Structure and properties of the anions MF₄⁻, MCl₄⁻ and MBr₄⁻ (M = C, Si, Ge)

Abstract

Density functional theory (DFT), Møller–Plesset (MP2) and coupled cluster with single and double substitutions including non-iterative triple excitations (CCSD(T)) calculations on the anions MX₄⁻, with M = C, Si, Ge and X = F, Cl, Br, show that GeF₄⁻, SiCl₄⁻, GeCl₄⁻ and SiBr₄⁻ prefer a C_2v conformation, but CCl₄⁻ is an elongated C_3v structure. CBr₄⁻ has T_d symmetry in MP2, but is slightly more stable in elongated C_3v form with DFT and CCSD(T). GeBr₄⁻ has T_d symmetry. CF₄⁻ and SiF₄⁻ are unstable with respect to loss of an electron. Vertical electron affinities (EAs) are negative also for CCl₄ and SiCl₄, and close to zero for GeF₄ and SiBr₄. Adiabatic EAs range from 0.47 eV for SiCl₄ to 1.78 eV for GeBr₄. The lowest excited states at T_d symmetry are ²T₂ resonances with energies of 2.1–3.5 eV, resulting from excitation of the a₁ singly occupied molecular orbital to vacant t₂ orbitals. Vertical excitation energies (VEEs) and vibrational frequencies are given for the most stable anionic geometries. Comparison with experimental VEEs for CCl₄⁻ is made. From dissociation energies of MX₄, MX₄⁻, MX₃ and MX₃⁻, appearance energies of X⁻, MX₃⁻, X₂⁻ and MX₂⁻ were calculated. Most were found to be in reasonable agreement with experimental values. Theoretical spin densities and g-factors have been compared with experimental results available for CCl₄⁻, SiCl₄⁻ and GeCl₄⁻.

Keywords:

• tetrahalide anions of carbon, silicon and germanium
• tetrahedral or lower symmetry
• appearance energies of X⁻, MX₃⁻
• excited states and resonance states
• spin densities and g-factors

Acknowledgements

The author thanks Dr Pablo J. Bruna for reading the manuscript, and for making valuable comments and suggestions. Provision of computing facilities by ACE-net is gratefully acknowledged.

Supplemental data

Supplemental data for this article can be accessed http://dx.doi.org/10.1080/00268976.2014.976601.

Additional information

Funding

The author is indebted to NSERC (Canada) for providing financial support in the form of discovery grants over many years.