Quantifying weak hydrogen bonding in uracil and 4-cyano-4′-ethynylbiphenyl:: A combined computational and experimental investigation of NMR chemical shifts in the solid state

Weak hydrogen bonding in uracil and 4-cyano-4'-ethynylbiphenyl, for which single-crystal diffraction structures reveal close CH center dot center dot center dot O=C and C-CH center dot center dot center dot N=C distances, is investigated in a study that combines the experimental determination of H-1, C-13, and N-15 chemical shifts by magic-angle spinning (MAS) solid-state NMR with first-principles calculations using plane-wave basis sets. An optimized synthetic route, including the isolation and characterization of intermediates, to 4-cyano-4'-ethynylbiphenyl at natural abundance and with C-13=(CH)-C-13 and N-15=C labeling is described. The difference in chemical shifts calculated, on the one hand, for the full crystal structure and, on the other hand, for an isolated molecule depends on both intermolecular hydrogen bonding interactions and aromatic ring current effects. In this study, the two effects are separated computationally by, first, determining the difference in chemical shift between that calculated for a plane (uracil) or an isolated chain (4-cyano-4'-ethynylbiphenyl) and that calculated for an isolated molecule and by, second, calculating intraplane or intrachain nucleus-independent chemical shifts that quantify the ring current effects caused by neighboring molecules. For uracil, isolated molecule to plane changes in the 1H chemical shift of 2.0 and 2.2 ppm are determined for the CH protons involved in CH center dot center dot center dot O weak hydrogen bonding; this compares to changes of 5.1 and 5.4 ppm for the NH protons involved in conventional NH center dot center dot center dot O hydrogen bonding. A comparison of CH bond lengths for geometrically relaxed uracil molecules in the crystal structure and for geometrically relaxed isolated molecules-reveals differences of no more than 0.002 angstrom, which corresponds to changes in the calculated H-1 chemical shifts of at most 0.1 ppm. For the C=CH center dot center dot center dot N=C weak hydrogen bonds in 4-cyano-4'-ethynylbiphenyl, the calculated molecule to chain changes are of similar magnitude but opposite sign for the donor C-13 and acceptor N-15 nuclei. In uracil and 4-cyano-4'-ethynylbiphenyl, the CH hydrogen-bonding donors are sp(2) and sp hybridized, respectively; a comparison of the calculated changes in H-1 chemical shift with those for the spa hybridized CH donors in maltose (Yates et al. J. Am. Chem. Soc. 2005, 127, 10216) reveals no marked dependence on hybridization for weak hydrogen-bonding strength.