Noncovalent interactions · Electronic structure
Halogen bonds are not purely electrostatic — the orbital term does real work
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Short answer
Energy decomposition attributes a substantial covalent component to halogen bonds, which are less electrostatic but often more orbital-stabilised than the corresponding hydrogen bonds.
What happened
Wolters and Bickelhaupt computed DX···A⁻ and DH···A⁻ complexes for D, X, A = F, Cl, Br and I at ZORA-BP86/TZ2P and split the interaction energy into electrostatic and orbital terms. Hydrogen bonds weaken from −53 to −18 kcal mol⁻¹ going from FH···F⁻ to FH···I⁻. Fluorine-bonded DF···A⁻ complexes partly invert that trend, strengthening from Cl⁻ to I⁻.
Why it matters
Halogen bonding is usually taught through the σ-hole: a patch of positive electrostatic potential on the halogen. That picture explains the geometry but, on this analysis, not the energy — which means electrostatic-potential maps alone are a weak basis for predicting halogen-bond strength.
Evidence
- Study type
- Relativistic DFT (ZORA-BP86/TZ2P) with energy decomposition analysis on model complexes
- Sample
- Gas-phase DX···A⁻ and DH···A⁻ model complexes for D, X, A = F, Cl, Br, I
- Journal
- ChemistryOpen · peer reviewed
- Replication
- Not assessed in this corpus; the covalent contribution to halogen bonding remains actively debated
- Limitations
- Gas-phase model trihalides at fixed geometries, with no solvent or thermal sampling. Energy decomposition schemes are method-dependent by construction.
What this connects to
Sources
The 2 studies this explanation is built from, by the role each plays. Every source links to PaperFren’s explanation of it and to the original paper.
Primary study
- 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.
What it does not showLimitations
These are gas-phase model trihalides, not condensed-phase crystal engineering or protein halogen bonds with thermal sampling.
PaperFren explanationStudy with cards and a quizOriginal paper (DOI)cc by
Supporting evidence
- Anion–π contacts are common in the PDB
A PDB-wide search finds anion–π interactions in most protein structures, with Asp/Glu carboxylates packing on aromatics and frequent cation–π partners opposite.
What it does not showLimitations
This is a structural census, not a measured binding free energy for a designed host; predicted cooperativity is inferred from geometry, not from a new titration series.
PaperFren explanationStudy with cards and a quizOriginal paper (DOI)cc by
Before
The σ-hole model treats halogen bonding as an essentially electrostatic attraction between a positive region on the halogen and a Lewis base.
Now
Orbital mixing carries a large share of the binding energy, and the trend across halide acceptors can invert. These are gas-phase model trihalides, so the argument is about where the energy comes from, not a prediction for crystals or protein sites.