Local nuclear dynamics of protons in a molecular water pipe.

Atoms in molecules that are self-assembled exhibit unique local dynamical features. In the presence of self-assembly, the vibrational contributions to thermodynamic potentials, and in consequence, the chemical reactivity, are completely different from their counterparts in bulk materials. Of all the assemblies, water channels or single-file water wires have attracted a lot of attention due to their biological relevance. In our recent work, we focused our attention on the serendipitous discovery that an exact 2:1 ratio of water and 2,2,6,6-tetramethylpiperidine (TMP) self-assembles into a 3D structure that exhibits molecular water channels. Overall, the long tube-like rods mirror the internal structure of the compound and can be described as a pseudo-tetragonal arrangement of TMP molecules that host, in the middle, a channel filled with water molecules, the whole assembly held together by hydrogen bonding interactions. A high degree of local disorder in the water–TMP complex structure and relatively isotropic atomic displacement parameters of hydrogen atoms in water channels let us suggest water transport in the channels via proton transfer with a Grotthuss-type mechanism. Motivated by this, we propose to concurrently employ incident neutron energy-dependent transmission (NT) and NCS on Vesuvio to investigate the interplay between the local structure and dynamics in TMP/H2O and TMP/D2O systems at 10K and 100K.

分子自组装体系中的原子展现出独特的局域动力学特性。在自组装条件下，对热力学势产生贡献的振动行为进而影响化学反应活性，与块体材料中的对应行为完全迥异。在各类组装体中，水通道或单链水线因兼具重要的生物学意义而广受关注。
在我们近期的研究中，我们聚焦于一项意外发现：水与2,2,6,6-四甲基哌啶（2,2,6,6-tetramethylpiperidine, TMP）以严格2:1的比例自组装形成含有分子水通道的三维结构。整体而言，长管状杆状结构与该化合物的内部结构相契合，可被描述为以准四方排列方式排布的TMP分子，其中心区域容纳一条填充水分子的通道，整个组装体通过氢键相互作用得以稳定维系。
水-TMP复合结构存在高度局部无序，且水通道内氢原子的原子位移参数呈现显著各向同性，据此我们推测通道内水分子通过格罗图斯（Grotthuss）型质子转移机制实现输运。受此研究结果启发，我们提出将入射中子能量依赖透射（NT）与Vesuvio装置上的NCS相结合，以探究10K与100K温度下TMP/H₂O及TMP/D₂O体系的局域结构与动力学之间的相互作用。