Headen T F, Cullen P L, Patel R, Taylor A, Skipper N T
ISIS Neutron Facility, STFC Rutherford Appleton Laboratory, Harwell Campus, Didcot, Oxon OX11 0QX, UK.
Phys Chem Chem Phys. 2018 Jan 24;20(4):2704-2715. doi: 10.1039/c7cp06689a.
Total neutron scattering has been used in conjunction with H/D and *N/N isotopic substitution to determine the detailed liquid-state structures of pyridine and naphthalene. Analysis of the data via an empirical potential-based structure refinement method has allowed us to interrogate the full six-dimensional spatial and orientational correlation surfaces in these systems, and thereby to deduce the fundamental effects of a heteroatom and aromatic core-size on intermolecular π-π interactions. We find that the presence of a nitrogen heteroatom, and concomitant dipole moment, in pyridine induces surprisingly subtle departures from the structural correlations observed in liquid benzene: in both cases the most probable local motif is based on perpendicular (edge-to-face) intermolecular contacts, while parallel-displaced configurations give rise to a clear shoulder in the correlation surface. However, the effect of the heteroatom is revealed through detailed analysis of the intermolecular orientational correlations. This analysis shows a tendency for neighbouring pyridine molecules to direct one meta- and one para-hydrogen towards the neighbouring aromatic π-orbitals in edge-to-face configurations, while head-to-tail alignment of adjacent nitrogen atoms is favoured in face-to-face configurations. In contrast to this, increasing aromatic core size from one to only two rings has a clear and profound effect on the π-π interactions and liquid structure. Our experiments show that naphthalene-naphthalene contacts are dominated by parallel-displaced configurations, akin to those found in graphite. This marks a fundamental difference with the structure of liquid benzene, in which perpendicular geometries are favoured. Furthermore, it is remarkable to note that in the systems studied, the most favoured spatio-orientational configurations observed in the liquid state are not predicted from ab initio calculations and/or solid state crystallographic studies. This highlights the need for caution when extrapolating the results of crystallographic and computational studies to aromatic interactions in liquids and disordered systems.
全中子散射已与氢/氘（H/D）和氮/氮（*N/N）同位素取代相结合，用于确定吡啶和萘的详细液态结构。通过基于经验势的结构精修方法对数据进行分析，使我们能够探究这些体系中完整的六维空间和取向相关曲面，从而推断杂原子和芳环核心尺寸对分子间π-π相互作用的基本影响。我们发现，吡啶中氮杂原子的存在及其伴随的偶极矩，导致其与液态苯中观察到的结构相关性出现令人惊讶的细微偏差：在这两种情况下，最可能的局部结构基元均基于垂直（边对面）分子间接触，而平行错位构型在相关曲面中产生明显的肩部。然而，通过对分子间取向相关性的详细分析揭示了杂原子的影响。该分析表明，在边对面构型中，相邻吡啶分子倾向于将一个间位氢和一个对位氢指向相邻的芳环π轨道，而在面对面构型中，相邻氮原子的头对头排列更受青睐。与此形成对比的是，芳环核心尺寸从一个环增加到仅两个环，对π-π相互作用和液体结构有明显而深刻的影响。我们的实验表明，萘-萘接触主要由平行错位构型主导，类似于在石墨中发现的构型。这标志着与液态苯结构的根本差异，液态苯中垂直几何构型更受青睐。此外，值得注意的是，在所研究的体系中，液态下最有利的空间取向构型并非从头算计算和/或固态晶体学研究中预测得到。这突出了在将晶体学和计算研究结果外推至液体和无序体系中的芳香相互作用时需要谨慎的必要性。
