Diau Eric W G, Kötting Carsten, Sølling Theis I, Zewail Ahmed H
Amos Noyes Laboratory of Chemical Physics, California Institute of Technology, Pasadena, CA 91125, USA.
Chemphyschem. 2002 Jan 18;3(1):57-78. doi: 10.1002/1439-7641(20020118)3:1<57::AID-CPHC57>3.0.CO;2-F.
Time-dependant density functional theory (TDDFT) and ab initio methods (CASSCF and CASMP2) are applied here for the investigation of the excited-state potential energy surfaces of ketones studied experimentally in the accompanying paper, number IV in the series. The aim is to provide a general and detailed physical picture of the Norrish type-I reaction from S0 and S1 potentials (papers I and II) and from higher-energy potentials (papers III and IV). Particular focus here is on reactions following excitation to the 3s, 3p, and 3d Rydberg state and to the (nz-->pi*) and (pi-->pi*) valence states. It is shown that the active orbitals in the CASSCF calculations can be chosen so that accurate results are obtained with a small active space. Dynamic corrections of the state-specific CASSCF energies at the multireference MP2 level do not improve the results for the Rydberg states but are significant for the valence states. The geometries of the Rydberg states are similar to the ground state; the S1 and other valence states are not. A common property of the valence states is the elongated CO bond and the pyramidalization of the carbonyl carbon atom. As a consequence, these valence states cross all Rydberg states along the CO stretching coordinate and provide an efficient pathway down to the 3s Rydberg states (S2) through a series of conical intersections (CIs). The nonadiabatic coupling vector of the CI between the (pi-->pi*) and the 3s Rydberg states guides energy channeling into the asymmetric CC-stretching mode. The energy demand for the CC bond breakage (Norrish type-I) on the S2 surface is lower than that of the CI leading to the S1 state. This CC bond breakage leads to a linear excited state acetyl radical (3s Rydberg). Crossing a small barrier the 3s acyl radical can access a CI leading either to a second CC bond breakage or to a hot ground-state acetyl radical. The barriers for the Norrish type-I reaction on the various excited-state surfaces can be rationalized within the framework of valence-bond theory. The dynamic picture of the Norrish type-I reactions is now clear: The excitation to high-energy states leads to the nonconcerted breakage of the alpha-CC bonds by an "effective downhill" potential in space involving the active excitation center CO, CC stretching, and CCO bending nuclear motions, but not, as usually thought, a direct repulsive potential along the CC bond. In our accompanying paper (part IV), it is shown that the results from the experimental investigations of Norrish type-I reactions on the femtosecond timescale are consistent with these theoretical results.
本文采用含时密度泛函理论（TDDFT）和从头算方法（CASSCF和CASMP2），对系列论文第四篇（即随附论文）中实验研究的酮类化合物的激发态势能面进行了研究。目的是从S0和S1势能面（论文I和II）以及更高能量的势能面（论文III和IV）出发，提供Norrish I型反应的全面而详细的物理图景。这里特别关注激发到3s、3p和3d里德堡态以及(nz→π*)和(π→π*)价态后的反应。结果表明，在CASSCF计算中可以选择活性轨道，从而在较小的活性空间内获得准确的结果。在多参考MP2水平对特定态CASSCF能量进行的动态校正，对于里德堡态并没有改善结果，但对价态却很重要。里德堡态的几何结构与基态相似；S1和其他价态则不然。价态的一个共同特征是CO键伸长以及羰基碳原子的锥形化。因此，这些价态沿着CO伸缩坐标与所有里德堡态交叉，并通过一系列锥形交叉点（CIs）提供了一条通向3s里德堡态（S2）的有效途径。(π→π*)和3s里德堡态之间锥形交叉点的非绝热耦合矢量引导能量进入不对称CC伸缩模式。S2势能面上CC键断裂（Norrish I型）所需的能量低于导致S1态的锥形交叉点的能量。这种CC键断裂会产生一个线性激发态乙酰基自由基（3s里德堡态）。越过一个小势垒，3s酰基自由基可以进入一个锥形交叉点，该交叉点要么导致第二个CC键断裂，要么导致一个热基态乙酰基自由基。在价键理论的框架内，可以合理解释各种激发态势能面上Norrish I型反应的势垒。现在，Norrish I型反应的动态图景已经清晰：激发到高能态会导致α-CC键通过空间中一个“有效下坡”势能非协同断裂，该势能涉及活性激发中心CO、CC伸缩和CCO弯曲核运动，而不是像通常认为的那样沿着CC键存在直接的排斥势能。在我们的随附论文（第四部分）中，表明在飞秒时间尺度上对Norrish I型反应进行实验研究的结果与这些理论结果一致。
