Proton Transfer, Hydrogen Bonding, and Disorder: Nitrogen Near-Edge X‑ray Absorption Fine Structure and X‑ray Photoelectron Spectroscopy of Bipyridine–Acid Salts and Co-crystals

The sensitivity of near-edge X-ray
absorption fine structure (NEXAFS)
spectroscopy to Brønsted donation and the protonation state of
nitrogen in the solid state is investigated through a series of multicomponent
bipyridine–acid systems alongside X-ray photoelectron spectroscopy
(XPS) data. A large shift to high energy occurs for the 1s →
1π* resonance in the nitrogen K-edge NEXAFS with proton transfer
from the acid to the bipyridine base molecule and allows assignment
as a salt (CNH+), with the peak ratio providing
the stoichiometry of the types of nitrogen species present. A corresponding
binding energy shift for CNH+ is observed in the
nitrogen XPS, clearly identifying protonation and formation of a salt.
The similar magnitude shifts observed with both techniques relative
to the unprotonated nitrogen of co-crystals (CN) suggest that
the chemical state (initial-state) effects dominate. Results from
both techniques reveal the sensitivity to identify proton transfer,
hydrogen bond disorder, and even the potential to distinguish variations
in hydrogen bond length to nitrogen.