Monoprotonated diamines can crystallize in three general motifs:  salt-bridged, cyclic, or clustered, as illustrated below (where A- symbolizes the negatively charged counterion).  Monoprotonated cis-1,5-cyclooctanediamine crystallizes in a salt-bridged motif, where the 8-membered ring adopts a chair-chair conformation with both amino groups equatorial and intermolecular NH^…N hydrogen bonds in addition to the salt bridge.

Crystallization in the cyclic or clustered motifs requires that a proton bridging between nitrogens prevail over hydrogen bonds between protonated amino groups and the anion.  In linear tertiary diamines, for instance, the observed motif depends on chainlength.  The 1:1 salt between N,N,N',N'-tetramethylputrescine, Me₂N(CH₂)₄NMe₂, and triflic acid forms a cyclic structure, while the corresponding salt of N,N,N',N'-tetramethylcadaverine, Me₂N(CH₂)₅NMe₂,  forms a dimeric cluster in a tęte-bęche orientation.  Computations at all levels predict that a central barrier to proton transit exists both in the cyclic and in the tęte-bęche structures.

The X-ray structure of the tęte-bęche triflate salt of monoprotonated Me₂N(CH₂)₅NMe₂ shows two equal N-N distances of 2.75 Ĺ.  The counterions lie on the opposite sides of the nitrogens, with the triflate oxygens >3.6 Ĺ away from the nitrogens.  Thus, no salt bridging occurs in the tęte-bęche dimer. 

The X-ray structure of the cyclic, monoprotonated triflate salt of Me₂N(CH₂)₄NMe₂  displays an internal N-N distance of 2.66Ĺ.  The triflate oxygens lie more than 4 Ĺ away from the midpoint between the nitrogen atoms, indicating that salt bridging does not occur here, either. 

The position of the bridging hydrogen in monoprotonated Me₂N(CH₂)₄NMe₂  was determined by solid-state NMR of crystals synthesized with increasing levels of deuterium substitution of the methyl and methylene groups.  The chemical shift of the bridging proton is more than 13 ppm downfield from the positions of the CH protons.  The monoprotonated salt of the perdeuterated diamine exhibits a ¹⁵N-¹H dipolar coupling constant at room temperature of 5300 ± 300 Hz, much smaller than would be predicted if the bridging proton were localized on one nitrogen. 

According to theory, the dipolar coupling constant is proportional to the expectation value <1/r³>, where r stands for the N-H bondlength.  If the proton were bouncing between two equilibrium positions (as expected for a double-well potential where the barrier height is on the order to kT above the zero point level), the predicted coupling constant would still be significantly larger than the observed value.  The N-H bondlength corresponding to the measured ¹⁵N-¹H dipolar coupling constant is r = 1.320 ± 0.025 Ĺ, almost exactly half of the N-N distance.  This implies that the most probable position of the proton is midway between the two nitrogens, even though that position corresponds to a potential energy maximum.