Electrostatic interactions and conformations of zwitterionic pyridinium alkanoates

Conformations of flexible zwitterionic omega-pyridinium alkanoates (PBn) with n methylene units in the tether and their hydrates and hydrochlorides are studied in the solid state by X-ray diffraction, in aqueous 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 in crystals have a conformation with the N+... O intramolecular distance of ca. 2.7 Angstrom, while PB3 . 2H(2)O and PB10 . 3H(2)O have a trans-zigzag conformation and are arranged antiparallel. Structures of isolated molecules of omega-pyridinium alkanoates (PBn) and their dihydrates (PBn . 2H(2)O) and hydrochlorides (PBn . HCl) optimized using the PM3, SAM1, and DFT methods are significantly different from those observed 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 with 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 units in the tether. The H-1 and C-13 NMR data suggest that polymethylene chains in PBn contain more folded (gauche) conformations than do sodium salts of carboxylic acids without a charged N+ atom. The SCRF calculations predict slightly longer N+... C-c distances than those derived by Chevalier and Perchec for trimethylammonium carboxylates from C-13 NMR spectra. This suggests that the SCRF model underestimates contribution of the gauche conformers in aqueous solutions.