Isomeric sensitivity of the C 1s spectra of xylenes

Oscillator strengths for C 1s inner-shell excitation of the isomeric xylenes (ortho-, meta- and para-) have been derived from electron energy loss spectra recorded under scattering conditions dominated by electric dipole transitions. The lineshape of the C 1s --> pi* transition is found to be dependent on the substitution pattern, with para-xylene exhibiting two components, ortho-xylene a single asymmetric peak, and meta-xylene a single symmetric peak at an instrumental resolution of 0.35 eV. Improved virtual orbital ab initio calculations were carried out on all three species, providing spectral shapes in reasonable agreement with experiment. The calculations indicate that the transition responsible for the splitting which is observed only in the C 1s spectrum of para-xylene is associated with the methyl-substituted ring carbons, for which there is a somewhat larger chemical shift in the para relative to the ortho- or meta-isomers. These results are compared to other recent studies of C 1s spectroscopy of di-substituted benzenes (dimethylphthalates and nitroanilines) in order to investigate trends in the sensitivity of C 1s spectroscopy to isomeric substitution patterns. (C) 1998 Elsevier Science B.V. All rights reserved.