Blake Anastasia V, Wei Haochuan, Donahue Courtney M, Lee Kyounghoon, Keith Jason M, Daly Scott R
The University of Iowa, Department of Chemistry, E331 Chemistry Building, Iowa City, IA 52242, USA.
Department of Chemistry, Colgate University, 13 Oak Drive, Hamilton, NY 13346, USA.
J Synchrotron Radiat. 2018 Mar 1;25(Pt 2):529-536. doi: 10.1107/S1600577518000528. Epub 2018 Feb 20.
P K-edge X-ray absorption near-edge structure (XANES) spectroscopy is a powerful method for analyzing the electronic structure of organic and inorganic phosphorus compounds. Like all XANES experiments, P K-edge XANES requires well defined and readily accessible calibration standards for energy referencing so that spectra collected at different beamlines or under different conditions can be compared. This is especially true for ligand K-edge X-ray absorption spectroscopy, which has well established energy calibration standards for Cl (CsCuCl) and S (NaSO·5HO), but not neighboring P. This paper presents a review of common P K-edge XANES energy calibration standards and analysis of PPhBr as a potential alternative. The P K-edge XANES region of commercially available PPhBr revealed a single, highly resolved pre-edge feature with a maximum at 2146.96 eV. PPhBr also showed no evidence of photodecomposition when repeatedly scanned over the course of several days. In contrast, we found that PPh rapidly decomposes under identical conditions. Density functional theory calculations performed on PPh and PPh revealed large differences in the molecular orbital energies that were ascribed to differences in the phosphorus oxidation state (III versus V) and molecular charge (neutral versus +1). Time-dependent density functional theory calculations corroborated the experimental data and allowed the spectral features to be assigned. The first pre-edge feature in the P K-edge XANES spectrum of PPhBr was assigned to P 1s → P-C π* transitions, whereas those at higher energy were P 1s → P-C σ*. Overall, the analysis suggests that PPhBr is an excellent alternative to other solid energy calibration standards commonly used in P K-edge XANES experiments.
P K 边 X 射线吸收近边结构（XANES）光谱法是分析有机和无机磷化合物电子结构的有力方法。与所有 XANES 实验一样，P K 边 XANES 需要定义明确且易于获取的校准标准用于能量参考，以便能够比较在不同光束线或不同条件下收集的光谱。对于配体 K 边 X 射线吸收光谱法尤其如此，该方法已为 Cl（CsCuCl）和 S（Na₂SO₄·5H₂O）建立了完善的能量校准标准，但相邻的 P 却没有。本文综述了常见的 P K 边 XANES 能量校准标准，并分析了 PPh₃Br 作为潜在替代物的情况。市售 PPh₃Br 的 P K 边 XANES 区域显示出一个单一的、高度分辨的边前特征峰，最大值位于 2146.96 eV。在几天的反复扫描过程中，PPh₃Br 也没有显示出光分解的迹象。相比之下，我们发现 PPh₃ 在相同条件下会迅速分解。对 PPh₃ 和 PPh₃⁺ 进行的密度泛函理论计算表明，分子轨道能量存在很大差异，这归因于磷氧化态（III 与 V）和分子电荷（中性与 +1）的差异。含时密度泛函理论计算证实了实验数据，并使得光谱特征得以归属。PPh₃Br 的 P K 边 XANES 光谱中的第一个边前特征峰被归属为 P 1s → P-C π* 跃迁，而较高能量处的特征峰则为 P 1s → P-C σ* 跃迁。总体而言，分析表明 PPh₃Br 是 P K 边 XANES 实验中常用的其他固体能量校准标准的极佳替代品。
