Structural, Rotational, Vibrational, and Electronic Properties of Carbon Cluster Anions Cn- (n = 3-13)

Abstract

The structural, rotational, and vibrational properties of Cn- clusters (n = 3-13) have been investigated by means of density functional theory (DFT/B3LYP) and, whenever possible, coupled cluster (CC) theory along with the aug-cc-pVDZ basis set. These properties are compared with those of their neutral counterparts and of the corresponding cations. The linear and merely cumulenic chains undergo a substantial increase of the bond-length alternation and an increase of size upon adiabatic electron attachment. In addition, most chains (C5, C7, C8, C9, and C10) become slightly bent in their anionic form because of Renner-Teller effects. The structural outcomes of such processes on carbon rings are far more varied and can be rationalized solely through a topological analysis of the frontier orbitals. Both for the linear and cyclic species, IR spectra and rotational moments provide specific markers of these complex structural variations. Closed anionic clusters such as C5-, C9-, and C13- are even-twisted cumulenic rings. The highest occupied levels of these rings relate to orbitals with a particularly exquisite bonding pattern, which explains, among other effects, significant departures from planarity. It has been noticed that d diffuse functions are essential for a sound description of the Renner-Teller distortions of Cn- chains and of the nonplanar nature of the C13- ring. The linear anionic chains exhibit a much stronger IR activity, as well as a systematically greater propensity to bind an extra electron, than the cyclic isomers. Among the rings, the adiabatic electron affinities (AEAs) of the C9 and C13 species are strikingly high, whereas the lowest value of AEA coincides with the cumulenic C10 species. For the anionic chains and the larger rings, the most intense IR absorption lines have vibrational frequencies ranging from 1600 to 2200 cm-1.