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Vibronic and spin-orbit coupling effects in the absorption spectra of pyrazine: A quantum chemical approach

Dinkelbach Fabian (Institute of Theoretical and Computational Chemistry, Heinrich-Heine-University Düsseldorf, Düsseldorf, Germany)
Marian Christel M. (Institute of Theoretical and Computational Chemistry, Heinrich-Heine-University Düsseldorf, Düsseldorf, Germany)

Derivatives of dipole transition moments between spin–orbit coupled (SOC) multireference configuration interaction wave functions have been used in conjunction with vibrational frequencies from density functional theories to compute vibronic S1←S0 (11B3u11Ag ) and T1←S0 (13B3u←11Ag) absorption spectra in Herzberg–Teller approximation. The experimentally known spectra are well reproduced. The calculations reveal unexpectedly small spin–orbit couplings between the 13B3u (3nπ*) state and nearby optically bright 1B2u (1ππ*) states, thus explaining the absence of the 1b1g10 (v10a10 ) fundamental in the vibrational fine-structure of the T1←S0 transition. Adiabatically, two triplet states are found below the S1 state. The out-of-plane distorted T2 minimum results from a pseudo Jahn–Teller interaction between two 3ππ* states of B1u and B2u symmetry. At the D2h-symmetric S0 and S1 minimum geometries, the latter states are located well above S1. The S1 and T2 potentials intersect at geometries far away from the Franck–Condon region. This explains the apparently contradictory results that the linewidth in the higher energy regime above the T1←S0 origin suddenly broadens while no trace of a second triplet state, located energetically below the S1 origin, could be identified in phosphorescence excitation spectra of the ultracold isolated pyrazine molecule.

Keywords: density functional theory, multireference configuration interaction, Herzberg-Teller coupling, singlet-triplet transitions, azabenzenes