The C₄ cluster and its ionic counterparts

Prompted by inconsistencies among experimental findings and discrepancy among calculated results, a comprehensive computational study of the neutral C₄ molecular cluster and its ionic counterparts C₄^- and C₄⁺ was carried out at both MP2 and DFT (B3LYP) levels using the correlation consistent basis set aug-cc-pVQZ. A large number of initial structures for each were considered and converged geometries verified with subsequent vibrational frequency calculations. Singlet, triplet, and quintet states were considered for the neutral cluster, with doublet and quartet states for the anion and cation. Converged geometries for neutral C₄ can be grouped into linear (cumulenic and acetylenic), rhomboidal, tower and star-shaped categories, with the two linear and rhomboidal structures lowest in energy at the singlet and triplet states. For the C₄^- and C₄⁺ ions, lowest energy geometries are similarly linear, rhomboidal, and tower-shaped. Intriguingly, however, C₄⁺ includes a low-lying cis-bent near-linear geometry, in possible agreement with earlier experimental results for C₄ trapped at low temperature in inert matrices. Differences between results of the two computational levels and comparisons to experiments will be discussed.