Structures and aromaticities of doped P4Mq (M = S, Se, q = 0; M = Si, Ge, q = 2−) species from theoretical study

Structural optimizations and harmonic vibrational frequency analyses of the doped P4Mq (M = S, Se, q = 0; M = Si, Ge, q = 2−) species were performed using B3LYP, B3PW91, PW91PW91, and [MP2(full)]/6-311+G(3df) theory levels. The results show that all P4Mq ground states adopt the singlet planar pentagonal structures (C2V, 1A1). Valence molecular orbital analyses indicate that each lowest isomer has three doubly occupied, delocalized π-MOs. The computed Wiberg bond index and bond lengths of all adjacent P-P and P-M (M = S, Se, Si, Ge) bonds are intermediate of the corresponding values for single-bond and double-bond. The big negative magnetic susceptibility anisotropies and nucleus-independent chemical shifts together with the presence of six delocalized valance π electrons (thus satisfying the 4n + 2 electron counting rule) confirm that the above P4S, P4Se, P4Si2−, and P4Ge2− grounds exhibit strong aromatic characteristics.