Weak Intermolecular CH center dot center dot center dot N Hydrogen Bonding: Determination of (CH)-C-13-N-15 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-15/C-13 spin-echo magicangle spinning (MAS) solid-state nuclear magnetic resonance (NMR) experiments are applied to "view" intermolecular CH center dot center dot center dot N hydrogen bonding in two selectively labeled organic compounds, 4-[N-15] cyano-4'-[C-13(2)] ethynylbiphenyl (1) and [N-15(3),C-13(6)]-2,4,6-triethyny1-1,3,5-triazine (2). The synthesis of 2-N-15(3),C-13(6) is reported here for the first time via a multistep procedure, where the key element is the reaction of [N-15(3)]-2,4,6-trichloro-1,3,5-triazine (5) with [C-13(2)]-[(trimethylsilyeethynyl]zinc chloride (8) to afford its immediate precursor [N-15(3), C-13(6)]-2,4,6-tris-trimethylsilypethynyl]-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 K-2h(CN) (29.0 X 1019 kg m(-2) s(-)2 A(-2) (1) and 27.9 X 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 center dot center dot center dot O hydrogen -bonding interactions in uracil ((KNO)-K-2h: 28.1 and 36.8 X 10(19) kg m(-2) s(-2)A(-2)). Moreover, the GIPAW calculations show a clear correlation between increasing zhicN (and 3hkN) coupling and reducing C(H)center dot center dot center dot N and H center dot center dot center dot-N hydrogen-bonding distances, with the Fermi contact term accounting for at least 98% of the isotropic (2h)J(CN), coupling.