Bradford Stephen D, Ge Yingbin, Zhang Jie, Trejo Marisol, Tronrud Dale, Kong Wei
Department of Chemistry, Oregon State University, Corvallis, OR 97331, USA.
Department of Chemistry, Central Washington University, Ellensburg, WA 98926, USA.
Phys Chem Chem Phys. 2022 Nov 23;24(45):27722-27730. doi: 10.1039/d2cp04492g.
We perform electron diffraction of 1,4-dichlorobenzene (CHCl, referred to as 2ClB) embedded in superfluid helium droplets to investigate the structure evolution of cluster growth. Multivariable linear regression fittings are used to determine the concentration and the best model structures of the clusters. At a droplet source temperature of 22 K with droplets containing on average 5000 He atoms, the fitting results agree with the doping statistics modeled using the Poisson distribution: the largest molecular clusters are tetramers, while the abundances of monomers and dimers are the highest and are similar. Molecular dimers of 2ClB are determined to have a parallel structure with a 60° rotation for the Cl-Cl molecular axes. However, a better agreement between experiment and fitting is obtained by reducing the interlayer distance that had been calculated using the density functional theory for dimers. Further calculations using the highest level quantum mechanical calculations prove that the reduction in interlayer distance does not significantly increase the energy of the dimer. Cluster trimers adopt a dimer structure with the additional monomer slanted against the dimer, and tetramers take on a stacked structure. The structure evolution with cluster size is extraordinary, because from trimer to tetramer, one monomer needs to be rearranged, and neither the trimer nor the tetramer adopts the corresponding global minimum structure obtained using high level coupled-cluster theory calculations. This phenomenon may be related to the fast cooling process in superfluid helium droplets during cluster formation.
我们对嵌入超流氦滴中的1,4 - 二氯苯（CHCl，简称为2ClB）进行电子衍射，以研究团簇生长的结构演化。使用多变量线性回归拟合来确定团簇的浓度和最佳模型结构。在液滴源温度为22 K且液滴平均含有5000个氦原子的情况下，拟合结果与使用泊松分布建模的掺杂统计结果一致：最大的分子团簇是四聚体，而单体和二聚体的丰度最高且相近。确定2ClB的分子二聚体具有平行结构，Cl - Cl分子轴旋转60°。然而，通过减小使用密度泛函理论计算的二聚体层间距离，实验与拟合之间获得了更好的一致性。使用最高水平的量子力学计算进行的进一步计算证明，层间距离的减小不会显著增加二聚体的能量。团簇三聚体采用二聚体结构，额外的单体倾斜于二聚体，而四聚体呈现堆叠结构。团簇结构随尺寸的演化非同寻常，因为从三聚体到四聚体，一个单体需要重新排列，并且三聚体和四聚体都没有采用使用高水平耦合簇理论计算得到的相应全局最小结构。这种现象可能与团簇形成过程中超流氦滴的快速冷却过程有关。
