Symmetry of O-H-O and N-H-N hydrogen bonds in 6-hydroxy-2-formylfulvene and 6-aminofulvene-2-aldimines.

The symmetry of the hydrogen bonds in 6-hydroxy-2-formylfulvene and two N,N'-diaryl-6-aminofulvene-2-aldimines is probed by the NMR technique of isotopic perturbation. Observed deuterium-induced 13C NMR isotope shifts at several positions can be attributed to a combination of an intrinsic shift and the perturbation of a tautomeric equilibrium. The most dramatic are at the aldehydic or aldiminic carbon signals, where the observed isotope shift for the unlabeled carbon is +376 or +223 ppb. This large downfield shift is contrary to the small upfield shift expected for a four-bond intrinsic shift and can be attributed only to a perturbation shift. Therefore these intramolecular hydrogen bonds are asymmetric, the proton resides in a double-minimum potential surface, and each molecule exists as a pair of rapidly interconverting tautomers, regardless of solvent. The symmetry of the hydrogen bond is not governed only by the O-O or N-N distance. It is proposed that symmetric hydrogen bonds can be observed in crystalline phases but not as yet in solution because the disorder of the solvation environment induces an asymmetry of the hydrogen bond, whereas a crystal can guarantee a symmetric environment. These results provide no insight into the source of the stabilization attributed to low-barrier hydrogen bonds if they lack the special feature of symmetry.