Hydrogen bonding. Part 61. FT-NMR study of acetylcholine and tetrapropylammonium ion; conformation and stoichiometry of hydration of tetrapropylammonium ion in aqueous solution

Abstract

Comparison of the 300 MHz 1H FT-NMR spectrum of the tetrapropylammonium ion with that of acetylcholine, 1H decoupling experiments, and consideration of coupling constants, observed rotation of methyl groups, and molecular models demonstrate that the tetrapropylammonium ion in aqueous solution assumes an S 4 conformation in which the methyl group and the R 3N + group in each chain are partially trans about the CH 2CH 2 bond, with an NCCC dihedral angle in the neighborhood of 155°. An all-gauche model which we had previously suggested is ruled out as modeling shows that severe steric hindrance would prohibit methyl rotation. D 2d and S 4 models in which the propyl chains are perfectly staggered in a trans conformation are also ruled out by steric considerations. The development of splitting of the β-CH 2 absorption with dilution, which was previously ascribed to 14N to β-CH 2 coupling in a gauche model, is now shown to arise from development of coupling of γ-CH 3 with β-CH 2. If 14N to β-CH 2 coupling is present in the tetrapropylammonium ion it is too small to be resolved in these spectra, as would be expected for a trans conformation. Since such coupling associated with onset of ion tumbling was not actually observed in our previous work, the stoichiometry of hydration of the tetrapropylammonium ion in solution must be reevaluated.