X-ray, FTIR and ab initio HF studies of OHO hydrogen bond in 4-dimethylamino-2,6-dimethylpyridine N-oxide 4-toluenesulphonate. Proton transfer in O center dot H center dot O complexes and correlation between the O center dotcenter dot center dot O, C-O and N-O distances and Delta pK(a)

Complex of 4-dimethylamino-2,6-dimethylpyridine N-oxide with 4-toluenesulph onic acid (4-NMe2-2,6-Me-2-PyOHOTs) was studied by X-ray diffraction, FTIR spectroscopy and ab initio HF calculations. The crystal is monoclinic, spac e group P2(1)/c with a = 8.527(2), b = 26.200(5), c = 7.4910(10) Angstrom, beta = 91.22(3)degrees. The proton is transferred from the acid to the N-ox ide and oxygen atoms of the SO3 group show strongly elongated ellipsoids. T he average O ... O distance is 2.539(10) Angstrom and the O(4)-H ... O(1) a ngle is 168(8)degrees. The HF/G-31G(d,p) method predicts hydrogen bonded io n pair with slightly longer hydrogen bond than that in the crystal. The FTI R spectrum of the complex shows a broad band at ca. 2360 cm(-1) caused by t he OH stretching vibration. Relations between geometrical parameters of the OHO bridge, based on the results of X-ray diffraction studied for 93 hydro gen bonded complexes of N-oxides with phenols and carboxylic acids, acids s alts of carboxylic acids, and basic salts of N-oxides and betaines covering a broad Delta pK(a) range from -11.56 to 5.0, are proposed. (C) 1999 Elsev ier Science B.V. All rights reserved.