ELECTROSTATIC INTERACTIONS AND CONFORMATIONS OF ZWITTERIONIC PYRIDINIUM ALKANOATES

Conformations of flexible zwitterionic omega-pyridinium alkanoates (PB n) with n methylene units in the tether and their hydrates and hydroch lorides are studied in the solid state by X-ray diffraction, in aqueou s solution by FT-IR and H-1, C-13, and N-14 NMR spectroscopies, and in the gas phase by PM3, SAM1, and DFT calculations. PB1 and PB1 . H2O i n crystals have a conformation with the N+... O intramolecular distanc e of ca. 2.7 Angstrom, while PB3 . 2H(2)O and PB10 . 3H(2)O have a tra ns-zigzag conformation and are arranged antiparallel. Structures of is olated molecules of omega-pyridinium alkanoates (PBn) and their dihydr ates (PBn . 2H(2)O) and hydrochlorides (PBn . HCl) optimized using the PM3, SAM1, and DFT methods are significantly different from those obs erved in the crystals. In crystals, when n greater than or equal to 2, as a result of electrostatic interactions in the crystal lattice, the positively charged center (N+ atom) interacts with negative carboxyl groups, water molecules, or chloride ions of the neighboring molecules (intermolecular charge compensation), while in the gas phase only wit h their own (intramolecular charge compensation). In aqueous solutions , similarly as in the crystalline state, distances between the charged centers increase monotonically with increasing number of methylene un its in the tether. The H-1 and C-13 NMR data suggest that polymethylen e chains in PBn contain more folded (gauche) conformations than do sod ium salts of carboxylic acids without a charged N+ atom. The SCRF calc ulations predict slightly longer N+... C-c distances than those derive d by Chevalier and Perchec for trimethylammonium carboxylates from C-1 3 NMR spectra. This suggests that the SCRF model underestimates contri bution of the gauche conformers in aqueous solutions.