STRUCTURE, CONFORMATION AND HYDROGEN-BONDING OF SOME PYRIDINIUMPROPIONATE COMPLEXES

Crystals of N-(2-carboxyethyl)pyridinium sulphate, [C5H5N+ CH2CH2COOH] (2)SO42-, are orthorhombic, Pna2(1), a = 15.392(1), b = 16.672(1), c = 7.0770(4) Angstrom, Z=. The SO42- anion forms two hydrogen bonds with two COOH groups, the O ... O distances being 2.510(3) and 2.570(3) An gstrom, and the OHO angles 142(5) and 173(4)degrees. The gauche confor mation of the N+(1)-C(7)-C(8)-C(9) linker fragment is stabilized mainl y by Coulombic attractions between the charged nitrogen and oxygen ato ms. Structures of isolated molecules of N-(2-carboxyethyl)pyridinium m onohydrate, N-(2-carboxyethyl)pyridinium chloride, N-(2-carboxyethyl)p yridinium cation, N-(2-carboxyethyl)pyridinium sulphate and N-(2-carbo xyethyl)trimethylammonium bromide optimized using the PM3 and DFT meth ods are significantly different from those observed in the crystals be cause of electrostatic interactions in the crystal lattice. In the cry stal, the positively charged N+ groups interact both with their ''own' ' carboxyl groups (intramolecular compensation of charge) and with nei ghbouring molecules (intermolecular charge compensation), whereas in t he gas phase only intramolecular charge compensation is possible. FTIR spectra show that an additional methylene unit between the charged gr oups increases the symmetry of the COO group and reduces its proton-ac ceptor properties, without affecting the hydrogen bonds in compounds N -(2-carboxyethyl)pyridinium hemihydrobromide, N-(2-carboxyethyl)pyridi nium hemitetrafluoroborate, N-(2-carboxyethyl)pyridinium hemiperchlora te and N-(2-carboxyethyl)pyridinium heminitrate (2:1 homoconjugated co mplexes). (C) 1998 Elsevier Science B.V. All rights reserved.