Solid energy calibration standards for P K-edge XANES: electronic structure analysis of PPh 4 Br.

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 (Cs2 CuCl4 ) and S (Na2 S2 O3 ·5H2 O), but not neighboring P. This paper presents a review of common P K-edge XANES energy calibration standards and analysis of PPh4 Br as a potential alternative. The P K-edge XANES region of commercially available PPh4 Br revealed a single, highly resolved pre-edge feature with a maximum at 2146.96 eV. PPh4 Br also showed no evidence of photodecomposition when repeatedly scanned over the course of several days. In contrast, we found that PPh3 rapidly decomposes under identical conditions. Density functional theory calculations performed on PPh3 and PPh4 + 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 PPh4 Br 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 PPh4 Br is an excellent alternative to other solid energy calibration standards commonly used in P K-edge XANES experiments.