The "invisible" ¹³C NMR chemical shift of the central carbon atom in [(Ph₃PAu)₆C]²⁺: A theoretical investigation

The experimental ¹³C NMR chemical shift of the central carbon atom in the octahedral [(Ph₃PAu)₆C]²⁺ cluster was investigated on the basis of relativistic density functional calculations. In order to arrive at independent model conclusions regarding the value of the chemical shift, a systematic study of the dependence of the cluster structure on the phosphine ligands, the chosen density functionals, and the basis set size was conducted. The best structures obtained were then used in the NMR calculations. Because of the cage-like cluster structure a pronounced deshielding of the central carbon nucleus could have been expected. owever, upon comparison with the ¹³C NMR properties of the related complex [C{Au[P(C₆H₅)₂(p-C₆H ₄NMe₂)]}₆]²⁺, Schmidbaur et al. have assigned a signal at δ= 135.2 ppm to the interstitial carbon atom. Our calculations confirm this value in the region of the aromatic carbon atoms of the triphenylphosphine ligands. The close-lying signals of the 108 phenyl carbon atoms can explain the difficulties of assigning them experimentally.