Hydrogen bonds: Scientists find new mechanism

Bernd Winter and colleagues, from BESSY, Max-Born-Institut, Uppsala University, and MPI für Dynamik und Selbstorganisation, report in the current issue of Nature how water solvates its intrinsic hydroxide (OH-) anion. Unraveling this behavior is important to advance the understanding of aqueous chemistry and biology.

Using a resonance (photo) core-electron spectroscopy technique, with sub ten-femtosecond temporal resolution, and employing synchrotron radiation in conjunction with a liquid microjet, the researchers find that OH- is capable of donating a transient hydrogen bond to a neighboring water molecule. Their experiment thus disproves the classical, so-called proton-hole picture, assuming that OH- is a hydrogen-bond acceptor only.

The weak OH- hydrogen donor bond is responsible for a distinct intensity pattern in the electron spectra, and is connected with a unique energy transfer (intermolecular Coulombic decay) between the oxygen 1s core-excited hydroxide ion and a neighboring water molecule. It is the first time such a process is observed in an aqueous system. To confirm that the measurements exclusively probe the weak OH- hydrogen donor bond at such high sensitivities the team has conducted comparative measurements of halide ions in water.

They find that chloride and isoelectronic fluoride do not exhibit this energy-transfer channel, which corroborates recent structural diffusion models for the unusually migration of the hydroxide ion in water. The work marks a step forward into studying very fast dynamical processes in water and aqueous solutions.

Citation: E. F. Aziz et al., Nature 455, 89 - 91 (04 Sep 2008), doi: 10.1038/nature07252

Source: Max-Born-Institut