Photoelectron spectroscopy and theoretical study of AlnC5-/0 (n=1-5) clusters: structural evolution, relative stability of star-like clusters, and planar tetracoordinate carbon structures

The AlnC5- (n = 1-5) clusters were detected in the gas-phase and were investigated via mass-selected anion photoelectron spectroscopy. The structures of AlnC5-/0 (n = 1-5) were explored by theoretical calculations. It is found that the structures of AlC5-/0 and Al2C5-/0 are linear while those of Al3C5-/0, Al4C5-/0, and Al5C5-/0 are two-dimensional. The most stable structures of AlC5-/0 and Al2C5-/0 are linear with the Al atoms attached to the ends of C-5 chain. The most stable structures of Al3C5-/0 can be viewed as three Al atoms interacting with a nonlinear C-5 chain. The most stable structure of Al4C5- anion is a planar structure composed of a C-2 unit, a C-3 unit, and two Al-2 units, while that of the neutral Al4C5 cluster has four Al atoms connected to different positions of a distorted C-5 chain. The global minimum structures of Al5C5-/0 are planar structures composed of an Al4C quadrilateral, two C-2 groups, and an Al atom connected to two C-2 groups. Planar tetracoordinate carbon (ptC) has been identified in the structures of both anionic and neutral Al5C5. It is worth mentioning that the star-like structure of Al5C5- is slightly higher in energy than the ground state structure. The comparison of theoretical calculations with the experimental spectra indicates the star-like structure of Al5C5- may also appear in our experiments.