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首页> 外文期刊>Chemistry: A European journal >The elusive structure of CrCl2 - A combined computational and gas-phase electron-diffraction study
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The elusive structure of CrCl2 - A combined computational and gas-phase electron-diffraction study

机译：CrCl2的难以捉摸的结构-结合计算和气相电子衍射研究

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Chromium dichloride poses a challenge to the structural chemist. Its different forms of aggregation and association display all well-known structural distortions induced by vibronic interactions. The monomeric molecule has a Renner-Teller distorted bent geometry, the crystal exhibits strong Jahn-Teller distortion, and the oligomers have slightly distorted four-membered-ring structures due to the pseudo-Jahn-Teller effect. In this paper we report on the low-energy structures of the monomer and its clusters, Cr2Cl4, Cr3Cl6, and Cr4Cl8, from unrestricted Kohn-Sham (broken-symmetry) density functional calculations. CrCl2 was also investigated at higher level, including coupled-cluster and state-average CASSCF computations. The global minima of the gas-phase clusters consist of two-dimensional, antiferromagnetically coupled chains of CrCl2 units forming four-membered, doubly bridged Cr2Cl2 rings, closely resembling the solid-state structure of alpha-CrCl2. Each Cr atom in these chains has spin quantum number S=2. This suggests that the CrCl2 nucleation starts very early on the structural chain motif found in the solid. There is only a very small change in energy from the antiferromagnetically to the ferromagnetically coupled Cr atoms, which indicates little spin-coupling between the metal centers. There is an approximately constant change in energy, about 50 kcal mol(-1), with every new CrCl2 unit during cluster formation. Information about the structure of these clusters was used in the re-analysis of high-temperature electron-diffraction data. The vapor at 1170 K contained about 77% monomeric molecules, 19% dimers, and a small amount of trimers. Monomeric CrCl2 was found to be bent with a bond angle of 149(10)degrees, in good agreement with our computations, which resulted in a Renner-Teller distortion of the lowest-energy 511 9 electronic state into the bent B-5(2) ground state. The vibrational spectrum of chromium dichloride is discussed and the thermodynamics of cluster formation from 1000-2000 K is examined.
机译：二氯化铬对结构化学家构成了挑战。其不同形式的聚集和缔合显示了由振动相互作用引起的所有众所周知的结构变形。单体分子具有Renner-Teller扭曲的弯曲几何形状，晶体表现出很强的Jahn-Teller畸变，并且由于拟Jahn-Teller效应，低聚物的四元环结构略有扭曲。在本文中，我们通过不受限制的Kohn-Sham（破坏对称）密度泛函计算，报告了单体及其簇的低能结构Cr2Cl4，Cr3Cl6和Cr4Cl8。还对CrCl2进行了更高级别的研究，包括耦合聚类和状态平均CASSCF计算。气相团簇的整体最小值由CrCl2单元的二维反铁磁耦合链组成，形成四元双桥Cr2Cl2环，非常类似于α-CrCl2的固态结构。这些链中的每个Cr原子具有自旋量子数S ＝ 2。这表明CrCl2成核非常早就在固体中发现的结构链基序上开始。从反铁磁耦合到铁磁耦合的Cr原子的能量变化很小，这表明金属中心之间几乎没有自旋耦合。在簇形成过程中，每个新的CrCl2单元的能量都有大约恒定的变化，约为50 kcal mol（-1）。有关这些团簇结构的信息用于高温电子衍射数据的重新分析。 1170 K时的蒸气包含约77％的单体分子，19％的二聚体和少量的三聚体。发现单体CrCl2弯曲时的键角为149（10）度，与我们的计算结果吻合良好，这导致最低能量511 9电子态的Renner-Teller畸变成弯曲的B-5（2） ）基态。讨论了二氯化铬的振动光谱，并研究了1000-2000 K范围内团簇形成的热力学。

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