In situ Solid-State Reactions Monitored by X-ray Absorption Spectroscopy: Temperature-Induced Proton Transfer Leads to Chemical Shifts

The dramatic colour and phase alteration with the solidstate,
temperature-dependent reaction between squaric acid and
4,4’-bipyridine has been probed in situ with X-ray absorption
spectroscopy. The electronic and chemical sensitivity to the local
atomic environment through chemical shifts in the near-edge X-ray
absorption fine structure (NEXAFS) reveals proton transfer from the
acid to the bipyridine base through the change in nitrogen
protonation state in the high-temperature form. Direct detection of
proton transfer coupled with structural analysis elucidates the nature
of the solid-state process, with intermolecular proton transfer
occurring along an acid-base chain followed by a domino effect to
the subsequent acid-base chains leading to the rapid migration
along the length of the crystal. NEXAFS thereby conveys the ability
to monitor the nature of solid-state chemical reactions in situ, without
the need for a priori information or long-range order.