The structures and stabilities of PN and its 27 isoelectronic analogues, CS, SiO, BCl, AlF, BeAr MgNe SN+ PO+ CCl+ SiF+, BAr+, AlNe+, SO2+, NCl2+, PF2+, CAr2+, SiNe2+, OCl3+, SF3+, NAr3+, PNe3+, FCl4+, OAr4+, SNe4+, FAr5+, ClNe5+, and ArNe6+, have been examined by ab initio molecular orbital theory. The CASSCF/6-311G(MC)(d) level was used to determine the ground-state potential energy curves and spectroscopic constants for the 28 diatomic systems. Equilibrium structures were also obtained with the 6-311G(MC)(d) basis set at the MP3 and ST4CCD levels, and dissociation energies were determined at the MP4/6-311 + G(MC)(2df) and MP4/6-311 + G(MC)(3d2f) levels. For the neutral and monocation analogues of PN, the calculated equilibrium geometries (at MP3/6-311G(MC)(d)) and dissociation energies (at MP4/6-311 + G(MC)(3d2f)) are in very good agreement with available experimental values. All the dication analogues of PN, namely, SO2+, NCl2+, PF2+, CAr2+, and SiNe2+, are predicted to be experimentally observable species. Of these, the SO2+, NCl2+, and CAr2+ dications are calculated to be kinetically stable species, with large barriers associated with the exothermic charge-separation reactions, while the PF2+ and SiNe2+ dications are predicted not only to be kinetically stable but also to be thermodynamically stable species. Despite the inherent strong Coulomb repulsion, the triply charged ions SF3+ and PNe3+ display remarkable stability toward extremely exothermic fragmentation reactions and are predicted to be experimentally accessible species in the gas phase. The OCl3+ and NAr3+ trications, on the other hand, although having short equilibrium bond lengths, lie in shallow potential wells. Except for the neon-containing species, all the doubly and triply charged ions are characterized by short equilibrium bond lengths. In the case of SO2+, the calculated S-O bond length may well be the shortest between any first-row atom and second-row atom. The fragmentation mechanism for the multiply charged diatomics can readily be understood in terms of avoided-crossing models. © 1990 American Chemical Society.
