2ΔH(D) and 1ΔN(D) isotope effects on nuclear shielding of ammonium ions in complexes with crown ethers and cryptands

One-bond deuterium isotope effects on nitrogen nuclear shielding, 1ΔN(D) and two-bond isotope effects at 1H nuclear shielding, 2ΔH(D), have been investigated in a series of inclusion complexes. The hosts comprise, SC-24, [2.2.2], [2.2.1], [2.2.1] D, [2.2] DD, K5, 18-crown-6 and 18-crown-6 tetracarboxylic acid. The structure of the host [2.2.1] is discussed based on isotope effects and 1H chemical shifts, and an interesting exchange reaction is observed between CDCl3 and ammonium ions in the [2.2.1] complex. The counterion dependence is shown to be zero for ammonium ions fully included in cages such as SC-24 and [2.2.2] and for 18-C-6(COOH)4. For cryptands and podands such as [2.2.1] and K5 a weak counterion dependence of 2ΔH(D) opposite to that found for ammonium ions in water is observed, while 1ΔN(D) and 15N chemical shifts depend very strongly on the counterions, as also found for ammonium ions in water solution. The anilinium ion shows effects similar to those observed for the ammonium ion. 1ΔN(D) isotope effects correlate well with δN and 2ΔH(D) correlate with δNH. The correlation between 1ΔN(D) and 2ΔH(D) is different for ammonium ions in water and ions included in crowns, cryptands or podands. This shows that different mechanisms are operating. The 2ΔH(D) isotope effects and δNH chemical shifts depend on the distance to the nearest acceptor (oxygen or nitrogen), and they also depend on the type of acceptor. 2ΔH(D) isotope effects are found to be a good gauge of N ... N or N ... O distances of the inclusion complexes. The 2ΔH(D) isotope effects vary with temperature, but the dramatic changes in the 1H NMR spectra of the host are not reflected in the isotope effects. The one-bond couplings, 1J(N,H), correlate with 2ΔH(D). The variations are much larger for the cryptands than found in water solutions. (au) (50 refs.)