The intensity of the research into small carbon clusters has increased dramatically since the discovery of fullerenes with particular emphasis on their role in fullerene formation. Fourier transform mass spectrometry has proven a very productive method for studying carbon cluster structure. Reactions involving carbon cations are quite extensive with many reagents being utilised and the resulting reactions and structure of the gas phase ions being well understood. Studies of anions have been much more limited. McElvany [1] has investigated the reactivity of C4- to C13- with F2 and C2N2 and Pozniak et al. [2] has performed a study on photodctachment and photodissociation of C4- to C13- utilising CCl4, an electron scavenger.
Our research has focussed on ion/molecule reactions of carbon anions (Cn-, n = 4 - 14, odd numbered clusters 15 ¾ n ¾ 29) with H2S using Fourier transform mass spectrometry. Kinetic data, where possible, was obtained and this as well as reaction pathways was used to determine possible structural isomers of the gas phase clusters.
The equipment used for the experiments consisted of a spectrospin CMS-47 Fourier transform mass spectrometer coupled with a spectra physics Nd-YAG laser. Carbon clusters were produced by direct laser ablation of a partially graphitised petroleum coke sample. This sample, rather then graphite, was ablated because it gave a large range of ncgative ion carbon clusters C4- to C29-. Direct laser ablation of graphite will only produce carbon anions up to C13-.
The carbon anions Cn-, (n = 4 - 9, 11, 13) were found to be reactive to H2S and the major product formed from these reactions was HS-. Difrerences were observed in t olher reaction products produced from the even and odd-numbered carbon clusters. The even-numbered clusters only formed the addition products, CnH- and CnHS- as well as C8H3S- from C8-, The odd-numbered carbon clusters formed many more reaction products than the even clusters with both addition and dissociation reactions occurring. This dramatic difference in the reaction channels of the even and oddnumbered clusters is consistent with linear structures of a cumulenic nature for odd clusters and a polyacetelenic nature for even clusters. A decrease with increasing size was observed in the rate constants obtained for C4- to C9-.
The reactivity of C11- and C13- suggest that two isomer were present with one isomer unreactive towards H2S. These results are suggestive of a linear and cyclic structure co-existing for C11- and C13-.
A dramatic loss in signal was observed from the anions C10- and C12- in the presence of H2S with no reaction products or signal being observed after 0.4 seconds. This result is unexplained and further research needs to be performed on these ions.
The larger anions C14- and the odd-numbered clusters from C15- to C29- were unreactive to H2S indicating that only non-reactive cyclic structures and no reactive linear structures exist for these anions.