Protonation sites on unsymmetrical 1,3-diamines: a nuclear magnetic resonance study of 3-aminotropanes

The proton NMR spectra of the isomeric 3 alpha- and 3 beta-aminotropanes we re recorded in aqueous solution over the pH range 2-14 and the resonances o f both diamines were completely assigned. The pH dependence of the chemical shifts of the protons a to the amino groups was used to determine which of the amino groups is more basic in each isomer. The conformations of the un protonated, monoprotonated and diprotonated species were also deduced from the H-1 NMR spectra. For both 3-aminotropane isomers, the tertiary amino gr oup is more basic than the primary amino group, and the chair form with an equatorial N-methyl group is the minimum energy conformation. Copyright (C) 1999 John Wiley & Sons, Ltd.