The nature of hydrogen in x-ray photoelectron spectroscopy: General patterns from hydroxides to hydrogen bonding

Important progressive alterations in chemical bonding are often realized through correlations with shifts in the x-ray photoelectron spectroscopy (XPS) binding energies of key elements. For example, there are useful general XPS shifting schemes for such systems as oxides, nitrides, halides, and even various functional groups in organics. Very general patterns, based upon location in the periodic table, exist for many of these materials even when the structure is not strongly considered. Unfortunately, apparently because of the lack of direct XPS detection of hydrogen, there seems to be no general statements in the literature for describing hydrogen-containing compounds, despite the fact that synergistic shifts obviously exist in the XPS spectra of elements attached to hydrogen (e.g., for M–O–H vs M–O–M units, where M is a typical metal). While not attempting a complete review paper, in the present work we use XPS shifting patterns to evolve a series of interrelated covalency/ionicity arguments to help explain the progressive, periodic changes in XPS peak locations for such common cases as M–O–H- and M–N–H-containing systems. These arguments are followed by consideration of the less dramatic XPS shifting patterns exhibited by metal and metalloid hydrides, including organic bonding. The formalism concludes with a discussion of hydrogen bonding detected by XPS. After a select review of the infrequent use made by others to attribute XPS peak shifts to hydrogen bonding, we consider in some detail two cases recently published by members of our group. One case involves the formation of –N–H–N– bonds in proton sponge organic systems, while the other uses XPS to examine the formation of surface oriented –O–H–––O– bonds in the adsorption of peptides on oxidized metals. In the present article, the XPS patterns for these two seemingly divergent cases are explained by closely related arguments that may have far reaching generalities.