Isomers of Small Carbon Cluster Anions: Linear Chains with up to 20 Atoms.

The structure of small carbon cluster anions, Cn(-) (4 </= n </= 20), was investigated with the use of ion chromatography. With this technique, both the existence and the relative amounts of possible structural isomers can be determined. More than 99% of the ions C(5)(-) to C(9)(-) were found to be linear under these experimental conditions. Starting with C(10)(-), a monocyclic isomer was observed, and linear and moncyclic structures coexisted from C(10)(-) through at least C(20)(-). This result is in contrast to previous work on positive ions, which showed the existence of linear isomers from C(5)(+) to C(10)(+), with linear and cyclic isomers coexisting only from C(7)(+) to C(10)(+). Above C(10)(+), no linear clusters were observed.