HYDROGEN-BONDING .61. FT-NMR STUDY OF ACETYLCHOLINE AND TETRAPROPYLAMMONIUM ION - CONFORMATION AND STOICHIOMETRY OF HYDRATION OF TETRAPROPYLAMMONIUM ION IN AQUEOUS-SOLUTION

Comparison of the 300 MHz H-1 FT-NMR spectrum of the tetrapropylammoni um ion with that of acetylcholine, H-1 decoupling experiments, and con sideration of coupling constants, observed rotation of methyl groups, and molecular models demonstrate that the tetrapropylammonium ion in a queous solution assumes an S-4 conformation in which the methyl group and the R(3)N(+) group in each chain are partially trans about the -CH 2-CH2- bond, with an N-C-C-C dihedral angle in the neighborhood of 155 degrees. An all-gauche model which we had previously suggested is rul ed out as modeling shows that severe steric hindrance would prohibit m ethyl rotation. D-2d and Sq models in which the propyl chains are perf ectly staggered in a trans conformation are also ruled out by steric c onsiderations. The development of splitting of the beta-CH2 absorption with dilution, which was previously ascribed to N-14 to beta-CH2 coup ling in a gauche model, is now shown to arise from development of coup ling of gamma-CH3 with beta-CH2. If N-14 to beta-CH2 coupling is prese nt in the tetrapropylammonium ion it is too small to be resolved in th ese spectra, as would be expected for a trans conformation. Since such coupling associated with onset of ion tumbling was not actually obser ved in our previous work, the stoichiometry of hydration of the tetrap ropylammonium ion in solution must be reevaluated.