Topological Effects in Vibronically Coupled Degenerate Electronic States: A Case Study on Nitrate and Benzene Radical Cation

[Image: see text] We carry out detailed investigation for topological effects of two molecular systems, NO(3) radical and C(6)H(6)(+) (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 [J. Phys. Chem. A2017,121, 6314–6326]. In the case of NO(3) radical, the nonadiabatic coupling terms (NACTs) among the low-lying five electronic states, namely, X̃(2)A(2)(′) (1(2)B(2)), A~(2)E″ (1(2)A(2) and 1(2)B(1)), and B~(2)E′ (1(2)A(1) and 2(2)B(2)), 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 NO(3) 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.