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

Abstract

Weak hydrogen bonds are increasingly hypothesized 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 nuclear magnetic resonance (NMR) experiments are applied to "view" intermolecular CH···N hydrogen bonding in two selectively labeled organic compounds, 4-[¹⁵N] cyano-4'-[¹³C₂] ethynylbiphenyl (<b>1</b>) and [¹⁵N₃,¹³C₆]-2,4,6-triethynyl-1,3,5-triazine (<b>2</b>). The synthesis of <b>2-</b>^<b>15</b><b>N</b>_<b>3</b>,^<b>13</b><b>C</b>_<b>6</b> 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 (<b>5</b>) with [¹³C₂]-[(trimethylsilyl)ethynyl]zinc chloride (<b>8</b>) to afford its immediate precursor [¹⁵N₃,¹³C₆]-2,4,6-tris[(trimethylsilyl)ethynyl]-1,3,5-triazine (<b>9</b>). Experimentally determined hydrogen-bond-mediated ^2h<i>J</i>_CN couplings (4.7 ± 0.4 Hz (<b>1</b>) and 4.1 ± 0.3 Hz (<b>2</b>)) are compared with density functional theory (DFT) gauge-including projector augmented wave (GIPAW) calculations, whereby species-independent coupling values ^2h<i>K</i>_CN (29.0 × 10¹⁹ kg m^-2 s^-2 A^-2 (<b>1</b>) and 27.9 × 10¹⁹ kg m^-2 s^-2 A^-2 (<b>2</b>)) quantitatively demonstrate the <i>J</i> 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<i>K</i>_NO: 28.1 and 36.8 × 10¹⁹ kg m^-2 s^-2 A^-2). Moreover, the GIPAW calculations show a clear correlation between increasing ^2h<i>J</i>_CN (and ^3h<i>J</i>_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<i>J</i>_CN coupling.