A THEORETICAL-STUDY OF LINEAR CARBON CLUSTER MONOANIONS, CN(-), AND DIANIONS, CN(2-) (N=2-10)

A large number of carbon cluster monoanions, C(n)-, have now been detected by negative ion photoelectron spectroscopy. In addition, evidence for carbon cluster dianions, C(n)2-, as small as C-7(2-) has been obtained mass spectrometrically. In this research we report results of theoretical calculations of structures and energetics of formation of linear carbon cluster monoanions and dianions containing up to ten carbon atoms. A number of different electronic states have been investigated. Self-consistent field (SCF) theory, many-body perturbation theory, and coupled-cluster theory including triple excitations have been used with basis sets containing polarization and diffuse functions. Considerably larger basis sets have also been used in calculations on some of the smaller species. For the monoanions, the observed electron detachment energies and the even-odd alternation thereof are well reproduced by the calculations. For the dianions, the even numbered species are found to be more easily formed than the odd numbered species, in accord with the intensity pattern observed in the mass spectrometric experiments, and with the availability of partially occupied pi-orbitals. C-10(2-) is established to be vertically and adiabatically stable to electron loss, while C-8(2-) is found to be vertically stable but adiabatically unstable to electron loss. Improved calculations may be sufficient to make C-8(2-) also stable to adiabatic electron loss. C-7(2-) and C-9(2-) are both found to be unstable to vertical electron loss, although both have negative highest occupied molecular orbital (HOMO) eigenvalues and C-9(2-) is stable to vertical electron loss at the SCF level. The geometry changes resulting from the addition of two electrons are significant, especially for the even numbered clusters. Addition of two electrons to the partially occupied pi-orbitals of the latter leads to strong single-triple bond alternation, which may be rationalized by noting that the dianions are products of double deprotonation of HC2nH. Such an "accordion" mechanism may have a role in the ability of carbon clusters to conduct electricity.