Negative Get-Togethers

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Science  25 Mar 2011:
Vol. 331, Issue 6024, pp. 1495
DOI: 10.1126/science.331.6024.1495-a

When transition-metal salts dissolve in water, it's not unusual for a number of the counterions to remain coordinated. In contrast, alkali metal ions such as sodium or potassium weren't traditionally thought to behave this way; dissolution of their salts conjures images of a sea of water molecules keeping anions and cations thoroughly apart. Recently, however, precise spectroscopic studies have been uncovering a more complicated scenario for these simple salts, in which cation and anion continue to influence one another in solution. In one such study, Bian et al. now present evidence that concentrated solutions of alkali thiocyanates (SCN) exhibit anion clustering. The authors used two-dimensional infrared spectroscopy to measure vibrational energy transfer between isotopically light and heavy SCN solutes as a probe of their proximity, and they found that in 10 M KSCN, over 90% of the anions gathered in clusters. Rotational anisotropy measurements implicated cluster sizes of approximately 18 anions. Diluting the solution reduced the apparent proportion of anions that gathered in clusters, as did shifting to smaller cations (lithium or sodium); cesium—the largest stable alkali—correspondingly induced the highest clustering proportion.

Proc. Natl. Acad. Sci. U.S.A. 108, 10.1073/pnas.1019565108 (2011).

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