Studying the Hemibond: High-Level Ab Initio Calculations on Complexes of Atomic Fluorine with Halogenated Organic Molecules

Atomic fluorine F (²P3/2) is known to form complexes with perhalogenated solvents like carbon tetrachloride or chlorofluorocarbons. Here these complexes are studied at a very high ab initio level employing CCSD(T) theory. The results show that the fluorine atom undergoes a localized three-electron bonding interaction with the halogen lone pairs, resulting in a doubly occupied σ-type orbital and a half-filled σ*-type orbital, giving a bond order of one-half. Bonding strengths range from almost negligible when the halogen atom bonded to the fluorine atom also is fluorine (0.7 kcal mol⁻¹ for the CF₄ complex) to significant when the halogen is iodine (19.6 kcal mol⁻¹ for bonding to the iodine atom in methyl iodide). The degree of charge transfer from the organohalogen compound to the fluorine atom calculated varies significantly from 0.57 elementary units in the case of the methyl iodide complex to exactly zero for the carbon tetrafluoride complex. For the complexes with chlorofluorocarbon molecules, a negative charge at the fluorine atom of only ca. 0.05 elementary units is calculated, indicating that these highly reactive complexes have very little charge transfer character. An analysis of the bonding situation via the theory of Atoms in Molecules (AIM) reveals bond critical points (BCPs) between the fluorine atom and the closest halogen atom in the organohalogen compound. The Laplacian of the electron density at the halogen-fluorine BCP is always positive, indicating that the complexes should be considered to be of the Lewis acid/Lewis base type.