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

## TL;DR

This paper uses advanced quantum calculations to study how atomic fluorine bonds with halogenated molecules, revealing unique bonding characteristics and strength variations.

## Contribution

The study provides a detailed ab initio analysis of fluorine-halogen bonding interactions and their strength and charge transfer properties.

## Key findings

- Fluorine forms a three-electron bond with halogen lone pairs, resulting in a bond order of one-half.
- Bonding strength varies significantly, from 0.7 to 19.6 kcal/mol depending on the halogen involved.
- AIM analysis confirms Lewis acid/base bonding with positive Laplacian values at bond critical points.

## Abstract

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.