The diradicaloid electronic structure of dialumenes: a benchmark study at the Full-CI limit

Abstract

Multiply-bonded main group compounds of groups 13–15 are attracting significant interest not only because they provide fundamental insight into the nature of metal–metal bonding, but also for their potential in small molecule bond activation and catalysis. This includes dialumenes, neutral Al(I) compounds that contain AlAl double bonds, which display high reactivity owing to their intrinsic diradicaloid character. The electronic structure of the simplest dialumene, Al₂H₂, is here analyzed up to a practical Full-CI limit using DMRG and selected CI methods for the bond dissociation energy (BDE), geometry and properties of the electron density (difference density, ELF). Acquiring Full-CI reference values for the simplest dialumene (but possessing the highest diradical character) allows for a rigorous benchmarking of simpler correlated wavefunction theory (WFT) methods and density functional methods in treating the electronic structure of such systems. Single-reference coupled cluster theory using a RHF reference is found to reliably converge to the Full-CI limit and CCSD(T) is fully capable of capturing the diradical character, while multi-reference methods offer no clear advantages. Density functional methods struggle to fully describe the electronic structure complexity although non-hybrid functionals such as TPSS come close. Solving the inverse Kohn–Sham problem for a Full-CI-quality density revealed minimal density-driven errors in the TPSS-calculated BDE unlike high-percentage hybrids such as M06-2X. No density functional, however, predicts accurate relative energies. Fractional-occupation density plots at the TPSS level correlate well with WFT-based diradical character metrics, a useful result for determining diradical character in larger systems.