Strong static and dynamic Jahn-Teller and pseudo-Jahn-Teller effects in niobium tetrafluoride.

We present a first-principles study of the static and dynamic aspects of the strong Jahn-Teller (JT) and pseudo-JT (PJT) effects in niobium tetrafluoride, NbF, in the manifold of its electronic ground state, E, and its first excited state, T. The complex topography of the full-dimensional multi-sheeted adiabatic JT/PJT surfaces is analyzed computationally at the complete-active-space self-consistent-field (CASSCF) and multireference second-order perturbation levels of electronic structure theory, providing a detailed characterization of minima, saddle points, and minimum-energy conical intersection points. The calculations reveal that the tetrahedral (T) configuration of NbF undergoes strong JT distortions along the bending mode of e symmetry, yielding tetragonal molecular structures of D symmetry with T → D stabilization energies of about 2000 cm in the X̃E state and about 6400 cm in the ÃT state. In addition, there exists strong X̃E-ÃT PJT coupling via the bending mode of t symmetry, which becomes important near the crossing seam of the X̃E and ÃT potential energy surfaces. A five-state five-mode JT/PJT vibronic-coupling Hamiltonian is constructed in terms of symmetry-invariant polynomial expansions of the X̃E and ÃT diabatic potential energy surfaces in the e and t bending coordinates. The parameters of the Hamiltonian are determined by a least-squares fit of its eigenvalues to the CASSCF ab initio data. The vibronic spectra and the time evolution of adiabatic electronic population probabilities are computed with the multi-configuration time-dependent Hartree method. The complexity of the spectra reflects the effects of the exceptionally strong E × e and T × e JT couplings and (E + T) × (e + t) PJT coupling. The time evolution of the populations of the adiabatic electronic states after the initial preparation of the ÃT state reveals the femtosecond nonadiabatic dynamics through a multidimensional seam of conical intersection. These results represent the first study of the static and dynamical JT/PJT effects in the X̃E and ÃT electronic states of NbF.