Weak, Broken, but Working-Intramolecular Hydrogen Bond in 2,2′-bipyridine

From an academic and practical point of view, it is desirable to be able to assess the possibility of the proton exchange of a given molecular system just by knowing the positions of the proton acceptor and the proton donor. This study addresses the difference between intramolecular hydrogen bonds in 2,2 & PRIME;-bipyridinium and 1,10-phenanthrolinium. Solid-state N-15 NMR and model calculations show that these hydrogen bonds are weak; their energies are 25 kJ/mol and 15 kJ/mol, respectively. Neither these hydrogen bonds nor N-H stretches can be responsible for the fast reversible proton transfer observed for 2,2 & PRIME;-bipyridinium in a polar solvent down to 115 K. This process must have been caused by an external force, which was a fluctuating electric field present in the solution. However, these hydrogen bonds are the grain that tips the scales precisely because they are an integral part of a large system of interactions, including both intramolecular interactions and environmental influence.

Automated summary

From an academic and practical point of view, it is desirable to be able to assess the possibility of proton exchange in a molecular system based on the positions of the proton acceptor and donor. This study investigates the differences in intramolecular hydrogen bonds in 2,2'-bipyridinium and 1,10-phenanthrolinium. Experimental and computational evidence demonstrates that these hydrogen bonds are weak, with energies of 25 kJ/mol and 15 kJ/mol, respectively. The observed fast reversible proton transfer in 2,2'-bipyridinium cannot be attributed to these hydrogen bonds or N-H stretches, but rather to an external force of fluctuating electric fields present in the solution.