Quantum Chemical Investigations of Relativistic Effects in Halogen Compounds

Quantum chemical investigations into halogen compounds subject to relativistic effects have advanced understanding of bonding, reactivity and spectroscopic properties across the periodic table. As atomic number increases, scalar and spin–orbit relativistic contributions alter orbital energies, bond lengths and electron densities in fluorine through astatine derivatives. Methodological developments include four-component Dirac–Coulomb and two-component Douglas–Kroll–Hess Hamiltonians as well as relativistic effective core potentials. These approaches reveal how spin–orbit coupling modulates halogen bonding, charge‐shift phenomena and covalent character in heavy halides. Studies on iodine species show up to a 10 % contraction of bond distances and mixed aromaticity in cyclic systems, while astatine complexes highlight the competition between charge‐shift bonding and σ-hole electrostatics. Relativistic effects have practical implications for materials design, radiopharmaceutical chemistry and catalysis, influencing both thermochemical stabilities and kinetic barriers. Topological analyses based on the quantum theory of atoms in molecules (QTAIM) and electron localisation functions provide insight into contact interactions and delocalisation indices. The integration of correlated methods, such as relativistic coupled-cluster theory, with density functional approximations calibrated for heavy elements has enabled quantitative predictions of spin-orbit splittings and spectroscopic shifts. Such investigations underscore the global significance of incorporating relativistic quantum mechanics in accurately modelling the chemistry of heaviest halogens.

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Quantum Chemical Investigations of Relativistic Effects in Halogen Compounds publication trend

The graph below shows the total number of articles in quantum chemical investigations of relativistic effects in halogen compounds across all publications each year (not limited to Nature Index journals).

Technical terms

Spin–orbit coupling: Interaction between an electron’s spin and its orbital motion around the nucleus, leading to energy level splitting.

Dirac–Coulomb Hamiltonian: A four-component relativistic operator accounting for kinetic and electron–nucleus interactions in heavy-atom calculations.

Douglas–Kroll–Hess Hamiltonian: A two-component approximation that decouples positive and negative energy states for efficient relativistic treatments.

Relativistic effective core potential: A simplified representation of core electrons that incorporates scalar and spin–orbit effects in heavy elements.

Quantum theory of atoms in molecules (QTAIM): A topological framework for analysing electron density distributions and bonding interactions.

Charge-shift bonding: A bonding motif where electron sharing is dominated by resonance between covalent and ionic structures rather than pure electrostatics.

σ-hole: A region of positive electrostatic potential opposite a covalent bond on halogen atoms, facilitating non-covalent interactions.

References

1. Astatine Facing Janus: Halogen Bonding vs. Charge-Shift Bonding. Molecules (2021).
2. Geometries, interaction energies and bonding in [Po(H 2 O) n ] 4+ and [PoCl n ] 4−n complexes. Physical Chemistry Chemical Physics (2022).