Topological effects in vibronically coupled degenerate electronic states: a case study on nitrate and benzene radical cation

Abstract

We carry out detailed investigation for topological effects of two molecular systems, NO₃ radical and C₆H₆⁺+ (Bz⁺) radical cation, where the dressed adiabatic, dressed diabatic, and adiabatic-via-dressed diabatic potential energy curves (PECs) are generated employing ab initio calculated adiabatic and diabatic potential energy surfaces (PESs). We have implemented beyond Born-Oppenheimer (BBO) theory for constructing smooth, single-valued, and continuous diabatic PESs for five coupled electronic states. In the case of NO₃ radical, the nonadiabatic coupling terms (NACTs) among the low-lying five electronic states, namely, X̂²A₂'(1²B₂), ²E"(1²A₂ and 1²B₁), and B̂²E'(1²A₁ and 2²B₂), bear the signature of Jahn–Teller (JT) interactions, pseudo JT (PJT) interactions, and accidental conical intersections (CIs). Similarly, Bz⁺ radical cation also exhibits JT, PJT, and accidental CIs in the interested domain of nuclear configuration space. In order to generate dressed PECs, two components of degenerate in-plane asymmetric stretching modes are selectively chosen for both the molecular species (Q_3x–Q_3y pair for NO3 radical and Q_16x–Q_16y pair for Bz⁺ radical cation). The JT coupling between the electronic states is essentially originated through the asymmetric stretching normal mode pair, where the coupling elements exhibit symmetric and nonlinear functional behavior along Q_3x and Q_16x normal modes.