Unusual organic molecules have been detected in the circumstellar gas and dust envelopes which surround red giant stars [1]. These include C₃, C₅ and the hydrocarbons C_nH (n=2-6), which have been identified around the bright carbon rich star IRC+10216 [1-3.] It is proposed that such neutrals are formed following photoionisation of acetylene, but the reaction sequences are not known [3]. We have been concerned with the synthesis of anions corresponding to these neutrals and in investigating their structures and reactivities. We have prepared C₃H^- and the linear isomer of C₅H^- in the gas phase by the following collision induced process.

The mass spectra of these ions have been studied, and ab initio calculations indicate structures which are difficult to visualise using standard valence bond theory. This work will be discussed in detail as well as current investigation into the synthesis and properties of the non-linear isomer of C₅H^- where the hydrogen resides on the central carbon.

1. H.Olofsson, 'Molecular Abundances in the Envelopes around Evolved Stars', in (O.G.Jorgensen, Ed) Molecules in the Stellar Environment, Lecture Notes in Physics, Springer Verlag, Heidelberg, 1994, pp. 114-133, and references cited therein.
2. P.F.Bernath, K.H.Hinkle and J.J.Keady, Science, 1989, 244, 562; K.H.Hinkle, 'Infrared Spectroscopy and the Molecules in Circumstellar Envelopes', in (O.G.Jorgensen Ed.) Molecules in the Stellar Environment, Lecture Notes in Physics, Springer Verlag, Heidelberg, 1994, pp. 99-114, and the references cited therein .
3. A.Omont, 'Polyynes and the Polycyclic Aromatic Molecules in the C-rich Circumstellar Envelopes', in (O.G.Jorgensen Ed.) Molecules in the Stellar Environment, Lecture Notes in Physics, Springer Verlag, Heidelberg, 1994, pp. 135-148, and the references cited therein.