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

Abstract

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.

Figure 1.

Molecular structures of (a) 1, (b) 2, (c) 3, and (d) 4.

Figure 2.

¹H MAS NMR spectra of (a) 1, (b) 2, (c) 3, and (d) 4. All spectra were obtained at 16.4 T with a sample spinning frequency of 30 kHz. In each case, a total of 16 transients were collected with a recycle delay of 60 s.

Figure 3.

Experimental (black trace) and simulated (red trace) ¹⁷O MAS NMR spectra of (a) 1, (b) 2, (c) 3, and (d) 4. For easy comparison, sub-spectra (green, purple, turquoise, and brown traces) from individual sites are also shown. The spectra shown in (a) and (d) were obtained at 18.8 T whereas those in (b) and (c) were acquired at 21.1 T. The sample spinning frequencies were (a) 17.00, (b) 31.25, (c) 22.00, and (d) 16.00 kHz, respectively. Other acquisition parameters are: (a) 4096 transients, 0.5 s recycle delay; (b) 3072 transients, 30 s recycle delay; (c) 3072 transients, 30 s recycle delay; (d) 4096 transients, 1 s recycle delay.

Figure 4.

Left panel: Experimental 2D ¹⁷O 3QMAS NMR spectra of (a) 2, (b) 3, and (c) 4. Right panel: Corresponding experimental (black trace) and simulated (red trace) slice spectra. The 2D spectra shown in (a) and (b) were obtained at 18.8 T whereas the 2D spectrum in (c) was acquired at 21.1 T. The sample spinning frequency was 16 kHz in all three cases. The signals marked as * are spinning sidebands. The red line shown in each 2D spectrum corresponds to the “chemical shift axis” where the f₁/f₂ slope is 1.

Figure 5.

Partial crystal structures of (a) 1, (b) 2, (c) 3, and (d) 4 to illustrate the CBHB networks in these compounds. Color coding for atoms: H (white), C (grey), N (light blue), O (red). Relevant HB lengths are shown.

Figure 6.

Illustration of the “¹H/¹⁷O chemical shift waves” observed in (a) 1, (b) 2, (c) 3, and (d) 4. The horizontal axis corresponds to the atomic positions of H (blue) and O (red) atoms along the CBHB network for each compound.

Scheme 1.

Several CBHB networks postulated by Huggins for carboxylic acids in the solid state.

Scheme 2.

Three examples of CBHB networks found in proteins: (a) 1k7c, (b) 4kxw, (c) 5mop.