Are helionitronium trications stable?

Abstract

, is an unusually stable heliumcontaining polyatomic ion. This result was based on second-order many-body perturbation (MP2) calculations that showed that strong binding should occur between the oxygen and helium atoms in the assumed singlet ground state. The dissociation energy with respect to NO ؉ and HeO 2؉ was predicted to be 7.95 eV. We show here by thorough multireference configuration interaction (MRCI) studies that the ground state for the helionitronium trication is a triplet 3 B1 state with He binding to the N atom (C2v). The HeOO bound structure of C s symmetry is not stable. Dissociation of the helionitronium trication occurs toward NO 2 2؉ and He ؉ , and the trication is bound by at most 0.25 eV. These results indicate that the helionitronium trication is unstable under ambient conditions. The discrepancies between our results and the previous study are explained by the strong multireference character of the wave function of the trication. Structural and energetic properties of helium-containing polyatomic cations are of great interest because they offer new possibilities and sources of superacids. There have been a number of studies (1-7) by 7.95 eV. The unusual stability of the helionitronium trication, HeNO 2 3ϩ catalyzed our interest in this class of novel species. In the present work we report results of more sophisticated calculations on the structure and stability of the helionitronium trication. MethodologyTo compute reliable energies and geometries for the multiple cationic species in question, it turned out that multireference ab initio techniques need to be applied as will be shown in the course of this article. All results for the complexes were obtained by the internally contracted multireference singles-doubles configuration interaction method (8) The multireference calculations [CASSCF͞complete active space configuration interaction (CASCI) and MCSCF͞ multireference configuration interaction (MRCI)] were carried out in two different schemes regarding active space and reference configurations. For the specification of the active space, it is the easiest to treat the fragments NO 2 and He separately. In all calculations the 1s orbitals of N and O were treated as core orbitals and were excluded from the correlation procedure. The space of valence orbitals is spanned by the He 1s orbital and three g ϩ , three u ϩ , two u , and one g orbital of the linear NO 2 fragment (NO 2 ϩ , NO 2 2ϩ , and NO 2 3ϩ all have linear equilibrium structures). This results in a full valence space of 13 active orbitals, containing 16 electrons. Careful test calculations by CASSCF showed that the highest valence u ϩ orbital is not needed to describe the static correlation in any of the treated processes and thus was excluded, resulting in an active space of 16 electrons in 12 orbitals (16e͞12o).The optimization of HeNO 2 3ϩ and the asymptotic fragments was carried out by the CASSCF͞CASCI approach, which required us to reduce further the active orbital space. For calculations of the complex this was accomplish...