Diamines acidity/basicity, hydrogen bonding

A further indication that the hydrogen bond in protonated 1,6-diaza-bicyclo[4.4.4]tetradecane [49] is strong is provided by the exceptionally low acidity the pA -value is ca. 25 (Alder, 1989). In some of the other diamines, for which inside-protonated forms could not be obtained by conventional proton-transfer reactions, it was not known whether this was due to a low rate of protonation or to low basicity of the amines towards inside protonation. The estimated pX -value for [49] at least makes it clear... 

Room et al. measured the basicities (in terms of the pK values of the conjugate acids HB+ of the bases B) of alkanediamines in gas phase, acetonitrile (AN), and THF, and computed gas-phase basicities for a number of monoamines and diamines using B3LYP/6-311-I-G calculations [505] They examined the intrinsic acidities of the compounds with attention to internal hydrogen bonding and steric strains. In THF, because of its low dielectric constant, they regarded the ions HB+ and A to be fully ion-paired. For the most part the pK, s in THF were intermediate to those in AN and H2O for triethylamine, for example, the pKj,s were 18.82 (AN)... 

---