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

Abstract

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 (1¹B_3u¬1¹A_g ) and T₁¬S₀ (1³B_3u¬ 1¹A_g) abs­orption spectra in Herzberg–Teller approximation. The expe­rimentally known spectra are well reproduced. The calculations reveal unexp­ectedly small spin–orbit couplings between the 1³B_3u (³nπ*) state and nearby optically bright ¹B_2u (¹ππ*) states, thus explaining the absence of the  () 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 B_1u and B_2u symmetry. At the D_2h-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.