Chromophore Analysis\nof Near-Edge X‑ray Absorption\nSpectra: Application to Four-Coordinate Phosphorus

This paper reports <i>X-ray chromophore</i> behavior:\nthe substituents of an X-ray-absorbing atom (here phosphorus) were\nfound to contribute consistent absorptions, in intensity and in relative\npeak position (transition energy). Linear relations for absolute peak\nposition, as a function of the sum of the phosphorus neighbor-atom\nelectronegativities (Σχ), have been found for ten chromophore\npeaks: P–F, P–O, P–Cl, P–N, P–C<sub>I</sub>, P–W<sub>I</sub>, P–C<sub>II</sub>(Ph), P–C<sub>II</sub>(pyr) P–C<sub>II</sub>(Fc), P–W<sub>II</sub>. The discovery was made by finding (i) surprisingly rich phosphorus\nK-edge X-ray absorption spectra [P X-ray absorption spectroscopy (XAS)]\nfor several four-coordinate-phosphorus compounds, including four new\nones involving the W(CO)<sub>5</sub> substituent, (ii) a vastly improved\ntime-dependent (linear-response-based) density-functional-theory prediction\nmethod for P XAS which provides unprecedented XAS peak-assignment\nability, and (iii) a fresh antibonding-orbital-based interpretive\nframework. The discovery, that selective X-ray excitation of substituents\nis possible, opens the door for studies of chemical reactivity implications\n(X-ray photochemistry), and improved studies of X-ray radiation exposure\neffects and damage.