Novel mechanisms for core level shifts in organic compounds

In the analysis of the shifts of the core level binding energies of atoms in molecules, it is standard to consider only the effective charges of the ionized atom and of its surroundings. In the present work, we give very strong evidence that another mechanism makes important contributions to core level binding energy shifts. This mechanism is the involvement of the deeper lying valence levels in the chemical bonding. In the present paper, we specifically consider the involvement of the N(2s) electrons in the bending for pyridine and pyrrole. We show that the the difference of this involvement in these two molecules is comparable in magnitude to the observed, approximate to 1 eV, difference of the N(1s) binding energies. We describe the 2s involvement as hybridization; evidence that this is an appropriate description is presented. For the first time, we have been able to directly determine the hybridization contribution to the chemical shift of the core level binding energies. It is likely that this mechanism needs to be considered, in general, for the proper analysis and interpretation of the core level chemical shifts measured with X-ray photoelectron spectroscopy because hybridization makes important contributions to the chemical bonding. (C) 1997 Elsevier Science B.V.