Trimesitylsilylium cation verification of a free silylium cation in solution by NMR chemical shift calculations

NMR chemical shift calculations at the SOS-DFPT/PW91/[9s6p2d/5s4pld/5s4pld/3s]//B3LYP/6-31G(d) level of theory were used to describe the trimesitylsilylium cation 1, recently synthesized in benzene solution and investigated by NMR spectroscopy. The conformation of cation 1 is characterized by mesityl rings rotated by 47° in a propeller-like form. Contrary to other silylium cations investigated, cation 1 forms a weak Van der Waals complex 3 with benzene rather than a Wheland σ-complex. The calculated 29Si NMR chemical shifts for 1 and 3 are 226 and 227 ppm, compared to the experimental value of 225.5 ppm. The agreement between calculated and measured NMR chemical shifts provides evidence that cation 1 presents the first free silylium cation synthesized.