Orientation of Nucleophilic Substitution in Perfluoroindene by ESCA

Abstract

WE have shown how the previously uncertain structure of allylpentachlorocyclopentadiene could be unambiguously established by ESCA¹, the assignment of structure following directly from inspection of the C₁₈ spectrum. We report here a more complex example of the application of ESCA to structure elucidation. In this case simple inspection of the relevant spectra was not sufficient to arrive at a solution to the problem and the observed spectrum was compared with spectra predicted for the possible alternatives; these were theoretically derived from computed CNDO/2 charge densities using Siegbahn's charge potential model². It has been established that for a wide variety of organic molecules an excellent correlation exists between experimentally determined shifts in the C₁₈ core binding energies and those derived from CNDO/2 SCF MO charge densities using the charge potential model^3–6. Further, we have shown that experimental core binding energies can be used to calculate charge densities for molecules which are too large for computational convenience⁷. We have established that in favourable cases it is possible to proceed in the reverse direction from charge densities to core binding energies and hence to C₁₈ spectra and that this may enable structural alternatives to be distinguished.