Effect of pH on the rotational conformations of 1,3‐diamino‐2‐hydroxypropane

The effect of pH on the rotational conformations of 1,3‐diamino‐2‐hydroxypropane in aqueous solution was investigated by proton NMR. Both the observed chemical shifts and coupling constants were used to calculate experimental pK_a values. The observed couplings were correlated with the expected couplings for the various possible staggered conformations to try to determine the pattern of conformations for the diamine and its conjugate acids. The best fits suggested a modest preference for the gauche–gauche conformation, especially at low pH, where the diprotonated hydroxydiamine predominates. In methanol, dimethyl sulfoxide and trichloromethane solutions, it was only possible to evaluate the conformational equilibria of the diamine. Slow proton exchange, which caused uncertainties in both chemical shifts and couplings for the monoprotonated and unprotonated diamine, nullified efforts to determine whether or not hydrogen bonding was important for these species in less polar solvents. Copyright © 2002 John Wiley & Sons, Ltd.