Beyond Born Oppenheimer (BBO)-Based Multi-Electronic State Diabatic Hamiltonians: Reactive Scattering Processes

When the electron-nuclear coupling among the electronic states of the reactive species are non-negligible, the calculated cross-sections and rate coefficients of tri-atomic chemical reactions can provide the mechanism of the reaction in conjunction with the experimental data. The construction of multi-state diabatic Hamiltonian is the inclusion of nonadiabatic coupling through Beyond Born-Oppenheimer theory, whereas the spin-orbit couplings could be incorporated as and when required. Once such diabatic Hamiltonians are derived and scattering calculations are performed, the reaction attributes generally show good agreement with the experimental data, otherwise, single surface adiabatic calculations depict deviations in many cases. The deviation and agreement with the experimental results is more pronounced over the energy range facing the conical intersection (CI) between the adiabatic potential energy surfaces. We have presented several examples of prototype reactive systems, where symmetry driven and accidental Jahn-Teller (JT)-type CI as well as Renner-Teller (RT) interaction influencing the reaction processes. Calculated cross-sections and rate coefficients are compared with experimentally measured ones to depict the necessity of inclusion of electron-nuclear coupling terms.