Structure, Stability, Spectroscopy and Chemical Bonding Analyses on the Novel X 6 (X = N, P, As, Sb, Bi) Molecules

ABSTRACT Inspired by the successful synthesis of zig‐zag C 2h ‐N 6 ( Nature 2025 , 642, 356–360), we systematically explored the structure, stability, infrared spectroscopy, chemical bonding and weak interaction of novel X 6 (X = N, P, As, Sb, Bi) molecules using relativistic DFT and CCSD(T) method. The results of global‐minimum structural searches indicate the N 6 prefers a trimer‐like C 3 geometry, while P 6 /As 6 tends to form the bag‐like C 2v structure, and Sb 6 /Bi 6 prefers the triangular prism D 3h structure energetically instead. Simulated infrared spectra reveal the trimer‐like N 6 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 N 6 , Sb 6 and Bi 6 possessing substantially wider frontier orbital energy separation relative to narrow‐gap bag‐like P 6 and As 6 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 N 6 , mixed σ and π bonding in P 6 and As 6 , to pure localized σ single‐bonded frameworks devoid of π orbital contributions in Sb 6 and Bi 6 . Quantified dissociation energy values for two representative uni‐molecular fragmentation pathways demonstrate monotonically increasing resistance to full decomposition from N 6 to Bi 6 . 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 N 6 and P 6 , moderate orbital splitting for As 6 , and decisive rearrangement of frontier orbital manifolds for Sb 6 and Bi 6 that dictates geometric stability preferences of heavy hexamer prism. This work systematically reveals the periodic evolution law of group VA X 6 molecules, providing key theoretical insights for the design and synthesis of novel pnictogen clusters.