First Molecular Anion Identified In Space

Once thought too fragile to exist in the severe environment of outer space, molecular anions may actually be ubiquitous there. For the first time, researchers have observed in astronomical clouds large amounts of a negatively charged molecule: the hexatriyne anion, or C₆H^- (Astrophys. J. 2006, 652, L141).

Detection of the molecule in interstellar space overturns the belief that radiation would quickly strip an anion of its extra electron, yielding a radical. The finding, reported by Michael C. McCarthy, Patrick Thaddeus, and colleagues at the Harvard-Smithsonian Center for Astrophysics, in Cambridge, Mass., also solves a spectral puzzle, identifying C₆H^- as the source of a mysterious set of rotational lines discovered in a molecular cloud over 10 years ago.

Observers say the addition of anions to the mix of known molecules in space will alter the field of interstellar chemistry. "I would say this is the most important observation since the first detection of positive ions in such clouds," says John P. Maier, a chemistry professor at the University of Basel, in Switzerland, who has studied the spectra of C₆H ^species.

"It's definitely a significant contribution, because it's likely to open up a new subgroup of observable species," says David E. Woon, an astrochemist at the University of Illinois, Urbana-Champaign.

During the past few decades, astronomers have discovered more and more complex molecules in the cold reaches of space. But until now, the discoveries have been limited to 130 neutral molecules and 14 cations.

The idea that anions might exist in space is not new. Back in 1940, German-American astronomer Rupert Wildt showed that the sun's opacity is due to H^-. But save for possible sightings of OCN^- in interstellar ice grains, no other astronomical anion has been detected.

The Harvard-Smithsonian team's search began with the 10-year-old collection of mystery spectral lines, designated B1377 by its discoverers, Kentarou Kawaguchi and colleagues at Okayama University, in Japan. The rotational spectrum of B1377 is very similar to that of the neutral C₆H, which is plentiful in some molecular clouds. It was therefore considered possible that the molecule that generated the B1377 lines was close in structure and composition to C₆H. Previous theoretical work also had suggested that C₆H^- might be the mystery molecule, but no laboratory spectra were available to confirm that.

The Harvard-Smithsonian scientists first studied two astronomical sources-a dense molecular cloud and a gaseous envelope around a carbon star. They then determined the rotational spectrum of C₆H^- in the lab and found that it matched the astronomical spectra.

The results suggest a new strategy for finding other anions, the researchers point out: Determine spectra for large anions in the lab, and look for those anions in densely populated molecular sources. In fact, Thaddeus says, since their discovery of C₆H^-, the group also has identified C₈H^- and C₄H^- in the lab, and astronomical searches are now under way.

Robert J. McMahon, a chemistry professor at the University of Wisconsin, Madison, who has collaborated previously with the Harvard-Smithsonian team, tells C&EN that the observation of C₆H^- in space "reinforces just how little we truly know about the chemistry of interstellar and circumstellar environments."