Highly accurate CCSD(T) homolytic Al–H bond dissociation enthalpies – chemical insights and performance of density functional theory

We obtain gas-phase homolytic Al–H bond dissociation enthalpies (BDEs) at the CCSD(T)/CBS level for a set of neutral aluminium hydrides (which we refer to as the AlHBDE dataset). The Al–H BDEs in this dataset differ by as much as 79.2 kJ mol⁻¹, with (H₂B)₂Al–H having the lowest BDE (288.1 kJ mol⁻¹) and (H₂N)₂Al–H having the largest (367.3 kJ mol⁻¹). These results show that substitution with at least one –AlH₂ or –BH₂ substituent exerts by far the greatest effect in modifying the Al–H BDEs compared with the BDE of monomeric H₂Al–H (354.3 kJ mol⁻¹). To facilitate quantum chemical investigations of large aluminium hydrides, for which the use of rigorous methods such as W2w may not be computationally feasible, we assess the performance of 53 density functional theory (DFT) functionals. We find that the performance of the DFT methods does not strictly improve along the rungs of Jacob's Ladder. The bestperforming methods from each rung of Jacob's Ladder are (mean absolute deviations are given in parentheses): the GGA B97-D (6.9), the meta-GGA M06-L (2.3), the global hybrid-GGA SOGGA11-X (3.3), the range-separated hybrid-GGA CAM-B3LYP (2.1), the hybrid-meta-GGA ωB97M-V (2.5) and the double-hybrid methods mPW2-PLYP and B2GP-PLYP (4.1 kJ mol⁻¹).