Structure, Stability, Spectroscopy and Chemical Bonding Analyses on the Novel X6 (X = N, P, As, Sb, Bi) Molecules.

Inspired by the successful synthesis of zig-zag C2h-N6 (Nature 2025, 642, 356-360), we systematically explored the structure, stability, infrared spectroscopy, chemical bonding and weak interaction of novel X6 (X = N, P, As, Sb, Bi) molecules using relativistic DFT and CCSD(T) method. The results of global-minimum structural searches indicate the N6 prefers a trimer-like C3 geometry, while P6/As6 tends to form the bag-like C2v structure, and Sb6/Bi6 prefers the triangular prism D3h structure energetically instead. Simulated infrared spectra reveal the trimer-like N6 exhibiting an isolated high-frequency triple-bond stretching band far separated from the low-frequency single-bond skeletal vibration regimes of heavier pnictogen hexamers. Canonical molecular orbital analyses reveal a non-monotonic periodic evolution of HOMO-LUMO electronic band gaps, with N6, Sb6 and Bi6 possessing substantially wider frontier orbital energy separation relative to narrow-gap bag-like P6 and As6 species. Multi-modal wavefunction analytical tools including AdNDP, QTAIM, ELF, IRI and RDG/NCI resolve continuous periodic evolution of covalent bonding motifs from discrete diatomic triple bonds in N6, mixed σ and π bonding in P6 and As6, to pure localized σ single-bonded frameworks devoid of π orbital contributions in Sb6 and Bi6. Quantified dissociation energy values for two representative uni-molecular fragmentation pathways demonstrate monotonically increasing resistance to full decomposition from N6 to Bi6. Additional SOC-CASSCF energy level calculations further quantify the progressive intensification of spin-orbit coupling relativistic effects down group 15, which exert negligible electronic perturbation on N6 and P6, moderate orbital splitting for As6, and decisive rearrangement of frontier orbital manifolds for Sb6 and Bi6 that dictates geometric stability preferences of heavy hexamer prism. This work systematically reveals the periodic evolution law of group VA X6 molecules, providing key theoretical insights for the design and synthesis of novel pnictogen clusters.