Modig Kristofer, Halle Bertil
Department of Biophysical Chemistry, Lund University, Box 124, SE-22100 Lund, Sweden.
J Am Chem Soc. 2002 Oct 9;124(40):12031-41. doi: 10.1021/ja026981s.
The nuclear magnetic shielding tensor is a sensitive probe of the local electronic environment, providing information about molecular structure and intermolecular interactions. The magnetic shielding tensor of the water proton has been determined in hexagonal ice, but in liquid water, where the tensor is isotropically averaged by rapid molecular tumbling, only the trace of the tensor has been measured. We report here the first determination of the proton shielding anisotropy in liquid water, which, when combined with chemical shift data, yields the principal shielding components parallel (sigma(parallel)) and perpendicular (sigma(perpendicular)) to the O-H bond. We obtained the shielding anisotropy sigma(parallel)-sigma(perpendicular) by measuring the proton spin relaxation rate as a function of magnetic induction field in a water sample where dipole-dipole couplings are suppressed by H/D isotope dilution. The temperature dependence of the shielding components, determined from 0 to 80 degrees C, reflects vibrational averaging over a distribution of instantaneous hydrogen-bond geometries in the liquid and thus contains unique information about the temperature-dependent structure of liquid water. The temperature dependence of the shielding anisotropy is found to be 4 times stronger than that of the isotropic shielding. We analyze the liquid water shielding components in the light of previous NMR and theoretical results for vapor and ice. We show that a simple two-state model of water structure fails to give a consistent interpretation of the shielding data and we argue that a more detailed analysis is needed that quantitatively relates the shielding components to hydrogen bond geometry.
核磁共振屏蔽张量是局部电子环境的灵敏探针，可提供有关分子结构和分子间相互作用的信息。水分子质子的磁屏蔽张量已在六方冰中测定，但在液态水中，由于分子快速翻滚使张量呈各向同性平均，仅测量了张量的迹。我们在此报告首次测定液态水中质子屏蔽各向异性，将其与化学位移数据结合，可得出与O-H键平行（σ平行）和垂直（σ垂直）的主屏蔽分量。我们通过测量质子自旋弛豫率获得屏蔽各向异性σ平行 - σ垂直，该弛豫率是在一个水样中作为磁感应场的函数进行测量的，在该水样中通过H/D同位素稀释抑制了偶极 - 偶极耦合。从0到80摄氏度确定的屏蔽分量的温度依赖性反映了液态中瞬时氢键几何结构分布上的振动平均，因此包含有关液态水温度依赖性结构的独特信息。发现屏蔽各向异性的温度依赖性比各向同性屏蔽的温度依赖性强4倍。我们根据先前关于蒸汽和冰的核磁共振及理论结果分析液态水的屏蔽分量。我们表明，简单的水结构二态模型无法对屏蔽数据给出一致的解释，我们认为需要进行更详细的分析，将屏蔽分量与氢键几何结构进行定量关联。
