Symmetrical hydrogen bridges, proton

The proton which is removed is a bridge proton as shown by experiments 18> using selectively deuterated hexaborane(lO) 209) and the structure thus derived is consistent with the boron-11 nmr spectrum of thenonahydrohexaborate(—1) ion 18.179) which exhibits two symmetrical doublets in the area ratio of five to one (with the smaller peak at higher field). Furthermore, the reaction of the ion with anhydrous hydrogen chloride generates hexaborane(lO) in nearly quantitative yield 1M79). 

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. 

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. 

Exo-endo additions of organic azides to the norbornene skeleton and other bridged bicyclic systems can be easily differentiated from the NMR spectra of the adducts. The endo protons (exo adducts) of the norbornane do not couple with the bridgehead hydrogens.386 The endo protons in position 3a and 7a of adduct 65 exhibit an AB spectrum. The lack of coupling with the bridgehead protons is a result of a dihedral angle of 82°.25,96 Based on this, the exo form of cycloadduct 66 exhibits symmetrical doublets centered at 8 4.60 and... 

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