Department of Chemistry, Imperial College London, UK.
Department of Chemical Engineering, Imperial College London, UK.
Faraday Discuss. 2017 Dec 14;206:183-201. doi: 10.1039/c7fd00155j.
Experimental near edge X-ray absorption fine structure (NEXAFS) spectra, X-ray photoelectron (XP) spectra and Auger electron spectra are reported for sulfur in ionic liquids (ILs) with a range of chemical structures. These values provide experimental measures of the atomic charge in each IL and enable the evaluation of the suitability of NEXAFS spectroscopy and XPS for probing the relative atomic charge of sulfur. In addition, we use Auger electron spectroscopy to show that when XPS binding energies differ by less than 0.5 eV, conclusions on atomic charge should be treated with caution. Our experimental data provides a benchmark for calculations of the atomic charge of sulfur obtained using different methods. Atomic charges were computed for lone ions and ion pairs, both in the gas phase (GP) and in a solvation model (SMD), with a wide range of ion pair conformers considered. Three methods were used to compute the atomic charges: charges from the electrostatic potential using a grid based method (ChelpG), natural bond orbital (NBO) population analysis and Bader's atoms in molecules (AIM) approach. By comparing the experimental and calculated measures of the atomic charge of sulfur, we provide an order for the sulfur atoms, ranging from the most negative to the most positive atomic charge. Furthermore, we show that both ChelpG and NBO are reasonable methods for calculating the atomic charge of sulfur in ILs, based on the agreement with both the XPS and NEXAFS spectroscopy results. However, the atomic charges of sulfur derived from ChelpG are found to display significant, non-physical conformational dependence. Only small differences in individual atomic charge of sulfur were observed between lone ion (GP) and ion pair IL(SMD) model systems, indicating that ion-ion interactions do not strongly influence individual atomic charges.
实验近边 X 射线吸收精细结构（NEXAFS）谱、X 射线光电子（XP）谱和俄歇电子谱报告了一系列具有不同化学结构的离子液体（ILs）中的硫。这些值提供了每个 IL 中原子电荷的实验测量值，并能够评估 NEXAFS 光谱和 XPS 探测硫相对原子电荷的适用性。此外，我们还使用俄歇电子能谱表明，当 XPS 结合能相差小于 0.5 eV 时，应谨慎对待关于原子电荷的结论。我们的实验数据为使用不同方法计算硫的原子电荷提供了基准。计算了孤离子和离子对的原子电荷，包括气相（GP）和溶剂化模型（SMD），考虑了广泛的离子对构象。使用三种方法计算原子电荷：使用基于网格的方法（ChelpG）计算静电势的电荷、自然键轨道（NBO）的布居分析和分子中的原子（AIM）方法。通过比较硫原子的实验和计算测量的原子电荷，我们提供了一个硫原子的顺序，范围从最负到最正的原子电荷。此外，我们还表明，基于与 XPS 和 NEXAFS 光谱结果的一致性，ChelpG 和 NBO 都是计算 ILs 中硫原子电荷的合理方法。然而，从 ChelpG 得到的硫原子电荷显示出显著的、非物理的构象依赖性。在孤离子（GP）和离子对 IL（SMD）模型系统之间，仅观察到硫的单个原子电荷有微小差异，表明离子-离子相互作用不会强烈影响单个原子电荷。
