Chromophore Analysis of Near-Edge X-ray Absorption Spectra: Application to Four-Coordinate Phosphorus

This paper reports X-ray chromophore behavior: the substituents of an X-ray-absorbing atom (here phosphorus) were found to contribute consistent absorptions, in intensity and in relative peak position (transition energy). Linear relations for absolute peak position, as a function of the sum of the phosphorus neighbor-atom electronegativities (Σχ), have been found for ten chromophore peaks: P–F, P–O, P–Cl, P–N, P–CI, P–WI, P–CII(Ph), P–CII(pyr) P–CII(Fc), P–WII. The discovery was made by finding (i) surprisingly rich phosphorus K-edge X-ray absorption spectra [P X-ray absorption spectroscopy (XAS)] for several four-coordinate-phosphorus compounds, including four new ones involving the W(CO)5 substituent, (ii) a vastly improved time-dependent (linear-response-based) density-functional-theory prediction method for P XAS which provides unprecedented XAS peak-assignment ability, and (iii) a fresh antibonding-orbital-based interpretive framework. The discovery, that selective X-ray excitation of substituents is possible, opens the door for studies of chemical reactivity implications (X-ray photochemistry), and improved studies of X-ray radiation exposure effects and damage.