The importance of 2s bonding contributions for the core level binding energies in organic compounds

The common interpretation of the shifts of the core level binding energies, BEs, of atoms in molecules is based on considerations of the effective charges of the atoms in the molecule. An isolated atomic ion will have a different core level BE from a neutral atom and core level BE shifts have been interpreted as reflecting the charge of a core ionized atom as though it were a free ion. Sometimes, the effect of the presence of electric fields due to the effective or nominal charge of the surrounding atoms is also taken into account. It is shown that, for organic compounds, the 2s involvement in the bonding has an important effect on the shift of core level binding energies. Following standard chemical interpretation, we refer to this contribution as 2s hybridization. Specifically, we consider the shift of the N(1s) binding energy in a series of four compounds. However, we expect this hybridization mechanism to be general for the proper interpretation of X-ray photoelectron spectroscopy.