Department of Applied Chemistry, National Chiao Tung University, Hsinchu 30010, Taiwan.
Chem Asian J. 2011 Nov 4;6(11):2977-85. doi: 10.1002/asia.201100472. Epub 2011 Sep 16.
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.
采用 DFT/TD-DFT、CASSCF 和 CASPT2 及相关量子化学计算方法，研究了吡嗪基态和最低四个单重激发态的几何、能量和光谱性质。通常认为由于偶极禁阻（1）A(u)←(1)A(g)跃迁，第二单重态 nπ*（1）A(u)态是暗态，但我们的新模拟表明，通过通过振子耦合，该态可以通过从相邻的 nπ*（1）B(3u)和 ππ*（1）B(2u)态强度借用，在吸收光谱中可见。势能面的扫描进一步表明，（1）A(u)态在后者的平衡附近与（1）B(2u)态相交，这意味着它参与了超快弛豫过程。
