The specificity of the [NHN]+ hydrogen bonds in protonated naphthalene proton sponges

A short review of data related to the structure and infra-red and 1H NMR spectra of protonated naphthalene proton sponges (DMAN-s) is presented. Protonated DMAN-s are characterized by a very low frequency of ν(NHN)+ stretching vibrations, ca. 500 cm−1, with an unusual 1H/2H isotope effect opposite to that commonly observed, reaching values above 2. The 2,7 derivatives showing the buttressing effect are analyzed. The shortest bridge was found for the 2,7-di-Si(CH3)3 derivative, which is close to the shortest bridge in protonated 1,6-diazabicyclo[4.4.4]tetradecane, for which a single minimum potential for proton motion was reported. However, in protonated DAMN-s a double minimum with a very low barrier (LBHB) is always present.