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

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. A 2017, 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 3 x - Q 3 y pair for NO 3 radical and Q 16 x - Q 16 y 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 3 x and Q 16 x normal modes.