The OHN hydrogen bonding in the adduct of 2,4-dinitrobenzoic acid with pyridine. Low temperature X-ray diffraction and IR spectroscopic studies

The crystal structure of the title compound contains a short asymmetric O . .. H-N hydrogen bond with an O ... N distance of 2.561(3)Angstrom at room t emperature and 2.547(3)Angstrom at 100 K. The difference electron density m aps calculated with the proton omitted show that some form of proton disord er between the two positions, one adjacent to the N atom and one adjacent t o the O atom, can not be excluded in this hydrogen bond. The IR spectra tak en in solid state and in various polar solvents show broad bands over the f requency range 400-1800 cm(-1) with two windows at 620 and 1000 cm(-1). No protonic absorption above 2000 cm(-1) is observed, which is characteristic of quasi-symmetric hydrogen bonds. The geometry of the carboxylic group and the position of the v(C=O) band indicates the strong engagement of the car boxylate group in OHN hydrogen bonding. It increases by going from solid st ate to solutions in less polar solvents. (C) 2000 Elsevier Science B.V. All rights reserved.