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

Abstract

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 best-performing 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).

A Systematic Exploration of B–F Bond Dissociation Enthalpies of Fluoroborane-Type Molecules at the CCSD(T)/CBS Level

“…In contrast, the worst-perfo method was shown to be the HGGA functional BH&HLYP, which afforde unacceptably large MAD of 45.6 kJ mol −1 and an LD of 54.2 kJ mol −1 . The poor perform of BH&HLYP in the computation of BDEs has been noted previously, for examp computation of Al-H [48], B-Cl [49], and N-X (X = F, Cl, and Br) bonds [54,55]. Seco all but four cases, it is evident that the functionals considered in this study te underestimate the BDEs (i.e., the MD values adopt negative values in most cases).…”

“…As previous studies have shown that the inclusion of empirical dispersion corrections may improve the performance of the underlying DFT functionals for the computation of various BDEs [48,49,55], we have additionally surveyed the effect of the inclusion of the Becke-Johnson D3 dispersion correction for twelve functionals (Table 4).…”

“…In particular, it has been demonstrated that for species with %TAE[(T)] ≤ 5%, post-CCSD(T) contributions are unlikely to exceed 2 kJ mol −1 . The same diagnostic has also been employed previously for the purposes of validation of datasets of the BDEs for a range of other chemical bonds, including, for example, S-F [46], C-Cl [47], Al-H [48], and B-Cl bonds [49].…”