¹H/¹⁷O Chemical Shift Waves in Carboxyl-Bridged Hydrogen Bond Networks in Organic Solids

We report solid-state ¹H and ¹⁷O NMR results for four ¹⁷O-labeled organic compounds each containing an extensive carboxyl-bridged hydrogen bond (CBHB) network in the crystal lattice: tetrabutylammonium hydrogen di-[¹⁷O₂]salicylate (1), [¹⁷O₄]quinolinic acid (2), [¹⁷O₄]dinicotinic acid (3), and [¹⁷O₂]Gly/[¹⁷O₂]Gly·HCl cocrystal (4). The ¹H isotropic chemical shifts found for protons involved in different CBHB networks are between 8.2 and 20.5 ppm, which reflect very different hydrogen-bonding environments. Similarly, the ¹⁷O isotropic chemical shifts found for the carboxylate oxygen atoms in CBHB networks, spanning a large range between 166 and 341 ppm, are also remarkably sensitive to the hydrogen-bonding environments. We introduced a simple graphical representation in which ¹H and ¹⁷O chemical shifts are displayed along the H and O atomic chains that form the CBHB network. In such a depiction, because wavy patterns are often observed, we refer to these wavy patterns as ¹H/¹⁷O chemical shift waves. Typical patterns of ¹H/¹⁷O chemical shift waves in CBHB networks are discussed. The reported ¹H and ¹⁷O NMR parameters for the CBHB network models examined in this study can serve as benchmarks to aid in spectral interpretation for CBHB networks in proteins.