All-Metal Aromaticity: Revisiting the Ring Current Model among Transition Metal Clusters

Area of Science:

• Computational chemistry
• Quantum chemistry
• Materials science

Background:

• Aromaticity in metal clusters is complex and debated.
• Local indices like nucleus-independent chemical shifts (NICS) have limitations.
• Understanding magnetic aromaticity is crucial for predicting cluster properties.

Purpose of the Study:

• To investigate the validity of the ring-current model versus local indices for assessing magnetic aromaticity in metal clusters.
• To provide computational evidence supporting the ring-current model.
• To propose a refined approach for evaluating magnetic aromaticity in transition-metal systems.

Main Methods:

• Employed two methods to estimate magnetically induced ring currents: Quantum Theory of Atoms in Molecules (QTAIM) and explicit calculation of magnetically induced current densities (MICD).
• Utilized QTAIM-based magnetizabilities to explain the aromaticity/antiaromaticity of specific 3d metallic clusters (Sc3(-), Cu3(+), Cu4(2-)).
• Validated QTAIM results by comparing with MICD computed using multiconfiguration self-consistent field (MCSCF) and density functional theory (DFT).

Main Results:

• QTAIM-based magnetizabilities successfully explain the two-zone aromaticity/antiaromaticity in studied metallic clusters.
• Calculated atomic and bond magnetizabilities show good agreement with explicitly computed MICD.
• Integrated MCSCF current strength susceptibilities and visual current density analysis confirm QTAIM interpretations.

Conclusions:

• The ring-current model, assessed via global indices like QTAIM magnetizabilities, is superior to local indices for determining magnetic aromaticity in metal clusters.
• A classical electromagnetic theory explanation for anomalous magnetic shielding in transition metal clusters is proposed.
• Magnetic aromaticity/antiaromaticity in transition-metal clusters warrants careful assessment using global indices.