Hennig Carsten, Schmatz Stefan
Institut für Physikalische Chemie, Universität Göttingen, Tammannstrasse 6, D-37077 Göttingen, Germany.
J Chem Phys. 2005 Jun 15;122(23):234307. doi: 10.1063/1.1924407.
The exothermic gas-phase bimolecular nucleophilic substitution (S(N)2) reaction Cl(-)+CH(3)Br (upsilon1',upsilon2',upsilon3')-->ClCH(3) (upsilon1,upsilon2,upsilon3)+Br- and the corresponding endothermic reverse reaction have been studied by time-independent quantum scattering calculations in hyperspherical coordinates on a coupled-cluster potential-energy surface. The dimensionality-reduced model takes four degrees of freedom into account [Cl-C and C-Br stretching modes (quantum numbers upsilon3' and upsilon3); totally symmetric modes of the methyl group, i.e., C-H stretching (upsilon1' and upsilon1) and umbrella bending vibrations (upsilon2' and upsilon2)]. Diagonalization of the Hamiltonian was performed employing the Lanczos algorithm with a variation of partial reorthogonalization. A narrow grid in the total energy was employed so that long-living resonance states could be resolved and extracted. While excitation of the reactant umbrella bending mode already leads to a considerable enhancement of the reaction probability, its combination with vibrational excitation of the broken C-Br bond, (0, 1, 1), results in a strong synergic effect that can be rationalized by the similarity with the classical transitional normal mode. Exciting the C-H stretch has a non-negligible effect on the reaction probability, while for larger translational energies this mode follows the expected spectatorlike behavior. Combination of C-Br stretch and symmetric C-H, (1,0,1), stretch does not show a cooperative effect. Contrary to the spectator mode concept, energy originally stored in the C-H stretching mode is by no means conserved, but almost completely released in other modes of the reaction products. Products are most likely formed in states with a high degree of excitation in the new C-Cl bond, while the internal modes of the methyl group are less important. Reactants with combined umbrella/C-Br stretch excitation, (0, 1, 1), may yield products with two quanta in the umbrella mode.
通过在超球坐标下基于耦合簇势能面的与时间无关的量子散射计算，研究了放热的气相双分子亲核取代（S(N)2）反应Cl(-)+CH(3)Br (υ1',υ2',υ3')-->ClCH(3) (υ1,υ2,υ3)+Br-以及相应的吸热逆反应。降维模型考虑了四个自由度[Cl-C和C-Br伸缩模式（量子数υ3'和υ3）；甲基的完全对称模式，即C-H伸缩（υ1'和υ1）和伞形弯曲振动（υ2'和υ2）]。采用带有部分重正交化变体的兰索斯算法对哈密顿量进行对角化。在总能量上使用了一个窄网格，以便能够分辨和提取长寿命的共振态。虽然反应物伞形弯曲模式的激发已经导致反应概率有相当大的提高，但其与断裂的C-Br键的振动激发（0, 1, 1）相结合，会产生强烈的协同效应，这可以通过与经典过渡正常模式的相似性来解释。激发C-H伸缩对反应概率有不可忽略的影响，而对于较大的平动能，该模式遵循预期的旁观者行为。C-Br伸缩和对称C-H伸缩（1,0,1）的组合没有显示出协同效应。与旁观者模式概念相反，最初存储在C-H伸缩模式中的能量绝不是守恒的，而是几乎完全在反应产物的其他模式中释放。产物最有可能在新的C-Cl键具有高度激发的状态下形成，而甲基的内部模式则不太重要。具有伞形/C-Br伸缩组合激发（0, 1, 1）的反应物可能会产生在伞形模式中有两个量子的产物。
