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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >Trajectory Investigations of the Dissociation Dynamics of Vinyl Bromide on an ab Initio Potential-Energy Surface
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Trajectory Investigations of the Dissociation Dynamics of Vinyl Bromide on an ab Initio Potential-Energy Surface

机译：从头算势能表面上乙烯基溴化物解离动力学的轨迹研究

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The reaction dynamics of vibrationally excited vinyl bromide have been investigated using classical trajectory methods on a global, analytic potential-energy hypersurface that is developed primarily by least-squares fitting of appropriately chosen functional forms to the results of ab initio electronic structure calculations. These calculations are carried out at the MP4 level of theory with all single, double, and triple excitations included. A 6-31G(d,p) basis set is employed for the carbon and hydrogen atoms. Huzinaga's (433321/4321/4) and a polarization f orbital with an exponent of 0.5 is used for the bromine atom. The present calculations focus upon the determination of the dependence of the potential upon the stretching coordinates for the bonded atoms in vinyl bromide, the C-C-H and C-C-Br bending coordinates and the dihedral angles. The couplings between these coordinates are also investigated. The total ab initio database is obtained by combining these results with previously reported studies of the vinylidene-acetylene system and our previous calculations of saddle-point geometries and energies for the various decomposition channels, reactant and product equilibrium structures, and vibrational frequencies. The analytic surface fitted to this data base (PES1) is then modified by adjustment of the potential curvatures at equilibrium to provide a better fit to the measured IR and Raman vibrational frequencies of vinyl bromide while simultaneously holding all other topographical features of the surface constant to the maximum extent possible. The surface so developed is labeled PES2. Finally, we have arbitrarily altered the reaction coordinate curvatures for three-center HBr and H_2 elimination to produce a third potential surface (PES3). The dissociation dynamics of vinyl bromide on each of these potential surfaces are investigated at several excitation energies in the range 4.5 to 6.44 eV. Total decomposition rate coefficients and product branching ratios are computed as a function of excitation energy. The HBr vibrational-state distribution is computed and found to be Boltzmann with an effective vibrational temperature of 7084 K on PES1. These results are in virtually exact agreement with recently reported measurements of this distribution. Finally, we have investigated the dissociation mechanisms for three-center H_2 and HBr elimination reactions. The results show that the dynamics are very similar on PES1, PES2, and PES3. Consequently, small variations in potential-energy curvatures at equilibrium and along the reaction coordinates do not exert significant influence upon the dissociation dynamics. However, some large qualitative variations between the dynamics on the ab initio surface and the more empirical surface previously employed are found to exist. We conclude that great care must be exercised when such surfaces are used to study reaction dynamics in polyatomic systems.
机译：振动激发的乙烯基溴的反应动力学已经使用经典的轨迹方法在全局解析势能超曲面上进行了研究，该曲面主要是通过对适当选择的功能形式进行最小二乘拟合从头算电子结构而得到的。这些计算是在理论MP4级别上进行的，其中包括所有单，双和三重激励。碳原子和氢原子采用6-31G（d，p）基组。溴原子使用Huzinaga氏（433321/4321/4）和指数为0.5的极化f轨道。目前的计算集中在确定电位对溴乙烯中键合原子的拉伸坐标，C-C-H和C-C-Br弯曲坐标以及二面角的依赖性。还研究了这些坐标之间的耦合。通过将这些结果与先前报道的亚乙烯基-乙炔体系的研究以及我们先前对各种分解通道，反应物和产物平衡结构以及振动频率的鞍点几何形状和能量的计算相结合，可以获得总的从头算数据库。然后，通过调整平衡时的势能曲率来修改适合该数据库（PES1）的分析表面，以更好地拟合所测得的乙烯基溴的IR和拉曼振动频率，同时使该表面的所有其他地形特征保持恒定。尽可能最大的程度。这样显影的表面被标记为PES2。最后，我们任意更改了三中心HBr和H_2消除的反应坐标曲率，以生成第三势能面（PES3）。在4.5至6.44 eV范围内的几种激发能下，研究了这些潜在表面上的乙烯基溴的解离动力学。计算总分解速率系数和产物支化比作为激发能的函数。计算出HBr的振动状态分布，发现其是在PES1上的有效振动温度为7084 K的玻尔兹曼。这些结果实际上与最近报道的该分布的测量结果完全一致。最后，我们研究了三中心H_2和HBr消除反应的离解机理。结果表明，动力学在PES1，PES2和PES3上非常相似。因此，在平衡时以及沿着反应坐标的势能曲率的微小变化不会对解离动力学产生重大影响。但是，发现在从头开始的表面动力学和先前采用的更经验的表面之间存在一些较大的质变。我们得出的结论是，当使用此类表面研究多原子系统中的反应动力学时，必须格外小心。

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