The foundation of molecular physics has long been the Born–Oppenheimer approximation, which neglects vibronic (non-adiabatic) coupling. However, in recent years it has been found that many chemical reactions, particularly photochemical ones, proceed when molecules are in the vicinity of a conical intersection (CI) of electronic states. In just such circumstances vibronic (non-adiabatic) coupling between the states becomes very significant for both the energetics and dynamics. This situation poses challenges for the computational chemist since most electronic structure calculations assume the validity of the Born–Oppenheimer approximation. Similarly, challenges for the experimental spectroscopist are posed because the ultrashort passage of molecules through CIs along reaction paths renders impossible the precise spectroscopic measurement of molecular parameters such as have been used traditionally to benchmark electronic structure calculations. Fortunately, another venerable theorem by Jahn and Teller provides a solution to this dilemma, as it guarantees that all nonlinear, polyatomic molecules that possess degenerate electronic states will distort leaving a CI at their high symmetry geometry within a potential that supports stationary states that can be probed by high resolution spectroscopy. Traditional adiabatic quantum chemistry calculations cannot describe this surface adequately because the non-adiabatic coupling diverges at the CI. However, calculations using a Hamiltonian, including both vibronic and spin-orbit coupling terms (parameterized by adiabatic electronic structure calculations), in a quasi-diabatic basis yield eigenvalues that can be compared directly to spectroscopic measurements of spin-vibronic energy levels. The corresponding eigenfunctions further allow the calculation of electron-spin and rotational structure contained within the spin-vibronic levels. This talk will compare the results of such calculations with detailed experimental observations of spectra involving Jahn-Teller active states of several reactive, chemical intermediates, including CH3O, NO3, and C5H5. .

Combining spectroscopic and computational characterizations of vibronically coupled molecular states

ACS Spring 2022

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