Density functional theory studies of S...X and Se...X (X = Br, I) halogen-bonding interactions are used to interpret the selection of selenium and iodine for thyroid hormone signaling. A new mechanism for dehalogenation in terms of halogen bonding is proposed. The activation barriers for deiodination of an aromatic iodide by MeSeH and MeSH (17.6 and 19.8 kcal/mol) are consistent with the relative rates of deiodination by iodothyronine deiodinase and its cysteine mutant.

Se···I Halogen bonding (XB) between triphenylphosphine selenide and organoiodides: 1,2-diiodotetrafluorobenzene (1,2-F4DIB), 1,4-diiodotetrafluorobenzene (1,4-F4DIB), and tetraiodoethylene (TIE) has been used to assemble molecules into finite adducts, chains, and two-dimensional layers. Phenyl embraces involving the triphenylphosphine groups extend these moieties into additional dimensions, leading in two cases (1,4-F4DIB and TIE) to open structures, with filled or partially filled cavities. Comparison of structural parameters for 26 reported zigzag chains of embracing Ph3XY reveals that this synthon can adapt its registry to accommodate widely diverse structures. The strengths of the XB interactions were analyzed using natural bond orbital (NBO) theory, which provides an estimate of the energy of a donor–acceptor interaction (ΔE d→a) through localization of the molecular orbitals. Structures of 1:1 and 1:2 XB complexes of Ph3PSe with 1,2-F4DIB and 1,4-F4DIB were optimized at the DFT(B97-1)/BSI level. The bond distance and angles obtained are in reasonably good agreement with the experimentally observed structures.