The Spin-vibronic Coupling Energy Levels In Molecular Spectroscopy Studied By Diabatic Model

The Born-Oppenheimer (BO) approximation is a fundamental approximation inmolecular physics. However, the BO approximation is broken down when the couplingbetween the electronic and nuclear motion becomes strong, for instance, the Jahn-Tellereffect or Renner-Teller effect in molecules. The diabatic model is an effective method ofdealing with such problems. The key point is to construct new diabatic electronic statesto reduce or even eliminate the nonadiabatic coupling terms. On the basis of previousstudies, we improved this model and did some researches on several C3vsymmetrymolecules with E etype Jahn-Teller effect. In addition to this, we studied theasymmetric isotopologues with special Jahn-Teller effect.(1) CH3Cl+(X2E) is the benchmark ion for Jahn-Teller effect research, which hasvery complex spectral structure because of the presence of the Jahn-Teller effect andspin-orbit couplings. The spin-vibronic energy levels of CH335Cl+/CH337Cl+(X2E) up to3500cm-1were calculated using diabatic model, and the ZEKE experimental resultswere assigned according to the theoretical predictions. All of the six vibrational modesand the two-mode couplings were taken into account. The calculated spin-orbit splitting(224.6cm-1) of the ground state is in good agreement with the experimental result (219±3cm-1). The root-mean-square errors of the ab initio calculation are20and18cm1,respectively.(2) As the isotopologues CH3Cl+and CD3Cl+maintain the same electronicstructure, but they have different vibrational frequencies and spectroscopic parameters.The spin-vibronic energy levels of CD335Cl+/CD337Cl+(X2E) up to3500cm-1werecalculated using diabatic model, and the ZEKE experimental results were assigned. Theroot-mean-square errors of the ab initio calculation are both8cm1. The above resultsshow that the diabatic model can provide highly accurate results for the case with strongspin-orbit coupling and Jahn-Teller effect.(3) The asymmetric isotopic substitution maintains the electronic structure of theJahn-Teller active molecules, but all vibrational modes break their geometric symmetry,which results in extremely complex vibrational structure. There are limited studies ofsuch cases due to their complexity. We expanded the diabatic model to the asymmetricisotopologues and calculated the spectroscopic parameters by introducing Duschinsky Rotation. The spin-vibronic energy levels of CH3O/CH2DO/CHD2O/CD3O (X2E) werecalculated for the first time. The spin-orbit splittings for the vibrational ground state are61.8/73.5/70.2/54.5cm-1, in good agreement with the reported experimental results62/75.1/72.0/54cm-1. Our study also shows that the larger spin-orbit splittings ofCH2DO/CHD2O are larger than those of CH3O/CD3O due to the fact that all theirvibrational modes are Jahn-Teller active.