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 C_2h-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 C_2v structure, and Sb₆/Bi₆ prefers the triangular prism D_3h 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.