The Role of the nπ* 1Au State in the Photoabsorption and Relaxation of Pyrazine

The geometric, energetic, and spectroscopic properties of the ground state and the lowest four singlet excited states of pyrazine have been studied by using DFT/TD-DFT, CASSCF, CASPT2, and related quantum chemical calculations. The second singlet n pi* state, (1)A(u), which is conventionally regarded dark due to the dipole-forbidden (1)A(u) <- (1)A(g) transition, has been investigated in detail. Our new simulation has shown that the state could be visible in the absorption spectrum by intensity borrowing from neighboring n pi* B-1(3u) and pi pi* B-1(2u) states through vibronic coupling. The scans on potential-energy surfaces further indicated that the (1)A(u) state intersects with the B-1(2u) states near the equilibrium of the latter, thus implying its participation in the ultrafast relaxation process.