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Weak hydrogen bonds are increasingly hypothesized to play key roles in a wide range of chemistry from catalysis to gelation to polymer structure. Here, 15 N/ 13 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-[ 15 N] cyano-4'-[ 13 C 2 ] ethynylbiphenyl ( 1 ) and [ 15 N 3 , 13 C 6 ]-2,4,6-triethynyl-1,3,5-triazine ( 2 ). The synthesis of 2-   15   N   3  ,  13   C   6  is reported here for the first time via a multistep procedure, where the key element is the reaction of [ 15 N 3 ]-2,4,6-trichloro-1,3,5-triazine ( 5 ) with [ 13 C 2 ]-[(trimethylsilyl)ethynyl]zinc chloride ( 8 ) to afford its immediate precursor [ 15 N 3 , 13 C 6 ]-2,4,6-tris[(trimethylsilyl)ethynyl]-1,3,5-triazine ( 9 ). Experimentally determined hydrogen-bond-mediated 2h  J  CN couplings (4.7 ± 0.4 Hz ( 1 ) and 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 2h  K  CN (29.0 × 10 19 kg m -2 s -2 A -2 ( 1 ) and 27.9 × 10 19 kg m -2 s -2 A -2 ( 2 )) quantitatively demonstrate the J 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 ( 2h  K  NO : 28.1 and 36.8 × 10 19 kg m -2 s -2 A -2 ). Moreover, the GIPAW calculations show a clear correlation between increasing 2h  J  CN (and 3h  J  CN ) 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 2h  J  CN coupling.

the journal of physical chemistry a

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