TRIMESITYLSILYLIUM CATION VERIFICATION OF A FREE SILYLIUM CATION IN SOLUTION BY NMR CHEMICAL-SHIFT CALCULATIONS

NMR chemical shift calculations at the SOS-DFPT/PW91/[7s6p2d/5s4pld/3s ]//B3LYP/6-31G(d) level of theory were used to describe the trimesityl silylium cation 1, recently synthesized in benzene solution and invest igated by NMR spectroscopy. The conformation of cation 1 is characteri zed by mesityl rings rotated by 47 degrees in a propeller-like form. C ontrary to other silylium cations investigated, cation 1 forms a weak Van der Waals complex 3 with benzene rather than a Wheland sigma-compl ex. The calculated Si-29 NMR chemical shifts for 1 and 3 are 226 and 2 27 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. (C ) 1997 Elsevier Science B.V.