Proton Transfer on the Edge of the Salt/Cocrystal Continuum: X-Ray Photoelectron Spectroscopy of Three Isonicotinamide Salts

X-ray photoelectron spectroscopy (XPS) has emerged as a technique that allows for characterization and classification of hydrogen bonding and proton transfer interactions in organic crystal structures, in a way that is complementary to crystallography by X-ray or neutron diffraction. Here, we analyze the nitrogen 1s core-level binding energies (BEs) of isonicotinamide (IN) systems with proton transfer between donor and acceptor groups at short distances. We show how a careful calibration of the BE scale places these salt systems correctly on the edge of the so-called salt–cocrystal continuum. We show how performing a fitting analysis of the data that is consistent with elemental analysis, expected stoichiometry, and quantification of adventitious carbon contamination facilitates the determination of absolute BEs with accuracy and reproducibility within ±0.1 eV. The determined N 1s core-level BEs of the protonated IN acceptors suggest that the local geometric arrangements of the donor, acceptor, and proton can influence the N 1s core-level BE significantly.