Alkali-Metal Trihalides: M+X3- Ion Pair or MX-X2 Complex?
Zhi Sun1, Kevin B Moore1, J Grant Hill2
1Center for Computational Quantum Chemistry , University of Georgia , Athens , Georgia 30602 , United States.
The Journal of Physical Chemistry. B
|December 13, 2017
Summary
Alkali-metal trihalides (MX3) exhibit distorted structures, challenging the simple ion pair model. Computational studies suggest they are better described as MX-X2 complexes, with localized M-X and X-X bonds.
Area of Science:
- Computational Chemistry
- Solid State Chemistry
- Quantum Chemistry
Background:
- Alkali-metal trihalides (MX3) are compounds with potential applications.
- Previous studies have explored their structures and bonding.
Purpose of the Study:
- To systematically investigate the structures and bonding of alkali-metal trihalides (MX3) using advanced computational methods.
- To determine the most stable isomeric structures and analyze the nature of chemical bonding.
Main Methods:
- Coupled-cluster (CCSD(T)) and density functional theory (B3LYP) calculations were employed.
- Isomer searches and vibrational frequency analyses were performed.
- Basis sets were optimized and validated against experimental data.
Main Results:
- A planar, asymmetric T-shaped C_s structure was identified as the global minimum for all MX3 species.
- Significant distortion of the X3- anion by the M+ cation was observed, with varying M-X and X-X bond lengths.
- Vibrational modes indicated localized M-X and X-X stretches, rather than free X3- anion vibrations.
Conclusions:
- Alkali-metal trihalides (MX3) are better described as MX-X2 complexes than M+X3- ion pairs.
- Bonding analyses support this alternative perspective, revealing low electron densities and natural bond orders between MX and X2 moieties.
- Thermochemical fragmentation data further supports the MX-X2 complex model for these compounds.
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