THE QUASI-SYMMETRICAL OHN AND ODN BRIDGES IN THE ADDUCTS OF 3,5-DIMETHYLPYRIDINE WITH 3,5-DINITROBENZOIC ACID

The crystal structure of the adduct of 3,5-dimethylpyridine and 3,5-di nitrobenzoic acid (DMP-DNB) has been determined at room temperature an d 80 K for both undeuterated and deuterated compounds, The monoclinic crystals are isomorphous, space group P2(1)/c and Z = 4. Very strong O HN hydrogen bonds are almost linear with fully disordered (1:1) bridge hydrogen atoms between oxygen and nitrogen atoms. This is well reflec ted in the difference in electron density maps the contours of which d epend both on cooling and deuteration, The intramolecular hydrogen bon d lengths are 2.550(2) Angstrom for the (OHN) and 2.563(2) Angstrom fo r (ODN) at room temperature and 2.529(2) Angstrom for (OHN) and 2.531( 2) Angstrom for (ODN) at 80 K. Therefore, there is a small but meaning ful isotope effect upon the O ... N hydrogen bridge length at room tem perature and no Ubbelohde isotope effect is observed at 80 K. The infr a-red spectra show very broad stretching protonic bands in the 200-160 0 cm(-1) range. The isotopic ratio upsilon(H)/upsilon(D) at room tempe rature is about 1.1. (C) 1998 Elsevier Science B.V.