N-H bond stretching in histidine complexes: A solid-state NMR study

N-H bond lengths in imidazolium-carboxylate pairs were studied as models for enzyme active site motifs. The bond lengths were measured using '2D 2φ-DipShift,' a solid-state NMR method wherein the N,H dipolar coupling is determined using heteronuclear dipolar spinning sideband patterns. Like the situation for compressed O-H⋯O systems, some complexes exhibit very short N⋯O distances and weaker N,H dipolar coupling. The weaker time-average N,H dipolar coupling for systems with short N⋯O hydrogen bonds can be most likely associated with an altered potential well for the proton, or a 'stretched' covalent bond, although other interpretations involving a double well potential are discussed. The range or variation in average bond lengths seen in this study is much narrower than that previously reported for O-H⋯O systems (1.01-1.07 Å for N-H vs 1.0-1.3 Å for O-H bonds).