Zhang Jian, Zhou Bin, Sun Zhen-Rong, Wang Xue-Bin
State Key Laboratory of Precision Spectroscopy, East China Normal University, Shanghai 200062, China.
Phys Chem Chem Phys. 2015 Feb 7;17(5):3131-41. doi: 10.1039/c4cp04687k. Epub 2014 Dec 17.
Proposed in theory and then their existence confirmed, anion-π interactions have been recognized as new and important non-covalent binding forces. Despite extensive theoretical studies, numerous crystal structural identifications, and a plethora of solution phase investigations, anion-π interaction strengths that are free from complications of condensed-phase environments have not been directly measured in the gas phase. Herein we present a joint photoelectron spectroscopic and theoretical study on this subject, in which tetraoxacalix[2]arene[2]triazine 1, an electron-deficient and cavity self-tunable macrocyclic, was used as a charge-neutral molecular host to probe its interactions with a series of anions with distinctly different shapes and charge states (spherical halides Cl(-), Br(-), I(-), linear thiocyanate SCN(-), trigonal planar nitrate NO3(-), pyramidic iodate IO3(-), and tetrahedral sulfate SO4(2-)). The binding energies of the resultant gaseous 1 : 1 complexes (1·Cl(-), 1·Br(-), 1·I(-), 1·SCN(-), 1·NO3(-), 1·IO3(-) and 1·SO4(2-)) were directly measured experimentally, exhibiting substantial non-covalent interactions with pronounced anion-specific effects. The binding strengths of Cl(-), NO3(-), IO3(-) with 1 are found to be strongest among all singly charged anions, amounting to ca. 30 kcal mol(-1), but only about 40% of that between 1 and SO4(2-). Quantum chemical calculations reveal that all the anions reside in the center of the cavity of 1 with an anion-π binding motif in the complexes' optimized structures, where 1 is seen to be able to self-regulate its cavity structure to accommodate anions of different geometries and three-dimensional shapes. Electron density surface and charge distribution analyses further support anion-π binding formation. The calculated binding energies of the anions and 1 nicely reproduce the experimentally estimated electron binding energy increase. This work illustrates that size-selective photoelectron spectroscopy combined with theoretical calculations represents a powerful technique to probe anion-π interactions and has potential to provide quantitative guest-host molecular binding strengths and unravel fundamental insights in specific anion recognitions.
阴离子-π相互作用在理论上被提出，随后其存在得到证实，现已被公认为一种新的重要非共价结合力。尽管有大量的理论研究、众多的晶体结构鉴定以及大量的溶液相研究，但不受凝聚相环境复杂性影响的阴离子-π相互作用强度尚未在气相中直接测量。在此，我们展示了一项关于该主题的光电子能谱和理论联合研究，其中四氧杂杯[2]芳烃[2]三嗪1，一种缺电子且具有空腔自调节功能的大环化合物，被用作电荷中性分子主体，以探测其与一系列具有明显不同形状和电荷状态的阴离子（球形卤化物Cl⁻、Br⁻、I⁻，线性硫氰酸盐SCN⁻，三角平面硝酸盐NO₃⁻，金字塔形碘酸盐IO₃⁻，以及四面体硫酸盐SO₄²⁻）之间的相互作用。通过实验直接测量了所得气态1∶1配合物（1·Cl⁻、1·Br⁻、1·I⁻、1·SCN⁻、1·NO₃⁻、1·IO₃⁻和1·SO₄²⁻）的结合能，显示出显著的非共价相互作用以及明显的阴离子特异性效应。发现Cl⁻、NO₃⁻、IO₃⁻与1的结合强度在所有单电荷阴离子中最强，约为30 kcal mol⁻¹，但仅约为1与SO₄²⁻之间结合强度的40%。量子化学计算表明，在配合物的优化结构中，所有阴离子都位于1的空腔中心，具有阴离子-π结合模式，其中1能够自我调节其空腔结构以容纳不同几何形状和三维形状阴离子。电子密度表面和电荷分布分析进一步支持了阴离子-π结合的形成。计算得到的阴离子与1的结合能很好地重现了实验估计的电子结合能增加情况。这项工作表明，尺寸选择性光电子能谱结合理论计算是探测阴离子-π相互作用的有力技术，有潜力提供定量的客体-主体分子结合强度，并揭示特定阴离子识别中的基本见解。
