Unusually Strong Cooperative Noncovalent Binding of Elemental Mercury by Protonated Melamine: A Density Functional and Coupled-Cluster Study

The noncovalent capture of elemental mercury (Hg⁰) is challenging due to its closed-shell electronic structure and weak affinity for most molecular hosts. Using density functional theory supported by wave function benchmarks, we examine the interaction of Hg⁰ with protonated melamine in the gas phase. Protonation is shown to dramatically enhance Hg⁰ binding, with triprotonated melamine accommodating three Hg⁰ atoms with average bond dissociation energies exceeding 20 kcal·mol⁻¹ per Hg atom. Additional Hg⁰ atoms bind above and below the triazine ring through π-associated interactions, giving rise to distinct binding motifs with a reduced level of incremental stabilization. Extensive validation against functional choice, basis set size, dispersion treatment, relativistic effects, and coupled-cluster benchmarks confirms the robustness of these energetic trends. Natural bond orbital, quantum theory of atoms in molecules, and noncovalent interaction analyses consistently indicate that the Hg⁰-melamine interactions are noncovalent and closed-shell in nature, dominated by cooperative electrostatic and polarization effects rather than covalent bonding. These findings reveal an unusual example of strong, cooperative, noncovalent Hg⁰ binding enabled by protonation and symmetry.