AZAAROMATIC CHLORIDES - A PRESCRIPTION FOR CRYSTAL-STRUCTURES WITH EXTENSIVE NITROGEN-CHLORINE DONOR-ACCEPTOR INTERACTIONS

The crystal structures of cyanuric chloride (1) 2,4,6,8-tetrachloropyr imido[5,4-d]pyrimidine (2), 9-methyl-2,6,8-trichloropurine (3), and 2, 4,6,7-tetrachloropteridine (4) display extensive networks of intermole cular nitrogen-chlorine donor-acceptor interactions. Compounds 1-3 for m layered structures in which two-dimensional N...Cl linked arrays are separated by approximately 3.3 angstrom, while crystals of compound 4 exhibit a herringbone structure interwoven with N...Cl contacts. The peripheral N=C-Cl subunits in these molecules appear to be particularl y prone to form such interactions in the solid state. All of the nitro gen and chlorine atoms in compounds 1 and 2 participate in N...Cl inte ractions (thus each molecule of 1 forms six N...Cl contacts and each m olecule of 2 forms eight), while the N...Cl arrays in 3 and 4 arc less complete. Ab initio calculations suggest that each of these weak bond s may be worth as much as 1.2 kcal/mol.