Spinning Bases

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Science  05 Dec 2008:
Vol. 322, Issue 5907, pp. 1437
DOI: 10.1126/science.322.5907.1437c

The pH of any sample of water is easily measured nowadays and reflects in broad terms the concentration of protons that have physically separated from charge-balancing counterions. These “free” protons are in fact still linked to water molecules, though, and the structural basis of the link—whether H3O+, (H2O) 2H+, or some higher cluster—remains a subject of intense research and debate. Similarly, the OH ion left behind when a proton is snatched out of H2O has its own complex interactions with the water molecules surrounding it. Thøgersen et al. have probed the nature of the OH solvation shell using ultrafast rotational anisotropy measurements. Specifically, they excited the anion using an ultraviolet pulse resonant with a charge-transfer-to-solvent transition (cleverly chosen to select the ion from among bulk water absorptions), and then tracked its rate in tumbling out of the laser plane across a range of temperatures. Above about 17°C, the OH rotates rather similarly to a water molecule, but below that temperature, the rate slows down significantly, suggesting that the ion's motion is restricted by a more tightly bound shell of solvent. — JSY

Chem. Phys. Lett. 466, 1 (2008).

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