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, −PO3H2, and −NH3+). 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,
−PO3H2, or −NH3+ 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.

氢键（Hydrogen-bonding）在从小分子到复杂生物分子的各类材料的结构与动力学过程中均发挥关键作用。氢核磁共振（1H NMR）已成为研究氢键的有力工具，这是因为质子各向同性化学位移与氢键相关的原子间距离存在依赖关系。在本研究中，我们展示了在高磁场（800兆赫兹）下采用超高速魔角旋转（ultrafast magic angle spinning）技术，可分辨一系列带有不同官能团（functional groups）（羧基−COOH、二氢膦酸基团−PO3H2以及铵根基团−NH3+）的结晶膦酸中的多个氢键位点。研究发现，通过X射线晶体学（X-ray crystallography）测得的O–H···X型氢键的相关距离，与氢键结合质子的质子化学位移之间存在一定变化趋势。针对上述结晶膦酸结构开展的密度泛函理论（Density Functional Theory）计算表明，实验观测到的质子化学位移随氢键距离的变化规律与理论预期趋势相符。进一步研究显示，尽管参与氢键作用的官能团为羧基、二氢膦酸基团或铵根基团，化学位移的变化趋势仍会因所涉及的官能团不同而产生显著差异。加深对各类官能团相关变化趋势的理解，将有助于在多种体系中通过质子化学位移准确测定氢键强度。