Assessment of DFT functionals for chiroptical properties of Fe(acac)$$_3$$ adduct: a study of M(acac)$$_3$$ (M = Fe and Co) stereoisomers

Abstract Context This study presents a systematic assessment of exchange-correlation functionals for predicting the chiroptical properties of transition metal complexes, focusing on the Fe(acac) $$_3\cdot $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mmultiscripts> <mml:mrow/> <mml:mn>3</mml:mn> <mml:mrow/> </mml:mmultiscripts> <mml:mo>·</mml:mo> </mml:mrow> </mml:math> CCl $$_4$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mmultiscripts> <mml:mrow/> <mml:mn>4</mml:mn> <mml:mrow/> </mml:mmultiscripts> </mml:math> adduct and its Co(acac) $$_3$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mmultiscripts> <mml:mrow/> <mml:mn>3</mml:mn> <mml:mrow/> </mml:mmultiscripts> </mml:math> analogue. Twenty functionals spanning the principal rungs of Jacob’s ladder—including GGA, meta-GGA, hybrid, range-separated hybrid, and double-hybrid families—were evaluated using time-dependent density functional theory (TD-DFT) to compute electronic circular dichroism (ECD) spectra. The theoretical results were benchmarked against experimental data for the $$\Delta /\Lambda $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>Δ</mml:mi> <mml:mo>/</mml:mo> <mml:mi>Λ</mml:mi> </mml:mrow> </mml:math> -Fe(acac) $$_3\cdot $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mmultiscripts> <mml:mrow/> <mml:mn>3</mml:mn> <mml:mrow/> </mml:mmultiscripts> <mml:mo>·</mml:mo> </mml:mrow> </mml:math> CCl $$_4$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mmultiscripts> <mml:mrow/> <mml:mn>4</mml:mn> <mml:mrow/> </mml:mmultiscripts> </mml:math> system. Structural validation via root-mean-square deviation (RMSD) analysis of optimized geometries against high-resolution X-ray crystallographic data (0.84 Å) demonstrated excellent agreement for the coordination core (RMSD = 0.082 Å). This result confirms the reliability of the theoretical model for primary metal–ligand interactions. Among all functionals investigated, the PBE functional incorporating dispersion correction (D3BJ) and scalar relativistic corrections via the Douglas–Kroll–Hess Hamiltonian (DKH2) provided the most accurate description, faithfully reproducing both the sign and the fine structural features of the experimental ECD spectrum. Relativistic effects proved essential for accurate modeling, particularly for transitions involving metal d -orbitals. The comparative analysis between high-spin (sextet) Fe(III) and low-spin (singlet) Co(III) complexes revealed distinct spectral signatures arising from their different electronic configurations, underscoring the critical role of d -electron count and ligand field strength in determining chiroptical properties. Method All calculations were performed with the ORCA quantum chemistry package. Geometry optimizations of the $$\Delta $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>Δ</mml:mi> </mml:math> and $$\Lambda $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>Λ</mml:mi> </mml:math> enantiomers of Fe(acac) $$_3$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mmultiscripts> <mml:mrow/> <mml:mn>3</mml:mn> <mml:mrow/> </mml:mmultiscripts> </mml:math> and its supramolecular adduct with CCl $$_4$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mmultiscripts> <mml:mrow/> <mml:mn>4</mml:mn> <mml:mrow/> </mml:mmultiscripts> </mml:math> were carried out using the M06 hybrid meta-GGA functional with the def2-SVP basis set, employing tight convergence criteria. Scalar relativistic effects were incorporated via the Douglas–Kroll–Hess Hamiltonian (DKH2). Harmonic vibrational frequency analyses confirmed the nature of all stationary points as local minima. For spectroscopic calculations, single-point TD-DFT calculations were performed at the optimized geometries using the def2-TZVPP basis set. One hundred excited states were computed to ensure adequate coverage of spectroscopically relevant transitions. Twenty exchange-correlation functionals were evaluated through the LibXC library, with atom-pairwise dispersion corrections (D3BJ) applied where appropriate. Rotational strengths were computed in both length and velocity representations to assess gauge invariance (good agreement between length and velocity gauges confirmed basis set convergence). Simulated ECD spectra were generated by Gaussian convolution of discrete transitions (FWHM = 50 cm $$^{-1}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mmultiscripts> <mml:mrow/> <mml:mrow/> <mml:mrow> <mml:mo>-</mml:mo> <mml:mn>1</mml:mn>