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首页> 外文期刊>The Journal of Chemical Physics >Nonadiabatic effects in C-Br bond scission in the photodissociation of bromoacetyl chloride
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Nonadiabatic effects in C-Br bond scission in the photodissociation of bromoacetyl chloride

机译：溴乙酰氯光解离中C-Br键断裂的非绝热作用

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Bromoacetyl chloride photodissociation has been interpreted as a paradigmatic example of a process in which nonadiabatic effects play a major role. In molecular beam experiments by Butler and co-workers [J. Chem. Phys. 95, 3848 (1991); J. Chem. Phys. 97, 355 (1992)], BrCH2C(O)Cl was prepared in its ground electronic state (S-0) and excited with a laser at 248 nm to its first excited singlet state (S-1). The two main ensuing photoreactions are the ruptures of the C-Cl bond and of the C-Br bond. A nonadiabatic model was proposed in which the C-Br scission is strongly suppressed due to nonadiabatic recrossing at the barrier formed by the avoided crossing between the S-1 and S-2 states. Recent reduced-dimensional dynamical studies lend support to this model. However, another interpretation that has been given for the experimental results is that the reduced probability of C-Br scission is a consequence of incomplete intramolecular energy redistribution. To provide further insight into this problem, we have studied the energetically lowest six singlet electronic states of bromoacetyl chloride by using an ab initio multiconfigurational perturbative electronic structure method. Stationary points (minima and saddle points) and minimum energy paths have been characterized on the S-0 and S-1 potential energy surfaces. The fourfold way diabatization method has been applied to transform five adiabatic excited electronic states to a diabatic representation. The diabatic potential energy matrix of the first five excited singlet states has been constructed along several cuts of the potential energy hypersurfaces. The thermochemistry of the photodissociation reactions and a comparison with experimental translational energy distributions strongly suggest that nonadiabatic effects dominate the C-Br scission, but that the reaction proceeds along the energetically allowed diabatic pathway to excited-state products instead of being nonadiabatically suppressed. This conclusion is also supported by the low values of the diabatic couplings on the C-Br scission reaction path. The methodology established in the present study will be used for the construction of global potential energy surfaces suitable for multidimensional dynamics simulations to test these preliminary interpretations. (c) 2006 American Institute of Physics.
机译：溴乙酰氯的光解离已被解释为非绝热效应起主要作用的过程的范例。在巴特勒及其同事的分子束实验中[J．化学物理95，3848（1991）; J.化学物理97，355（1992）]，制备了处于基态电子状态（S-0）的BrCH2C（O）Cl，并用248 nm的激光激发至其第一激发单重态（S-1）。随后发生的两个主要光反应是C-Cl键和C-Br键的断裂。提出了一种非绝热模型，其中，由于在S-1和S-2状态之间避免了交叉而形成的势垒处的非绝热重新交叉，强烈抑制了C-Br分裂。最近的降维动力学研究为该模型提供了支持。但是，对实验结果的另一种解释是，C-Br断裂的可能性降低是分子​​内能量重新分配不完全的结果。为了提供对该问题的进一步了解，我们通过使用从头开始的多结构微扰电子结构方法研究了溴乙酰氯的能量最低的六个单重态电子态。在S-0和S-1势能面上已确定了固定点（最小点和鞍点）和最小能量路径。四重绝热方法已用于将五个绝热激发电子态转换为非绝热表示。前五个激发单重态的绝热势能矩阵是沿着势能超表面的多次切割而构建的。光解离反应的热化学性质以及与实验翻译能量分布的比较强烈表明，非绝热作用主导了C-Br分裂，但是该反应沿着能量上允许的非绝热途径进行，直至激发态产物，而不是被绝热抑制。 C-Br分裂反应路径上的非绝热偶合值较低也支持该结论。在本研究中建立的方法将用于构造适合多维动力学模拟的全球势能面，以测试这些初步解释。 （c）2006年美国物理研究所。

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