Alkali-Metal Trihalides: M+X3- Ion Pair or MX-X2 Complex?
Alkali-Metal Trihalides: M+X3- Ion Pair or MX-X2 Complex?
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DOI：
10.1021/acs.jpcb.7b10005
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发表时间：
2017-12
期刊：
影响因子：
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通讯作者：
Zhi-Cheng Sun;Kevin B. Moore;J. Hill;K. Peterson;H. Schaefer;R. Hoffmann
中科院分区：
文献类型：
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作者：
Zhi-Cheng Sun;Kevin B. Moore;J. Hill;K. Peterson;H. Schaefer;R. Hoffmann
本文用耦合团簇方法对碱金属三卤化物MX_3（M = Li，Na，K，Rb，Cs; X = Cl，Br，I）进行了系统的研究。使用CCSD（T）对双原子实验结果的基准表明，选择用于预测可靠结构和谐波振动频率的加权核价基组（K，Rb和Cs的新基组）的性能令人满意。使用B3 LYP功能的异构体搜索产生一个平面，但不对称的T形C的结构作为全球最小的所有MX 3物种。更高水平的CCSD（T）计算表明，在各自的平衡几何结构中，M+阳离子对X3-阴离子有中度到强烈的扭曲。最明显的是，对于LiCl 3，两个Cl-Cl距离相隔0.786 μ m。即使对于M+X3模型中畸变最小的CsI 3，两个I-I距离也相差0.243 μ m。不需要太多的能量就能使母体阴离子沿着反对称伸展变形，所以这并不奇怪。MX3分子的振动的正常模式是在更好的协议与矩阵隔离实验比以前的计算。这些简正模表明，取代了“自由”X3-负离子的反对称和对称伸缩，计算了相对局域化和相互扰动的X-X和M-X伸缩。建议MX3系统可以被描述为MX-X2复合物而不是M+X3-离子对。这一观点得到了键合分析的支持，键合分析显示MX和X2部分之间的键合临界点和自然键级处的低电子密度。MX3断裂成MX + X2与M+ + X3-的热化学也支持这类分子中成键的另一种观点。
The alkali-metal trihalides MX3 (M = Li, Na, K, Rb, and Cs; X = Cl, Br, and I) are systematically studied using coupled-cluster methods. Benchmarks using CCSD(T) against diatomic experimental results suggest satisfactory performance for the weighted core-valence basis sets (new basis sets for K, Rb, and Cs) selected for predicting reliable structures and harmonic vibrational frequencies. An isomer search using the B3LYP functional yields a planar, yet asymmetric T-shaped C s structure as the global minimum for all MX3 species. Much higher level CCSD(T) computations show a moderate to strong distortion of the X3- anion by the M+ cation in the respective equilibrium geometries. Most obviously, for LiCl3, the two Cl-Cl distances are separated by 0.786 Å. Even for CsI3, the structure least distorted from the M+X3- model, the two I-I distances differ by 0.243 Å. It does not take much energy to distort the parent anions along an antisymmetric stretch, so this is no surprise. The normal modes of vibration of the MX3 molecules are in better agreement with matrix isolation experiments than previous calculations. And these normal modes reveal that, instead of the well-established antisymmetric and symmetric stretches of the "free" X3- anions, relatively localized and mutually perturbed X-X and M-X stretches are calculated. The suggestion emerges that the MX3 system may be alternatively described as an MX-X2 complex rather than the M+X3- ion pair. This perspective is supported by bonding analyses showing low electron densities at the bond critical points and natural bond orders between the MX and X2 moieties. The thermochemistry of fragmentations of MX3 to MX + X2 versus M+ + X3- also supports the alternative viewpoint of the bonding in this class of molecules.