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

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.