Weak intermolecular CH…N hydrogen bonding : determination of 13CH-15N hydrogen-bond mediated J couplings by solid-state NMR spectroscopy and first-principles calculations

Weak hydrogen bonds are increasingly hypothesised to play key roles in a wide range of chemistry from catalysis to gelation to polymer structure. Here, 15N/13C spin-echo magic-angle spinning (MAS) solid-state NMR experiments are applied to “view” intermolecular CH···N hydrogen bonding in two selectively labelled organic compounds, 4-[15N] cyano-4’-[13C2] ethynylbiphenyl (1) and [15N3,13C6]-2,4,6-triethynyl-1,3,5-triazine (2). The synthesis of 2-15N3,13C6 is reported here for the first time via a multistep procedure, where the key element is the reaction of [15N3]-2,4,6-trichloro-1,3,5-triazine (5) with [13C2]-[(trimethylsilyl)ethynyl]zinc chloride (8) to afford its immediate precursor [15N3,13C6]-2,4,6-tris[(trimethylsilyl)ethynyl]-1,3,5-triazine (9). Experimentally determined hydrogen-bond mediated 2hJCN couplings (4.7 ± 0.4 Hz (1), 4.1 ± 0.3 Hz (2)) are compared with density functional theory (DFT) gauge-including projector augmented wave (GIPAW) calculations, whereby species independent coupling values 2hKCN (28.1 (1), 26.1 (2) × 1019 kg m–2 s–2 A–2) quantitatively demonstrate the J couplings for these CH···N hydrogen bonds to be of a similar magnitude to those for conventionally observed NH···O hydrogen-bonding interactions in uracil (2hKNO: 28.1 and 36.8 × 1019 kg m–2 s–2 A–2).