Fluxional and aromatic behavior in small magic silicon clusters: a full ab initio study of Si(n), Si(n) (1-), Si(n)...

The structural, electronic, vibrational, optical, magnetic, and aromatic characteristics of Si(n), Si(n) (1-), Si(n) (2-), and Si(n) (1+), clusters have been calculated very accurately with a variety of high level ab initio techniques. These calculations have been performed with the aim to clarify existing ambiguities in the literature and to bring up the fluxional and aromatic characteristics of these species. The fluxional behavior, according to earlier conjecture of the present author, could be connected to the magic property. In addition such behavior could also explain the existence of conflicting results. The ab initio techniques include quadratic configuration interaction, coupled cluster, and multireference second order perturbation theory, together with density functional theory ("static" and time dependent) with the hybrid B3LYP functional. Various high quality correlation-consistent basis sets, ranging from 2Z up to 5Z quality, were employed. It is demonstrated that Si(6) is fluxional, fluctuating around a symmetric D(4h) structure. Si(10) is also fluxional but to a lesser degree, in contrast to Si(10) (1-) anion which is highly fluxional. For both clusters, in full agreement with Wade's and Lipscomb's rules for deltahedral boranes, the corresponding dianions have higher symmetry (O(h) and D(4d), respectively) and lower energy than the neutral clusters. The aromatic behavior of Si(6) fits better to a mixed conflicting aromaticity picture. This type of aromatic and fluxional behavior has also been observed in stable "magic" carbon clusters as C(6) and carbon fullerenes such as C(20). The present results, which support possible connection of fluxional and magic properties, are in excellent agreement with experimental measurements of ionization energies, electron affinities, and vibrationally resolved photoelectron spectra.