Unusually for a carboxylic acid, santonic acid does not form hydrogen-bondeddimers in the crystalline phase. Rather, it adopts a polymeric structure, with individual santonic acid molecules linked by intermolecular carboxyl-to-ketone hydrogen bonds.
^ abcA. P. J. Brunskill, H. W. Thompson and R. A. Lalancette (April 1999). "Santonic acid: catemeric hydrogen bonding in a γ,ε-diketo carboxylic acid". Acta Cryst.C55 (4): 566–568. doi:10.1107/S0108270198014231.