July 2011

Aluminum monofluoride

In 1987, Cernicharo & Guélin reported the discoveries of three diatomic chloride speceies, NaCl, KCl, and AlCl, as well as the tentative detection of aluminum monofluoride (AlF), all in the circumstellar shell of the carbon star IRC +10216 (also known as CW Leonis). Although the lines for three successive rotational transitions were observed, the features in question were weak and not confirmed with isotopic lines, as was the case for AlCl. Confirmation of AlF followed in 1994, when Ziurys et al. found three additional lines in the same source using the Caltech Submillimeter Observatory. AlF was also found in a second circumstellar shell, CRL 2688 (the Egg Nebula), by Highberger et al. in 2001.

While aluminium monofluoride is a closed-shell species (1Σ+ ground state), it is much less stable than trivalent AlF3. The availability of the 3s2 electrons for bonding is also reflected by the presence of low-lying 3Π and 1Π excited states of AlF that arise through recoupled pair bonding.

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