Vertical ionization energies of boron chelates: a DFT and electron propagator theory benchmark

A comprehensive comparative study of vertical ionization energies (VIEs), vertical electron affinities (VEAs), and Kohn-Sham HOMO and LUMO gaps for 41 medium-sized (up to 50 atoms) boron chelate complexes and their precursors is reported. The study combines electron propagator theory (EPT), density functional theory (DFT), and ultraviolet photoelectron spectroscopy (UPS). The EPT-based approaches included the outer-valence Green's function (OVGF), partial third-order propagator (P3), and approximately renormalized partial third-order (P3+) methods. The DFT calculations employed the ωB97X, CAM-B3LYP, M06-2X, and BHandHLYP functionals. Among the EPT methods, the P3/aug-cc-pVDZ combination yields the most accurate VIEs, with a mean deviation (MD) of 0.03 ± 0.20 eV. VIEs were further benchmarked across various geometry optimization methods, basis sets, and DFT functionals, including ωB97M-D4 and ωB97M-V. This analysis establishes that, within the DFT framework, the ωB97M-V functional incorporating the VV10 nonlocal correlation provides the closest agreement with experiment. For β-diketones and their thio- and imino-analogues, this method, combined with r²SCAN-3c geometries and the saug-ANO-pVTZ or def2-QZVPPD basis sets, achieves an accuracy (MD = 0.11 ± 0.18 eV) comparable to that of EPT methods. Across the series of studied compounds, the selected DFT method yields semi-quantitative agreement between theoretical and experimental VIEs (MD = 0.31 ± 0.25 eV). Furthermore, the r²SCAN-3c/ωB97M-V/def2-QZVPPD protocol is identified as optimal for calculating VEAs (MD = 0.18 ± 0.22 eV) and HOMO-LUMO gaps (MD = 0.11 ± 0.33 eV) of the studied complexes.