Tapavicza Enrico, Tavernelli Ivano, Rothlisberger Ursula, Filippi Claudia, Casida Mark E
Laboratory of Computational Chemistry and Biochemistry, BCH 4107 EPF Lausanne, CH-1015 Lausanne, Switzerland.
J Chem Phys. 2008 Sep 28;129(12):124108. doi: 10.1063/1.2978380.
We present a mixed time-dependent density-functional theory (TDDFT)/classical trajectory surface hopping (SH) study of the photochemical ring opening in oxirane. Previous preparatory work limited to the symmetric CC ring-opening pathways of oxirane concluded that the Tamm-Dancoff approximation (TDA) is important for improving the performance of TDDFT away from the equilibrium geometry. This observation is supported by the present TDDFT TDA/SH calculations which successfully confirm the main experimentally derived Gomer-Noyes mechanism for the photochemical CO ring opening of oxirane and, in addition, provide important state-specific information not easily accessible from experiments. In particular, we find that, while one of the lowest two excited states is photochemically relatively inert, excitation into the other excited state leads predominantly to rapid ring opening, cyclic-C(2)H(4)O-->()CH(2)CH(2)O(). This is followed by hopping to the electronic ground state where hot (4000 K) dynamics leads to further reactions, namely, ()CH(2)CH(2)O()-->CH(3)CHO-->()CH(3)+(*)CHO and CH(4)+CO. We note that, in the dynamics, we are not limited to following minimum energy pathways and several surface hops may actually be needed before products are finally reached. The performance of different functionals is then assessed by comparison of TDDFT and diffusion Monte Carlo potential energy curves along a typical TDDFT TDA/SH reaction path. Finally, although true (S(0),S(1)) conical intersections are expected to be absent in adiabatic TDDFT, we show that the TDDFT TDA is able to approximate a conical intersection in this system.
我们展示了一项关于环氧乙烷光化学开环反应的混合含时密度泛函理论（TDDFT）/经典轨迹表面跳跃（SH）研究。先前仅限于环氧乙烷对称碳 - 碳开环途径的预备性工作得出结论，即塔姆 - 丹科夫近似（TDA）对于在远离平衡几何构型时提高TDDFT的性能很重要。这一观察结果得到了当前TDDFT TDA/SH计算的支持，该计算成功地证实了主要从实验推导得出的环氧乙烷光化学碳 - 氧开环反应的戈默 - 诺伊斯机制，此外，还提供了从实验中不易获得的重要态特异性信息。特别是，我们发现，虽然最低的两个激发态之一在光化学上相对惰性，但激发到另一个激发态主要导致快速开环，即环状 - C(2)H(4)O→()CH(2)CH(2)O()。随后跃迁到电子基态，在那里热（4000K）动力学导致进一步反应，即()CH(2)CH(2)O()→CH(3)CHO→()CH(3)+(*)CHO和CH(4)+CO。我们注意到，在动力学过程中，我们不限于沿着最小能量路径，实际上在最终到达产物之前可能需要几次表面跳跃。然后通过比较沿着典型的TDDFT TDA/SH反应路径的TDDFT和扩散蒙特卡罗势能曲线来评估不同泛函的性能。最后，尽管绝热TDDFT中预计不存在真正的（S(0)，S(1)）锥形交叉点，但我们表明TDDFT TDA能够在该系统中近似一个锥形交叉点。
