Singlet-triplet separation induced by the Pseudo Jahn-Teller effect examples: The CX₂ and NX₂⁺ (X=H, Cl) molecules

A theoretical investigation with high-level multireference methods is performed to reveal the Jahn-Teller origin of the triplet and singlet bent structures for CH₂, CCl₂, NH₂⁺, and NCl₂⁺ molecules and rationalize the relative energy ordering of the lowest singlet and triplet states. At the highest-symmetry linear configurations of these triatomic systems, the Pseudo Jahn-Teller effect (PJTE) between the ground state 3σg- and excited state ³_Πu upon the bending mode, denoted as (3σg-+3Πu)⊗πu, is the reason of producing triplet ³B₁ bent structures, and the combined PJTE of (¹_δg + ¹_Πu) ⊗ _πu and Renner-Teller effect (RTE) of ¹_δg state result in the formations of singlet ¹A₁ and ¹B₁ minima. The PJTE of "δ-Π" plays a dominant role for the instability of degenerate Delta state which is subject to RTE. The spin-crossover induced by PJTE is responsible for the reversed order of singlet and triplet ground states in CH₂/NH₂⁺ and CCl₂/NCl₂⁺. The different singlet-triplet energy separations of those molecules can be rationalized from different Jahn-Teller vibronic coupling intensity at linear configurations.