Substituted benzenes as building blocks in near-edge X-ray absorption spectra

Calculations of NEXAFS (Near-Edge X-ray Absorption Fine Structure) spectra of fluorine substituted benzenes, C6H6-nFn, n = 1-6, and of aniline and phenol have been carried out for the purpose of elucidating systematic trends of these spectra on number, position and strength of substituents. It is found that the differential and average shifts of pi* NEXAFS spectra for the unconnected carbon atoms are very small, which make these spectra excellent fingerprints for a benzene-ring building block. The excitation energy for the connected carbon pi* spectra fingerprints the substitution strength and the relative intensity fingerprints the multiplicity of substitution, while the positional dependency of either the connected or the unconnected carbon spectra is too small to fingerprint the isomers. Previous assignments of the spectra are critically examined and in some cases revised. (C) 1997 Elsevier Science B.V.