On the molecular mechanism of non-radiative decay of nitrobenzene and the unforeseen challenges this simple molecule holds for electronic structure theory
On the molecular mechanism of non-radiative decay of nitrobenzene and the unforeseen challenges this simple molecule holds for electronic structure theory
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DOI：
10.1039/c4cp01232a
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发表时间：
2014-01-01
影响因子：
3.3
通讯作者：
Dreuw, Andreas
中科院分区：
文献类型：
--
作者：
Mewes, Jan-M.;Jovanovic, Vladimir;Dreuw, Andreas
在这项工作中，我们展示了单取代芳香族化合物硝基苯从亮单线态到电子基态的非辐射衰变的完整机理图。该机制涉及沿着硝基的三个主要内坐标的内部转换（IC）和系统间交叉（ISC），并一致地解释了实验结果。通过ISC从最低三重态的弛豫沿着硝基的面外弯曲坐标发生，而初始IC以及进入三重态流形的超快ISC沿着尚未考虑的对称NO拉伸和ONO弯曲模式发生。所提出的机制基于采用 ADC3、MOM-CCSD(T)、EOM-CCSD、DFT/MRCI 和 CAS-SCF/NEVPT2 水平理论的高级单参考和多参考电子结构计算，正如我们将证明的那样，这对于确保可靠且足够准确的硝基苯理论描述是绝对必要的。对三阶方法的需求可以追溯到硝基苯第二激发单线态约 50% 的大双激发特征。因此，二阶近似耦合簇 (CC2) 和部分偶数 (EOM-) CCSD 等二阶方法会产生定性错误的激发态图像。令人惊讶的是，基态几何的描述在 CC2 以及部分 CCSD 理论水平上已经存在问题。
In this work, we present a complete mechanistic picture of the non-radiative decay of the monosubstituted aromatic compound nitrobenzene from the bright singlet state to the electronic ground state. This mechanism involves internal conversion (IC) and inter-system crossing (ISC) along three dominating internal coordinates of the nitro group and consistently explains the experimental findings. Relaxation from the lowest triplet state via ISC occurs along the out-of-plane bending coordinate of the nitro group, while initial IC as well as ultrafast ISC into the triplet manifold take place along symmetric NO stretching and ONO bending modes that have not been considered yet. The proposed mechanism is based on high-level single-and multi-reference electronic structure calculations employing ADC3, MOM-CCSD(T), EOM-CCSD, DFT/MRCI and CAS-SCF/NEVPT2 levels of theory, which is, as we will demonstrate, absolutely necessary to assure a reliable and sufficiently accurate theoretical description of nitrobenzene. The need for third-order methods will be traced back to the large double-excitation character of about 50% of the second excited singlet state of nitrobenzene. As a result, second-order methods like approximate coupled-cluster of second order (CC2) and partially even (EOM-) CCSD yield a qualitatively wrong picture of the excited states. Surprisingly, already the description of the ground state geometries is problematic at the CC2 and partially also CCSD level of theory.