1 : 1 structure III with one strong halogen bond.
No takes yet. Share an insight, caveat, or question.
Raatikainen et al. (2009) studied this question.
Synapse has enriched 10 related papers. Consider them for comparative context: