Chemical trends in the near-edge X-ray absorption fine structure of monosubstituted and para-bisubstituted benzenes

Carbon ls (C-R) --> pi*(c=c) electronic transitions originating from the substituent-bonded carbon atom of a benzene ring show distinctive chemical shifts in their near-edge X-ray absorption fine structure (NEXAFS) spectra. We have systematically explored these chemical shifts through ab initio calculations and carefully calibrated experimental data for a wide range of molecules containing substituted benzene rings. The systematic disparity between experimental and calculated transition energies was used to develop a semiempirical correction for this class of transitions, allowing us to map calculated transition energies onto a corrected, experimental energy scale. The correction method was applied to a large set of calculated core C ls (C-R) --> pi*(c=c) transition energies, and used to prepare a chemically wide-ranging NEXAFS correlation diagram for the "C-R pi* band". We demonstrate the usefulness of this correlation diagram for the analytical application of NEXAFS spectroscopy and microscopy to organic materials.