# Halogen Bonds under Electric Field with Quantum Accuracy and Relativistic Basis Sets

## TL;DR

This study investigates how halogen bonds respond to electric fields using quantum calculations, showing that relativistic effects significantly influence their behavior.

## Contribution

The paper introduces a benchmark of relativistic basis sets for halogen bonds under electric fields, revealing their sensitivity compared to hydrogen bonds.

## Key findings

- Relativistic treatments reduce exaggerated field responses in halogen bonds.
- M06-2X functional shows accuracy comparable to correlated methods when using appropriate basis sets.
- Halogen bonds are more sensitive to electric fields than hydrogen bonds.

## Abstract

Halogen bonds (XBs)
are a cornerstone of supramolecular chemistry,
yet their response to external perturbations remains poorly investigated,
particularly in systems with heavy elements where relativistic effects
are significant. We benchmark two prototypical iodine-chloride X-bonded
complexes, ClI···N­(CH3)3 and
ClI···NCH, under electric fields (EFs) using quantum
chemical calculations up to CCSD and CCSD­(T). Relativistic basis sets,
including the all-electron jorge-TZP-DKH, are assessed against non-relativistic
and pseudopotential-based alternatives (def2-TZVP, SDD, LANL2DZ) for
their impact on XB geometries, binding energies, vibrational Stark
shifts, and electron density redistribution. Explicit relativistic
treatments substantially reduce the exaggerated field response otherwise
observed. Benchmarking M06-2X and B3LYP with various basis sets against
correlated methods confirms the accuracy of M06-2X, while showing
that the relativistic effects included in the basis set influence
the results more than the choice of functional itself. Symmetry-Adapted
Perturbation Theory (SAPT) indicates that electrostatics dominate
XB stabilization, with induction becoming more relevant under strong
positive fields. Overall, XBs prove markedly more sensitive to external
EFs than H-bonds across different field arrangements.