The crystal and molecular structure of tropolone

Tropolone, C₇H₆O₂, crystallizes in space group P2₁/c, with a = 7.135, b = 12.178, c = 7.122 Å, β = 99.63 ° and Z = 4. The crystal structure was solved by the symbolic addition procedure and refined by the least-squares method to give the final R value of 0.054 for the 853 observed reflexions. The molecule is essentially planar and exhibits a slight degree of bond alternation in the seven-membered ring. The hydroxyl group makes a bifurcated hydrogen bond with carbonyl oxygen atoms, of which one branch is intramolecular and the other intermolecular. The latter intermolecular branches form a hydrogen-bonded dimer. These characteristic hydrogen bonds seem to play a role in increasing the contribution from the dipolar ionic forms to the ground state of the tropolone molecule.