London Centre for Nanotechnology, Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, United Kingdom.
J Am Chem Soc. 2010 Apr 28;132(16):5735-42. doi: 10.1021/ja909084e.
High-resolution neutron diffraction has been used in conjunction with hydrogen/deuterium isotopic labeling to determine with unprecedented detail the structure of two archetypal aromatic liquids: benzene and toluene. We discover the nature of aromatic pi-pi interactions in the liquid state by constructing for the first time a full six-dimensional spatial and orientational picture of these systems. We find that in each case the nearest neighbor coordination shell contains approximately 12 molecules. Benzene is the more structured of the two liquids, showing, for example, a sharper nearest neighbor coordination peak in the radial distribution function. Superficially the first neighbor shells appear isotropic, but our multidimensional analysis shows that the local orientational order in these liquids is much more complex. At small molecular separations (<5 A) there is a preference for parallel pi-pi contacts in which the molecules are offset to mimic the interlayer structure of graphite. At larger separations (>5 A) the neighboring aromatic rings are predominantly perpendicular, with two H atoms per molecule directed toward the acceptor's pi orbitals. The so-called "anti-hydrogen-bond" configuration, proposed as the global minimum for the benzene dimer, occurs only as a saddle point in our data. The observed liquid structures are therefore fundamentally different than those deduced from the molecular dimer energy surfaces.
高分辨率中子衍射与氢/氘同位素标记相结合，以前所未有的细节确定了两种典型芳香液体：苯和甲苯的结构。我们通过首次构建这些系统的完整六维空间和取向图像，发现了液体状态下芳香π-π相互作用的本质。我们发现，在每种情况下，最近邻配位壳层大约包含 12 个分子。苯是两种液体中结构更复杂的一种，例如，径向分布函数中的最近邻配位峰更尖锐。从表面上看，第一层壳似乎各向同性，但我们的多维分析表明，这些液体中的局部取向序更加复杂。在较小的分子间距(<5 Å)下，存在平行的π-π接触的偏好，其中分子偏移以模拟石墨的层间结构。在较大的间距(>5 Å)下，相邻的芳环主要垂直，每个分子有两个 H 原子指向受体的π轨道。所谓的“反氢键”构型，被提议为苯二聚体的全局最小值，在我们的数据中仅作为鞍点出现。因此，观察到的液体结构与从分子二聚体能量表面推断出的结构根本不同。
