Halogen bonds are not purely electrostatic
Relativistic DFT shows halogen and hydrogen bonds share HOMO–LUMO covalency; halogen bonds have weaker electrostatics but stronger orbital mixing.
Source
Halogen Bonding versus Hydrogen Bonding: A Molecular Orbital Perspective
Study at a glance
- Design
- Computational / modelling — ZORA-BP86/TZ2P energy decomposition of trihalide halogen bonds vs hydrogen bonds
- N
- Molecular orbital theory study — no sample N
- Population
- DX···A− and DH···A− model complexes (D, X, A = F–I)
- Outcome
- Electrostatic vs orbital contributions distinguishing halogen and hydrogen bonds
Structured fields used in claim comparison tables when every cited study has a complete layer.
Inspect source passages
Halogen bonds are not purely electrostatic
These passages occur verbatim in the stored paper text. They support the points listed below; they do not verify every statement in the explanation.
FH···F− to FH···I− hydrogen-bond strength weakens from −53 to −18 kcal mol−1.
For example, from FH⋅⋅⋅F − to FH⋅⋅⋅I − , the hydrogen-bond strength (Δ E ) is weakened from −53 to −18 kcal mol −1 , while the r H⋅⋅⋅A − value increases from 1.159 to 2.319 Å, and the value of the bond stretch Δ r D–H is reduced from 0.226 to 0.049 Å (see ).
FF···A− fluorine-bond ΔE values are −52, −43, −44 and −48 kcal mol−1 along F−, Cl−, Br−, I−.
Here, Δ E for the fluorine-bond strength varies along A − =F − , Cl − , Br − and I − with values of −52, −43, −44 and −48 kcal mol −1 , respectively (see ).
What they did
Authors computed trihalide DX···A− and hydrogen-bonded DH···A− complexes (D, X, A = F, Cl, Br, I) with ZORA-BP86/TZ2P and decomposed interaction energies into electrostatic and orbital terms.
What they found
Both bond types have a sizable covalent HOMO–LUMO piece. Halogen bonds are less electrostatic (DX less polar than DH) but often more orbital-stabilized. FH···F− to FH···I− weakens from −53 to −18 kcal mol−1; DF···A− fluorine bonds are partly inverted along Cl−–I−.
The limits
What it doesn't show
These are gas-phase model trihalides, not condensed-phase crystal engineering or protein halogen bonds with thermal sampling.
Key terms
- Halogen bond
- Attractive DX···A interaction where X is a halogen acting as an electrophile toward a Lewis base.
- HOMO–LUMO charge transfer
- Donation from the acceptor lone pair into the D–X or D–H σ* orbital.
- ZORA
- Zeroth-order regular approximation for relativistic effects in DFT, important for iodine.
- Energy decomposition
- Splitting ΔE into electrostatic, Pauli, and orbital-interaction terms.
- Fluorine bond
- DF···A− contact; here its strength trend along A− is partly inverted.
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Quiz yourself
Halogen bonds in this DFT study are:
Common questions
Are halogen bonds purely electrostatic?
No—they have a sizable covalent HOMO–LUMO component, like hydrogen bonds.
How do they differ from H-bonds?
Weaker electrostatic attraction but often stronger orbital interaction.
What happens from F− to I− acceptors?
Bonds generally weaken as halide basicity falls.
What is unusual about DF···A−?
Strength can increase again from Cl− to I− (partial inversion).
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