An NMR Spectroscopic and Quantum Chemical Investigation of Hydrogen Bonding in Solids
Solid-state NMR spectroscopy is used to investigate strong hydrogen bonds in a variety of solids. NMR measurements of the 2H nuclear quadrupole coupling (CQ) and nuclear magnetic shielding tensors are performed on samples of trimethylammonium chloride (TMAC), protonated 1,8-bis(dimethylamino)napthalene (DMANH+), and potassium and sodium bifluoride. 2H CPMAS is used to obtain high quality spectra while reducing experimental time. From spectral simulations, values of 127, 36, 59 and 58 kHz are determined for the 2H CQ of TMAC, DMANH+ CF3SO3-, NaHF2 and KHF2, respectively. The 2H CPMAS spectrum of TMAC shows a minor secondary component resulting from a solid phase in which the trimethylammonium cation is experiencing precessional motion. At high temperature the 2H CPMAS spectrum of DMANH+ shows unexpected spinning sideband lineshapes because of residual dipolar coupling to 14N. The experimental 2H CQ values are corroborated by ab-initio and DFT calculations; for DMAN and the bifluorides the 2H CQ values are averaged over the potential energy surface to improve the computational quality. Large values of the isotropic chemical shift (>10 ppm) are observed for all of the hydrogen-bonded deuterons. To complement the 2H NMR work, other nuclei in the compounds of interest are investigated, for TMAC these include: 35Cl, 37Cl, 1H, 14N, 15N. The 35Cl CQ shows a small, but observable deuterium/proton isotope effect. Quadrupolar and chemical shift parameters for assorted nuclei in TMAC are calculated at various N-H distances, demonstrating the strong dependence of the chlorine and hydrogen parameters on the proton position. For DMANH+ the 15N CPMAS spectrum of a static sample of DMANH+-d1 provides a value for the average dipolar 15N-D coupling constant of 870±30 Hz, corresponding to a distance of 1.29 A. Spectra of the counterions in the bifluoride salts are obtained, providing CQ values of 123 kHz and 1.141 MHz for 39K and 23Na, respectively.