Emerging Concepts in Chemical Bonding

The particle-on-a-ring model provides a simple framework for understanding the stability, magnetic properties, and topology of π-conjugated systems. When a weak perpendicular magnetic field is applied to this model, the quantisation of angular momentum reveals the Hückel 4n+2 rule and ring currents, resulting in aromaticity. At larger magnetic fields, the same physics gives rise to a Berry phase, and Aharonov-Bohm oscillations between trivial and non-trivial (Möbius) topologies.
Molecular structure offers an alternative, field-free handle on the Berry phase. For example, geometrically twisting a carbon macrocycle into a Möbius geometry modifies the boundary conditions experienced by the delocalised electrons, affecting the molecular properties due to a change in topology.
This talk will present our recent work on the characterization of a molecule with half-Möbius topology, in which the orbital basis twists by 90° upon one circulation of the ring. We will also present preliminary results on the equivalence of magnetic flux and molecular structure in half-Möbius systems, using molecular transmission as a diagnostic.

The Molecular Electrostatic Potential (MEP) is widely used to understand chemical reactivity and noncovalent interactions. Nevertheless, obtaining a complete topological picture of the MEP, identifying all maxima, minima, and saddle points has remained challenging due to its slow long-range decay and sensitivity to computational setups.
In this work, we introduce a robust and efficient algorithm to exhaustively locate all critical points (CPs) of the MEP in three dimensions. Our approach combines Newton’s iterative method with a systematic, multi-resolution sampling of physical space that reaches far beyond the molecular boundaries. We test the algorithm on a model function with known topology and apply it to various molecular systems, including neutral molecules, ions, and noncovalent complexes from the S66 and IONIC-HB datasets. Additionally, we explore a tricubic interpolation of the MEP to boost computational speed while preserving topological accuracy.
Our findings show that the interpolated MEP reproduces the exact topology in 87% of the cases tested. Discrepancies are minor, appearing only near nuclei or in low-gradient regions (e.g., LiH, N₃⁻). The method satisfies the Poincaré–Hopf index for charged systems and achieves speedups of 2–7× (2.12× on average) compared to exact calculations. As an illustration, Figure 1 displays the full set of CPs for the sulfate anion (SO₄²⁻), featuring twelve (3,–1), eighteen (3,+1), and twelve (3,+3) critical points that reveal the rich topology of its MEP.
This work positions MEP topology as a practical and scalable tool for large-scale studies, offering deep insights into electrostatic-driven molecular recognition and noncovalent bonding patterns.

The formation of the chemical bond in some simple diatomic molecules of main-group atoms is analyzed using the difference of the electronic charge distribution (DECD) between the molecules and the respective atoms. The results are compared with the Laplacian distribution delta2ρ(r) of the molecules. It is shown that the curvature of the charge distribution does not provide a physically sound picture of the covalent bond, particularly in the case of polar molecules.

The nature of dative bonding remains one of the most persistent conceptual controversies in chemistry, despite more than a century of theoretical development since Lewis. We demonstrate that this apparent dichotomy largely originates from an overreliance on the electron-pair paradigm. By combining Generalized Valence Bond theory with complementary real-space and wavefunction-based descriptors, we show that a broad class of bonds traditionally labeled as dative or ionic are more naturally and consistently described as one-electron bonds.

Our central finding is that, in these interactions, only one electron of the nominal bonding pair actively delocalizes and provides essentially the full bonding stabilization, while its partner remains largely localized as a spectator. This asymmetric, single-electron delocalization motif emerges robustly across prototypical ionic, polar covalent, and donor–acceptor systems, and is revealed independently by orbital-based and orbital-invariant analyses. The resulting picture reconciles long-standing debates concerning the distinctiveness of dative bonding and provides a unified interpretation of bonding, dissociation pathways, and bond strength trends across heteropolar chemistry.

A century after G. N. Lewis introduced his foundational model of chemical bonding, its core principles remain central to our understanding of molecular structure. Traditionally, neighboring atoms in a Lewis structure are assumed to share stabilizing interactions. However, this assumption begins to falter at the boundary between covalent and noncovalent bonding. Our work reveals that, in certain molecules with the general formula MAX₃, where (M) is a metal, (A) a nonmetal, and (X) an electron-rich or electron-withdrawing group, the interaction between adjacent M and A atoms can be destabilizing. Remarkably, the overall structure is preserved due to strong interactions between electron-deficient metals and the X groups.1–6 This phenomenon gives rise to unconventional structures, such as the inverted LiCF₃, where lithium interacts preferentially with fluorine atoms over the central carbon in the gas phase. Our recent investigations suggest that these collective interactions extend beyond organometallic compounds and offer insight into anomalous bond dissociation energies and stability trends across a wide range of molecules, from organometallics to perhaloalkanes.

The chemical bond is commonly viewed as the result of stabilizing interactions, including electron-pair bonding and, for example, favorable HOMO–LUMO orbital interactions. However, the strength of a bond and the course of a reaction are determined not only by stabilization, but also by the extent of destabilizing closed-shell–closed-shell interaction, or Pauli repulsion. Over the past several years, our work has demonstrated that modulation of Pauli repulsion provides a general and powerful framework for understanding trends in bonding, stability, and reactivity across a wide range of molecular systems.

This lecture will illustrate how Pauli repulsion controls element–element single bonds in relation to atomic size and coordination number, and how it underlies the recently recognized phenomenon of the lone-pair-shielded radical (LPSR, see illustration). These examples reveal that subtle changes in occupied–occupied interactions can have profound consequences for bond strength and molecular structure. The same conceptual framework also sheds light on variations in the stability of organic radicals and the magnitude of reaction barriers.
Taken together, these findings highlight that chemical bonding is shaped not only by attractive forces, but equally by the modulation of repulsive ones. Recognizing and exploiting this balance opens new avenues for a deeper and more predictive understanding of molecular structure and reactivity.

Chemical bonding is a nonlocal phenomenon that binds atoms into molecules. Its ubiquitous presence in chemistry, however, stands in stark contrast to its ambiguous definition and the lack of a universal perspective for its understanding. In this talk, we rationalize and characterize chemical bonding through the lens of an equally nonlocal concept from quantum information: orbital entanglement. We introduce maximally entangled atomic orbitals (MEAOs), whose entanglement pattern is shown to recover both Lewis (two-center) and beyond-Lewis (multicenter) structures, with multipartite entanglement serving as a comprehensive index of bond strength. Our unifying framework for bonding analyses is effective not only for equilibrium geometries but also for transition states in chemical reactions and complex phenomena such as aromaticity. It also has the potential to elevate Hilbert-space atomic partitioning to match the prevalent real-space partitioning in the theory of atoms in molecules. Accordingly, our work opens new pathways for understanding fuzzy chemical concepts using rigorous, quantitative descriptors from quantum information.