An international team of researchers evaluated these dynamics using X-ray spectroscopy, which enables them to examine how individual atoms behave within a molecule, according to a study published in the Journal of the American Chemical Society.
Proton attraction. Image Credit: Stephan Thürmer
In aqueous systems, proton transfer is a basic process that occurs everywhere. A proton is lost by a molecule, which then joins forces with another molecule. Since the original model was proposed more than 200 years ago, researchers have been closely examining its mechanisms because of its importance in fields including biology, energy conversion, and electrochemistry.
Their experiments were carried out at the Japanese synchrotron radiation facility SPring-8. The study included researchers from Japan, Germany, Russia, Switzerland, and Sweden.
It is not uncommon for X-ray excitations to lead to ultrafast dissociation, where atoms rapidly leave the excited molecule. This time, however, we found the complete opposite: the local excitation instead attracts a proton, creating a new kind of state that we call associative.
Zhong Yin, Study Lead and Associate Professor, Tohoku University
Zhong Yin and colleagues selectively energized the aqueous hydroxyl ion (OH−) and studied how an associative state attracts a proton from adjacent water molecules. In addition to the substantial local decay, they discovered a shoulder spectral characteristic in Resonant Inelastic X-ray Scattering (RIXS). This technique monitors the energy loss of X-rays scattered by atoms to provide details about the molecular environment. This demonstrated an isotopic effect in aqueous OH−/OD−.
The researchers used advanced cluster calculations to determine that the smaller peak characteristic in aqueous OH− is caused by an associative state in which a proton approaches the OH−/OD− after resonant excitation.
This unexpected observation in the scattering process of the solvated hydroxide ion demonstrates that nuclear dynamics in RIXS can include associative states in addition to dissociative states seen in systems such as water and acetic acid.
Victor Kimberg, who led the theoretical research, adds that this finding clarifies the mechanism of proton transfer and broadens the scope of X-ray spectroscopy. RIXS’s atom-specific site selectivity is a significant advantage over other photon-based technologies, and it could be a suitable tool for exploring local characteristics and dynamics in solutions with a wide range of chemical and biological applications.
The Tohoku University Support Program funded the article processing charge (APC) for the study in FY2024.
Journal Reference:
Yin, Z. et. al. (2025) Observation of an Associative State in Aqueous Hydroxide. Journal of the American Chemical Society. doi.org/10.1021/jacs.4c13453