A Charge from Sulfur

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Science  16 Oct 2009:
Vol. 326, Issue 5951, pp. 340
DOI: 10.1126/science.326_340c

A zwitterion is essentially a pair of oppositely charged ions connected by a covalent bridge. A common example is the structure of an amino acid dissolved in water near neutral pH—the amino group strips a proton from the carboxylic acid group, leading to a positive ammonium and a negative carboxylate. In general, these highly polarized structures tend to form in solvents that can stabilize the separated charges. Heldebrant et al. have observed a rare instance in which a pure molecular liquid adopts zwitterionic character. The viscous yellow substance forms at room temperature on exposure of an amino alcohol, dibutylundecanolamine, to sulfur dioxide. Formally, the sulfur bonds to the terminal oxygen of the alcohol, forming a negatively charged sulfite group, while the proton formerly on that oxygen shifts to the opposite sinterminus, forming a positively charged ammonium center. The authors characterized the unusual liquid spectroscopically and discovered that the SO2 addition is reversible. Raising the temperature under reduced pressure leads to release of the gas and recovery of the amino alcohol precursor. Because the precursor does not react with CO2, the SO2 binding and release cycle might be applicable to selective removal and recovery of sulfur from combustion exhaust streams.

Energy Environ. Sci. 2, 10.1039/b916550a (2009).

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