A problem that has puzzled scientists for over forty years has been solved by Jack Dunitz and Bernd Schweizer, from ETH-Zurich in Switzerland.  Since its publication in 1964, the crystal structure of the molecule alloxan has posed an apparent problem for crystallographers and crystal engineers.  Alloxan, in the solid state, has a very high melting point, and remarkable stability.  In most solids, these attributes are thought to be the result of hydrogen bonding between the individual molecules in the solid.  However, crystallographic studies of alloxan revealed the lack of hydrogen bonding.  What's more, the molecule itself contains four carbonyl and two imino functional groups, which would be expected to participate readily in hydrogen bonding.

Dunitz and Schweizer looked again at the crystal structure of alloxan, and compared it to those of related molecules, of similar size and shape, but containing different functional groups.  They saw an obvious pattern, in that these molecules all pack in the solid state in helical chains, with a very similar arrangement between pairs of neighbouring molecules in these chains.  By looking at the weak bonding between each different type of neighbouring pair, Dunitz and Schweizer explain the packing forces in the solid state, and show that, as they say, 'there should be no problem about crystal structures unless we attempt to explain them in term of inappropriate terms borrowed from molecular bonding concepts'.