ISOMERIC SENSITIVITY OF THE C-1S SPECTRA OF XYLENES

Oscillator strengths for C 1s inner-shell excitation of the isomeric x ylenes (ortho-, meta- and para-) have been derived from electron energ y loss spectra recorded under scattering conditions dominated by elect ric dipole transitions. The lineshape of the C 1s --> pi transition i s 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 o ut on all three species, providing spectral shapes in reasonable agree ment with experiment. The calculations indicate that the transition re sponsible for the splitting which is observed only in the C 1s spectru m of para-xylene is associated with the methyl-substituted ring carbon s, for which there is a somewhat larger chemical shift in the para rel ative to the ortho- or meta-isomers. These results are compared to oth er recent studies of C 1s spectroscopy of di-substituted benzenes (dim ethylphthalates and nitroanilines) in order to investigate trends in t he sensitivity of C 1s spectroscopy to isomeric substitution patterns. (C) 1998 Elsevier Science B.V. All rights reserved.