A comparison between polar covalent bonding and hypervalent recoupled pair bonding in diatomic chalcogen halide species {O,S,Se} × {F,Cl,Br}

Abstract

The ²Π ground states and low-lying ⁴Σ^– excited states of the nine diatomic species formed by combining O, S, or Se with F, Cl, or Br were characterized with multireference configuration interaction calculations in order to compare and contrast the behaviour of the bonding in these two states of the chalcogen halides. For each of these species, the ²Π ground state is polar covalently bound via simple singlet coupling of unpaired electrons on each atom. But for each compound there is also a bound ⁴Σ^– state where bond formation requires recoupling the valence p ² pair of electrons on the chalcogen atom. This mode of bonding makes more electrons available for additional bond formation and is the basis of hypercoordination. The behaviour of S and Se differs significantly from that of O. Although the hypervalent ⁴Σ^– states of the diatomic chalcogen halides are bound for all three elements, the O species are only weakly bound and exhibit minimal recoupling compared with the corresponding S and Se compounds.

Keywords:

• hypervalency
• diatomic molecules
• excited states
• ab initio
• MRCI

Acknowledgements

Support for this work was provided by funding from the Distinguished Chair for Research Excellence in Chemistry at the University of Illinois at Urbana-Champaign.