Revisiting aromaticity and stability in the diboron actinide compound Pa₂B₂

Area of Science:

• Quantum chemistry
• Computational materials science
• Relativistic effects in heavy elements

Background:

• Heavy element clusters, especially actinides, offer unique insights into bonding and relativistic effects.
• Previous studies suggested unconventional aromaticity in the D2h-symmetric Pa2B2 cluster.
• Assessing aromaticity in heavy elements requires careful consideration of electronic structure and relativistic influences.

Purpose of the Study:

• To critically re-evaluate the proposed double Möbius-Craig aromaticity of the D2h-symmetric Pa2B2 cluster.
• To determine the true electronic structure and stability of the Pa2B2 cluster using advanced computational methods.
• To establish robust criteria for identifying aromaticity in heavy element systems.

Main Methods:

• Potential Energy Surface (PES) analysis to identify the most stable isomeric structure.
• Fully relativistic four-component Dirac-Coulomb calculations for Magnetically Induced Current Density (MICD) analysis.
• Multiconfigurational CASSCF(16,16) calculations to assess electronic configuration.
• Comparative analysis with the experimentally validated ReB4- cluster.

Main Results:

• The D2h-symmetric Pa2B2 structure is a higher-energy isomer; the global minimum adopts a distorted tetrahedral geometry.
• MICD analysis revealed no net diatropic ring current, but a weak paratropic response associated with a sigma Pa-Pa bond.
• CASSCF calculations confirmed the D2h structure is single-reference dominated, validating DFT results.
• The ReB4- cluster exhibited a strong diatropic ring current, confirming MICD's ability to detect genuine aromaticity.

Conclusions:

• The Pa2B2 cluster does not exhibit the claimed double Möbius-Craig aromaticity.
• Rigorous PES validation, multiconfigurational treatment, and fully relativistic analysis are crucial for assessing heavy element cluster aromaticity.
• There is a need to critically reassess purported unconventional aromatic motifs in heavy element chemistry due to potential mischaracterization.