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 3 Σ g - and excited state 3 Π u upon the bending mode, denoted as ( 3 Σ g - + 3 Π u ) ⊗ π u , is the reason of producing triplet 3B1 bent structures, and the combined PJTE of ( 1 Δ g + 1 Π u ) ⊗ π u and Renner–Teller effect (RTE) of 1 Δ g 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.