Structure and dynamics studies of the short strong hydrogen bond in the 3,5-dinitrobenzoic acid–nicotinic acid molecular complex

The molecular complex between 3,5-dinitrobenzoic acid and nicotinic acid (35DBNA) has been studied by variable temperature single crystal X-ray and neutron diffraction (30 to 300 K) and ab initio molecular dynamics, in order to investigate the dynamics and any proton migration in this system, which exhibits structural similarities with the well-known proton migration material 3,5-dicarboxylic acid. The refined structures clearly indicate a significant degree of proton transfer in the short NHO hydrogen bond, contrary to the previous description of 35DBNA as an organic adduct without proton transfer. This behaviour is consistent with the difference between the pKa values of 3,5-dinitrobenzoic acid and the ring nitrogen atom in nicotinic acid. Complementary ab initio MD simulations at 400 K show the key proton hopping across the NHO short hydrogen bond, spending short periods along the trajectory (8% of the simulation time) bonded to the O atom. Similar simulations performed on 3,5-dicarboxylic acid and 3,4-dicarboxylic acid show that the MD calculations correlate well with the experimental observations (or absence) of proton migration, and therefore suggest that they could be used as a predictive tool for investigating this phenomenon in short strong hydrogen bonds.