American Chemical Society
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S···π Chalcogen Bonds between SF2 or SF4 and C–C Multiple Bonds

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journal contribution
posted on 2015-06-04, 00:00 authored by Vincent de Paul N. Nziko, Steve Scheiner
SF2 and SF4 were each paired with a series of unsaturated hydrocarbons including ethene, ethyne, 1,3-butadiene, and benzene, in each case forming a chalcogen bond between the S atom and the carbon π-system. MP2 ab initio calculations reveal that the S atom is situated directly above one specific CC bond, even when more than one are present. The binding energies range between 3.3 and 6.6 kcal/mol. SF2 engages in a stronger, and shorter, noncovalent bond than does SF4 for all systems with the exception of benzene, to which SF4 is more tightly bound. cis-Butadiene complexes contain the shortest chalcogen bond, even if not necessarily the strongest. The internal S–F covalent bonds elongate upon formation of each chalcogen bond. The molecules are held together largely by charge transfer forces, particularly from the CC π-bonds to the σ*­(SF) antibonding orbitals. In the case of SF2, a sulfur lone pair can transfer charge into the π* MOs of the alkene, a back-transfer which is more difficult for SF4.