Hydrogen bridges proton sponge cations

Theoretical calculations of an [H3N H- NH3]+ system have shown that when the N - N distance is 2.75 A, the potential curve has two minima with a barrier of 10.9 klrnol with a decrease in the distance to ca 2.50 A, the barrier disappears. As seen in Table 5, the minimal N N distance for proton sponge cations is 2.54-2.55 A, i.e. none of the compounds has a symmetrical barrier-free H-bond. Indeed, in 80% of cases, the X-ray studies on salts of proton sponge 1 have revealed an asymmetric hydrogen bridge with the distances N—H and N - H in the ranges 0.84-1.27 A and 1.42-1.86 A, respectively. It is believed that such strong variations are due to the influence of the anion, either by an electric field effect or by structural changes in the crystal lattice. 

An interesting situation occurs in the dication of the doubly protonated proton sponge 35 2H+ and in the zwitterion of 4,5-dihydroxy-l,8-bis(dimethylamino)naphthalene (111). The asymmetry of both hydrogen bridges in these systems is clearly controlled by electrostatic factors in order to obtain maximal separation of the two positively charged centres in 35 2H+ and, in reverse, to gain an attraction between the cationic and anionic centres in 111. 

There are very specific changes of the hydrogen bridge geometry in the cations of 2,7-disubstituted proton sponges. The measurements of such systems were performed for five salts with cations 44 H+, 101-II1,102 H+, 105 H+ and 112 H+. The ortfio-substituents in these cations display a distinct steric pressure on the adjacent A-methyl groups, pushing... 

Lopez, C., Claramunt, R.M., Elguero, J., Denisov, G.S., and Limbach, H.H. (2010) Symmetrization of cationic hydrogen bridges of protonated sponges induced by solvent and counteranion interactions as revealed by NMR spectroscopy. 

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