Short halogen-carbon bonds in 1-chloro- and 1-bromo-1-nitro-2,3-difluorocyclopropanes: Synergistic 1,4-intramolecular interactions?

Cyclopropane is introduced in this computational study as a framework for obtaining contracted carbon-chlorine and carbon-bromine bond lengths, utilizing the M06-2X/aug-cc-pVTZ method/basis set combination. For chloro- and bromocyclopropane, placing a nitro group on the same carbon as the chlorine or bromine and two fluorines in the 2 and 3 positions closest to the larger halogen leads to three 1—4 intramolecular interactions with the chloro and bromo substituents, with computed C-Cl and C-Br bond lengths of 1.711 Å and 1.872 Å, respectively. Adding more fluorines to this framework does not lead to shorter carbon-chlorine or carbon-bromine bond lengths because of the what has been labeled the “Teflon” effect; as more fluorines are added they become less negative or even positive. The Coulombic interpretation of the above-mentioned bond contractions is supported by computed electrostatic potentials.