The elusive structure of CrCl2 - A combined computational and gas-phase electron-diffraction study

Brian Vest, Zoltán Varga, Magdolna Hargittai, Andreas Hermann, Peter Schwerdtfeger

Research output: Contribution to journalArticle

16 Citations (Scopus)

Abstract

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, Cr2Cl 4, 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 Cr 2Cl2 rings, closely resembling the solid-state structure of α-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 1170K 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)°, in good agreement with our computations, which resulted in a Renner-Teller distortion of the lowest-energy 5g electronic state into the bent 5B2 ground state. The vibrational spectrum of chromium dichloride is discussed and the thermodynamics of cluster formation from 1000-2000 K is examined.

Original languageEnglish
Pages (from-to)5130-5143
Number of pages14
JournalChemistry - A European Journal
Volume14
Issue number17
DOIs
Publication statusPublished - Jun 9 2008

Keywords

  • Chromium dichloride
  • Density functional calculations
  • Electron diffraction
  • Nucleation
  • Renner-Teller effect
  • Structure elucidation

ASJC Scopus subject areas

  • Catalysis
  • Organic Chemistry

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