Credit: © 2009 ACS

In confined environments, molecules adopt properties that are different from those in the bulk, and calculations have suggested that proton hydrates — clusters formed by a proton and water molecules— could form wires with high proton mobility when confined in nanochannels. These hold great promise for proton-conduction applications in fuel cells and biological channels, but few structures exhibit proton channels that are well-enough defined to enable verification of these predictions.

Now, Evgenii Stoyanov, Christopher Reed and co-workers at the University of California, Riverside, have observed1 the aggregation of proton hydrates into wires in carborane nanochannels. Crystallization of carborane acid H(CHB11I11) from a saturated aqueous solution led to a tubular structure, rather than the ionic proton-hydrate–carborane lattice formed with other carboranes. Crystallographic studies of the tubular structure showed that channels formed by carborane anions enclosed proton hydrates. Based on the distances between the oxygen atoms, the researchers were able to determine the position of the bridging proton in each cluster, and showed the presence of three types of clusters, in which one proton is associated with either two, three or four water molecules.

All three clusters featured longer oxygen–oxygen distances than discrete proton hydrates, which indicates a large delocalization of the positive charges. Furthermore, short distances between the oxygen atoms of two types of clusters revealed that they aggregated at the centre of the nanochannels to form true proton wires.