势能面的锥形交叉引起的光解产物角分布的转动相关

if two potential energy curves cross, the electronic state must beossing.
什么是锥形交叉
J.Chem.Phys. 69:3242-3249
J. Chem. Phys., Vol. 104,
• Causes a rapid transfer between electronic states by passing though the intersection.
• Molecular structure at the intersectrion (transition state) affect product angular distribution
势能面的锥形交叉引起的 光解产物角分布的转动相关
内容
I. 锥形交叉(conical intersection)
II. 势能面的锥形交叉对光解产物角分布的影响： 简单参数碰撞模型(impact parameter model)
III. 最近研究实例： 用简单参数碰撞模型解释ketene光解产物角分布 变化的转动相关
锥形交叉的重要性
• Until recently conical intersections were thought to be very rare, however over the last five years we have established that conical intersections are ubiquitous! In fact, many photochemical reactions have now been shown to involve conical intersections.
锥形交叉的特点：
• The main feature of a conical intersection is its non-adiabatic nature – that is, that the breakdown of the Born-Oppenheimer approximation (that electronic and nuclear motion separable)
• Conical intersections appear to govern a large variety of fundamental processes, such as internal conversion, intersystem crossing, electron transfer and photoinduced reactions
In a two coordinate system, intersection of the two surfaces can occur at one molecular geometry. If the potential energy surfaces are plotted as functions of the two coordinates, they form a cone centered at the degeneracy point. The name conical intersection comes from this observation. At the point of conical intersection the two surfaces are degenerate. The molecule can cross from one electronic state to another.
很多小分子的势能面存在锥形交叉
• NH3(for example) • H2O • NO2 • O3 • ……
锥形交叉对小分子的解离动力学有什 么样的影响？
NH3的势能面
Model system for nonadiabatic dissociation dynamics
This rule fails for polyatomic molecules:
two electronic states,even of the same symmetry are allowed to cross because of the additional degrees of freedom.
• Importance in the organic chemistry and photobiology. For example, the stability of DNA with respect to the UV irradiation is due to such conical interrection. The molecular wave packet excited to some electronic excited state by the UV photon follows the slope of the potential energy surface and reaches the conical intersection from above. At this point the very large vibronic coupling induces a non radiative transition (surface hopping) which leads the molecule back
to its electronic ground state.
势能面为什么会发生交叉:
Non-crossing rule for diatomic molecules: potential energy curves corresponding to electronic states of the same symmetry cannot cross.