A density functional study of the structures and energies of C _nP₅^- (n = 1-7) clusters

Using molecular graphics software, we have designed numerous models of C_nP₅^- (n=1-7). We carried out geometry optimization and calculation on vibrational frequency by means of the B3LYP density functional method. After comparing the total energies of the isomers, we found that the ground state structures are straight carbon chains with a P₄C ring connected at one end and a phosphorus atom at the other. The alternate behaviours in electron affinity, bond length, and incremental binding energy with odd and even "n" match the peak pattern observed in the laser-induced mass spectra of C_nP₅^- (n=1-7) and the results of experimental reactions. When n is even, the electron state is ¹A₁, whereas it is ³A₂ with odd n. The bond lengths of straight C_n chains show a polyacetylene-like structure. As for the C-P bonds within the straight chains, there is an alternate pattern in length, i.e., compared to the C-P bonds at even n, those at odd n show a bigger bond length.