Castillo J F, Aoiz F J, Bañares L
Departamento de Química Física I, Facultad de Química, Universidad Complutense, 28040 Madrid, Spain.
J Chem Phys. 2006 Sep 28;125(12):124316. doi: 10.1063/1.2357741.
An ab initio interpolated potential energy surface (PES) for the Cl+CH(4) reactive system has been constructed using the interpolation method of Collins and co-workers [J. Chem. Phys. 102, 5647 (1995); 108, 8302 (1998); 111, 816 (1999); Theor. Chem. Acc. 108, 313 (2002)]. The ab initio calculations have been performed using quadratic configuration interaction with single and double excitation theory to build the PES. A simple scaling all correlation technique has been used to obtain a PES which yields a barrier height and reaction energy in good agreement with high level ab initio calculations and experimental measurements. Using these interpolated PESs, a detailed quasiclassical trajectory study of integral and differential cross sections, product rovibrational populations, and internal energy distributions has been carried out for the Cl+CH(4) and Cl+CD(4) reactions, and the theoretical results have been compared with the available experimental data. It has been shown that the calculated total reaction cross sections versus collision energy for the Cl+CH(4) and Cl+CD(4) reactions is very sensitive to the barrier height. Besides, due to the zero-point energy (ZPE) leakage of the CH(4) molecule to the reaction coordinate in the quasiclassical trajectory (QCT) calculations, the reaction threshold falls below the barrier height of the PES. The ZPE leakage leads to CH(3) and HCl coproducts with internal energy below its corresponding ZPEs. We have shown that a Gaussian binning (GB) analysis of the trajectories yields excitation functions in somehow better agreement with the experimental determinations. The HCl(v'=0) and DCl(v'=0) rotational distributions are as well very sensitive to the ZPE problem. The GB correction narrows and shifts the rotational distributions to lower values of the rotational quantum numbers. However, the present QCT rotational distributions are still hotter than the experimental distributions. In both reactions the angular distributions shift from backward peaked to sideways peaked as collision energy increases, as seen in the experiments and other theoretical calculations.
利用柯林斯及其同事的插值方法[《化学物理杂志》102, 5647 (1995); 108, 8302 (1998); 111, 816 (1999); 《理论化学学报》108, 313 (2002)]构建了Cl + CH₄反应体系的从头算插值势能面（PES）。从头算计算采用单双激发理论的二次组态相互作用来构建PES。使用了一种简单的缩放全相关技术来获得一个PES，其给出的势垒高度和反应能量与高水平的从头算计算和实验测量结果高度吻合。利用这些插值PES，对Cl + CH₄和Cl + CD₄反应的积分和微分截面、产物转动振动态分布以及内能分布进行了详细的准经典轨迹研究，并将理论结果与现有的实验数据进行了比较。结果表明，Cl + CH₄和Cl + CD₄反应计算得到的总反应截面与碰撞能量的关系对势垒高度非常敏感。此外，由于在准经典轨迹（QCT）计算中CH₄分子的零点能（ZPE）泄漏到反应坐标上，反应阈值低于PES的势垒高度。ZPE泄漏导致CH₃和HCl副产物的内能低于其相应的ZPE。我们已经表明，对轨迹进行高斯分箱（GB）分析得到的激发函数在某种程度上与实验测定结果更吻合。HCl(v' = 0)和DCl(v' = 0)的转动分布对ZPE问题也非常敏感。GB校正使转动分布变窄并向较低的转动量子数偏移。然而，目前的QCT转动分布仍然比实验分布更热。在这两个反应中，随着碰撞能量增加，角分布从向后峰值变为侧向峰值，这与实验和其他理论计算结果一致。
