Weak Intermolecular CH···N Hydrogen Bonding: Determination of 13 CH- 15 N Hydrogen-Bond Mediated J Couplings by Solid-State NMR Spectroscopy and First-Principles Calculations

Weak hydrogen bonds are increasingly hypothesized to play key roles in a wide range of chemistry from catalysis to gelation to polymer structure. Here, N/ C spin-echo magic-angle spinning (MAS) solid-state nuclear magnetic resonance (NMR) experiments are applied to "view" intermolecular CH···N hydrogen bonding in two selectively labeled organic compounds, 4-[ N] cyano-4'-[ C ] ethynylbiphenyl ( ) and [ N , C ]-2,4,6-triethynyl-1,3,5-triazine ( ). The synthesis of , is reported here for the first time via a multistep procedure, where the key element is the reaction of [ N ]-2,4,6-trichloro-1,3,5-triazine ( ) with [ C ]-[(trimethylsilyl)ethynyl]zinc chloride ( ) to afford its immediate precursor [ N , C ]-2,4,6-tris[(trimethylsilyl)ethynyl]-1,3,5-triazine ( ). Experimentally determined hydrogen-bond-mediated couplings (4.7 ± 0.4 Hz ( ) and 4.1 ± 0.3 Hz ( )) are compared with density functional theory (DFT) gauge-including projector augmented wave (GIPAW) calculations, whereby species-independent coupling values (29.0 × 10 kg m s A ( ) and 27.9 × 10 kg m s A ( )) quantitatively demonstrate the couplings for these "weak" CH···N hydrogen bonds to be of a similar magnitude to those for conventionally observed NH···O hydrogen-bonding interactions in uracil ( : 28.1 and 36.8 × 10 kg m s A ). Moreover, the GIPAW calculations show a clear correlation between increasing (and ) coupling and reducing C(H)···N and H···N hydrogen-bonding distances, with the Fermi contact term accounting for at least 98% of the isotropic coupling.