Gold setting the "gold standard" among transition metals as a hydrogen bond acceptor - a theoretical investigation

The Au(i) atom of dimethylaurate (DMA) is shown to behave as a hydrogen-bond acceptor, providing theoretical evidence that it can act as a Lewis base. Calculations at the MP2/aug-cc-pVTZ-pp level of theory confirm that DMA forms hydrogen bonds decreasing in strength from -16.2 kcal mol^-1 to -2.4 kcal mol^-1 in the order HCN ≈ HF > H₂O > HCCH > NH₃ > CH₄, i.e. following the trend of decreasing proton acidity of the hydrogen-bond donor. The geometrical and Atoms in Molecules (AIM) parameters of the hydrogen-bonded adducts compare well to those obtained with the auride anion, a known hydrogen-bond acceptor. Relativistic effects are shown to play a dominant role in the formation of the hydrogen bonds with DMA: omission of these effects (confirmed using two different approaches) results in the loss of the hydrogen bond. Instead, the hydrogen-bond donor interacts with the carbon atom on one of the methyl ligands, yielding an adduct that is closely comparable to those found with the Cu and Ag analogues of DMA.