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•H•O complexes and correlation between the O•••O, C-O and N-O distances and ΔpKa

Complex of 4-dimethylamino-2,6-dimethylpyridine N-oxide with 4-toluenesulphonic 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, space 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-oxide and oxygen atoms of the SO3 group show strongly elongated ellipsoids. The average O ... O distance is 2.539(10) Angstrom and the O(4)-H ... O(1) angle is 168(8)degrees. The HF/G-31G(d,p) method predicts hydrogen bonded ion pair with slightly longer hydrogen bond than that in the crystal. The FTIR spectrum of the complex shows a broad band at ca. 2360 cm(-1) caused by the OH stretching vibration. Relations between geometrical parameters of the OHO bridge, based on the results of X-ray diffraction studied for 93 hydrogen bonded complexes of N-oxides with phenols and carboxylic acids, acids salts 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 Elsevier Science B.V. All rights reserved.