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

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.