Departamento de Química Física, Facultad de Ciencias Químicas, Universidad de Salamanca, 37008 Salamanca, Spain.
Phys Chem Chem Phys. 2011 Aug 14;13(30):13656-69. doi: 10.1039/c0cp02452j. Epub 2011 Jul 4.
A complete quantum study for the state-to-state Li + HF(v,j,m) → LiF(v',j',Ω') + H reactive collisions has been performed using a wave packet method, for different initial rotational states and helicity states of the reactants. The state-to-state differential cross section has been simulated, and the polarization of products extracted. It is found that the reactivity is enhanced for nearly collinear collisions, which produces a vibrational excitation of HF, needed to overcome the late barrier. It is also found that LiF(v' = 0) products are preferentially forward scattered, while vibrationally excited LiF(v' = 1 and 2) are backward scattered. These results are interpreted with a simple reaction mechanism, based on the late character and bent geometry of the transition state, originating from a covalent/ionic crossing, which consists of two steps: the arrival at the transition state and the dissociation. In the first step, in order to get to the saddle point some HF vibrational excitation is required, which favors head-on collisions and therefore low values of m. In the second step a fast dissociation of H atom takes place, which is explained by the ionic Li(+)F(-)H character of the bent transition state: the FH(-) is repulsive making that H depart rapidly leaving a highly rotating LiF molecule. For the higher energy analyzed, where resonances slightly contribute, the orientation and alignment of product rotational states, referred to as reactants frame (with the z-axis parallel to k), are approximately constant with the scattering angle. The alignment is close to -1, showing that j' is perpendicular to k, while starting from initial states with well defined rotational orientation, as states with pure m values, the final rotational are also oriented. It is also found that when using products frame (with the z'-axis parallel to k') the rotational alignment and orientation of products varies a lot with the scattering angle just because the z' axis changes from being parallel to anti-parallel to k when varying from θ = 0 to π.
已使用波包方法对 Li + HF(v,j,m) → LiF(v',j',Ω') + H 反应碰撞的态-态进行了完整的量子研究，针对反应物的不同初始旋转态和螺旋态。模拟了态-态微分截面，并提取了产物的极化。研究发现，对于近乎共线碰撞，反应性增强，从而使 HF 发生振动激发，这是克服后期势垒所必需的。还发现，LiF(v'=0)产物优先向前散射，而振动激发的 LiF(v'=1 和 2)则向后散射。这些结果通过基于过渡态的后期特征和弯曲几何形状的简单反应机制来解释，该机制源自共价/离子交叉，由两个步骤组成：到达过渡态和离解。在第一步中，为了到达鞍点，需要 HF 振动激发，这有利于迎头碰撞，因此 m 值较低。在第二步中，H 原子迅速离解，这可以用弯曲过渡态的离子 Li(+)F(-)H 特性来解释：FH(-)是排斥的，使 H 迅速离开，留下高度旋转的 LiF 分子。对于分析的较高能量，其中共振略有贡献，产物旋转态的取向和对准，称为反应物框架（z 轴平行于 k），与散射角大致保持不变。对准接近-1，表明 j'垂直于 k，而从具有明确定义旋转取向的初始态开始，例如具有纯 m 值的态，最终旋转也取向。还发现，当使用产物框架（z'轴平行于 k'）时，产物的旋转对准和取向随散射角变化很大，这是因为当从θ=0 变为π时，z'轴从平行变为反平行于 k。
