Stability and cooperativity of hydrogen bonds in dihydroxybenzoic acids.

Multiple temperature single crystal neutron diffraction studies of a new polymorphic form of 2,4-dihydroxybenzoic acid, and of the isomer 2,5-dihydroxybenzoic acid, are presented, together with static and dynamic solid-state density functional theory calculations. The studies present an unambiguous analysis of the potential for cooperativity and hydrogen atom disorder in the hydrogen bond network of both materials. The neutron diffraction experiments establish clearly that there is no disorder, and hence that no hydrogen bond cooperativity is present in either system at temperatures up to 150 K. The findings are supported by DFT calculations, from which lattice and hydrogen bond configuration energies are also obtained, together with computed vibrational spectra.