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Keywords:

  • computational chemistry;
  • electron localization function;
  • helium;
  • multiple bonds;
  • substituent effects

Abstract

This paper deals with the possibility of forming short and relatively strong carbon–helium bonds in small typical organic molecules through substitution of one or several H atoms by He+. A structural and energetics study (based on high-level calculations) of this unusual bonding, as well as a topological characterization of the resulting cations, is undertaken. Stable species generally requires substitution of about half of the hydrogen atoms for formation. Under these conditions, the number of such species appears to be potentially unlimited. “True” C[BOND]He bonds exhibit equilibrium distances ranging from 1.327 (C2H2He22+) to 1.129 Å (He2CO2+). The energies of neutral He releasing range from approximately 5 kcal mol−1 [He2CO2+, (Z)-C2H2He22+] to 25 kcal mol−1 (C2HHe33+), but remain most frequently around 10 kcal mol−1. However, most of He+-substituted hydrocarbons are metastable with respect to C[BOND]C cleavage, except derivatives of ethene. Atoms in molecules (AIM) and electron localization function (ELF) topological descriptors classify the C[BOND]He bond as a weak charge-shift interaction [S. Shaik, D. Danovich, B. Silvi, D. L. Lauvergnat, P. C. Hiberty, Chem. Eur. J. 2005, 11, 6358–6371] in agreement with a recent publication by Rzepa [S. H. Rzepa, Nat. Chem. 2010, 2, 390–393]. He2CO2+ is the only investigated compound that presents a C[BOND]He bonding ELF basin, which indicates a non-negligible covalent contribution to the bond. Other modifications in the electronic structure, such as the breaking of the triple bond in ethyne derivatives or the loss of aromaticity in C6H3He33+, are also nicely revealed by the ELF topology.