X-Ray Photoelectron Spectroscopy of Porphycenes: Charge Asymmetry Across Low-Barrier Hydrogen Bonds

It is ironic that although it is typically a technique that "sees" all elements other than hydrogen atoms, X-ray photoelectron spectroscopy (XPS) acts as a sensitive probe of low-barrier hydrogen bonds, as seen in a study of free-base tetrapyrroles. The nitrogen 1s XPS peaks never quite coalesce, even when the N⋅⋅⋅H⋅⋅⋅N hydrogen bonds are almost perfectly symmetrical, as is the case for dibenzo[cde,mno]porphycenes.