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 t�te-b�che orientation.� Computations at all levels predict that a central barrier to proton transit exists both in the cyclic and in the t�te-b�che structures.

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

The X-ray structure of the cyclic, monoprotonated triflate salt of Me₂N(CH₂)₄NMe_2� displays an internal N-N distance of 2.66�.� The triflate oxygens lie more than 4 � 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)₄NMe_2� 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 �, 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.