Assigning Oxidation States to Organic Compounds via Predictions from X-ray Photoelectron Spectroscopy: A Discussion of Approaches and Recommended Improvements

The traditional assignment of oxidation states to organic molecules is problematic. Accordingly, in 1999, Calzaferri proposed a simple and elegant solution that is based on the similar electronegativities of carbon and hydrogen: hydrogen would be assigned an oxidation state of zero when bonded to carbon. Here, we show that X-ray photoelectron spectroscopy, a core electron spectroscopy that is sensitive to oxidation states of elements, generally agrees with his suggestion. We also list the typical (IUPAC) rules for assigning oxidation states, review recent suggestions of Loock and Steinborn that are based on Pauling's earlier approach, discuss the traditional (IUPAC and Pauling-Loock-Steinborn) assignments of oxidation states to organic molecules, review Calzaferri's suggestion, introduce X-ray photoelectron spectroscopy (XPS), show the general agreement of Calzaferri's suggestion with XPS results, provide supporting examples from the literature, and discuss the limitations of Calzaferri's recommendation vis-a-vis XPS results. We conclude by recommending that either (i) Calzaferri's suggestion be implemented into the current IUPAC rules or (ii) the Loock definition be explanded to deal specifically with atoms with similar electronegativities.