Investigating hydrogen-bonded phosphonic acids with proton ultrafast MAS NMR and DFT calculations

Hydrogen-bonding plays a key role in the structure and dynamics of a wide range of materials from small molecules to complex biomolecules. ¹H NMR has emerged as a powerful tool for studying hydrogen-bonding because the proton isotropic chemical shift exhibits a dependence on the interatomic distances associated with the hydrogen bond. In the present work, we illustrate the use of ultrafast magic angle spinning at high magnetic field (800 MHz) for resolving multiple hydrogen-bonding sites in a set of crystalline phosphonic acids that contain various functional groups (-COOH, -PO ₃H ₂, and -NH ₃ ⁺). Trends are observed between the proton chemical shift of the hydrogen-bonded proton and the associated hydrogen-bonding distances (O-H⋯X) from X-ray crystallography. Density functional theory calculations conducted on the phosphonic acid structures illustrate that the experimental proton chemical shift dependence on hydrogen-bond distance agrees with the expected theoretical trends. Further, it is shown that the chemical shift trend varies considerably depending on the functional group participating in the hydrogen bonding, albeit a -COOH, -PO ₃H ₂, or -NH ₃ ⁺ moiety. An improved understanding of these trends for various functional groups should be useful for determining accurate hydrogen-bond strengths from the proton chemical shift in an array of systems.