Mechanism of proton transfer in the strong OHN intermolecular hydrogen bond

The structure of the complex of 3,5-dinitrobenzoaic acid with 3,5-dimethylpyridine was studied by neutron diffraction at 330, 300, 270, 240, 210, 180, 150, 120, 90, 60, and 30 K. The O center dot center dot center dot H bond length gradually changes from 1.403(10) angstrom at 300 K to 1.424(4) angstrom at 30 K. The proton shifts in the hydrogen bridge towards the acceptor nitrogen atom. Temperature-dependent changes in the strong OHN hydrogen bond are used to discuss the proton transfer mechanism.