Many VV-alkylpyridinium cations undergo collisionally activated dissociation in the gas phase to form pyridine and

Conformations of flexible zwitterionic ω-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 1 H, 13 C, and 14 N NMR spectroscopies, and in the gas phase by PM3, SAM1, and DFT calculations. PB1 and PB1‚H 2 O in crystals have a conformation with the N + ‚‚‚O intramolecular distance of ca. 2.7 Å, while PB3‚2H 2 O and PB10‚3H 2 O have a transzigzag conformation and are arranged antiparallel. Structures of isolated molecules of ω-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 g 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 1 H and 13 C 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 13 C NMR spectra. This suggests that the SCRF model underestimates contribution of the gauche conformers in aqueous solutions.

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The crystal structure of the ti!le compound has been determined by X-ray analysis. The intramolecular hydrogen bond lengths are 2.606(?) A for the (NHN)' bridge in protonated 1,8-bis(dimethyIamino)naphthalene cation (DMAN'H) and 2.401(4) A for the (OH0)-bridge in the hydrogen maleate anion (HM-). The H-bonds are asymmetrical and not strictly linear: NHN, 157(3)" and OHO, 170(5)". The geometries of the N-H. . .N and O-Ha . -0 bridges of the investigated cation and anion are dominated by the spherical repulsions of their constituent atoms.The overlapping bands in the absorbance IR spectra of potassium hydrogen (deuteron) maleate are separated in the second-derivative spectra. The strong mixing of the in-plane modes with skeletal modes in the hydrogen maleate ion causes a larger separation (Av w 115 cm-') of the v(C=O) bands in comparison with those in other type A acid salts containing intermolecular hydrogen bonds (Av z 20-35 cm-'). The observed lack of solvent effect on the IR absorption suggests that the hydrogen bonds in tetrabutylammonium hydrogen maleate and 1,8bis(dimethy1amino)naphthalene hydrogen maleate are not extremely polarizable. 'H and ' 3C NMR chemical shifts of the investigated compound were measured and identified in two-dimensional ( 20) experiments. The ' H NMR spectra show two narrow signals at ca. 19.5 and 18.7 ppm due to the OH0 and NHN protons, respectively. The structural parameters of the cation and anion were also determined by quantum-mechanical calculations with the semiempirical MNDO-PM3 method.

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1H and 13C NMR chemical shifts of a series of 1‐substituted pyridinium salts were measured in dimethyl sulfoxide. The effect of the quaternary pyridinium ring on the resonance of the methylene group of the substituent is reported. Increments of δ1H = 3.89 ± 0.16 ppm and δ13C = 47.5 ± 3.7 ppm and are estimated for replacing YCH2H with YCH2Py+. The effects of 1‐substituents on the pyridinium ring chemical shifts are discussed.