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 nπ* 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 nπ* (1)B(3u) and ππ* (1)B(2u) states through vibronic coupling. The scans on potential-energy surfaces further indicated that the (1)A(u) state intersects with the (1)B(2u) states near the equilibrium of the latter, thus implying its participation in the ultrafast relaxation process.

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http://dx.doi.org/10.1002/asia.201100472DOI Listing

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