THE 1-(TRIMETHYLSILYL)BICYCLOBUTONIUM ION - NMR-SPECTROSCOPY, ISOTOPEEFFECTS, AND QUANTUM-CHEMICAL AB-INITIO CALCULATIONS OF A NEW HYPERCOORDINATED CARBOCATION

The 1-(trimethylsilyl)bicyclobutonium ion is generated from (1'-(trime thylsilyl)cyclopropyl)methanol by reaction with SbF5. The NMR spectros copic data in SO2ClF/SO2F2 solution at -128 degrees C are in accord wi th a bridged puckered bicyclobutonium structure undergoing a 3-fold ra pid degenerate rearrangement that renders the two beta- and one gamma- methylene groups equivalent, leading to one averaged C-13-NMR signal f or the CH2 groups at 48.9 ppm. Conformational ring inversion is slow s o that two separate signals for the three averaged endo-CH2 (4.04 ppm) and three averaged exo-CH2 hydrogens (3.24 ppm) are observed. The deu terium equilibrium isotope effects for exe- and endo-CHD-labeled catio ns are different in sign and magnitude and are rationalized by differe nt endo- and exo-C-H bond force constants at the pentacoordinated carb on. NMR chemical shift calculations for the 1-silylbicyclobutonium ion and the (1'-silylcyclopropyl)methyl cation were performed with the GI AO-SCF and GIAO-MP2 methods. The experimental shifts are satisfactoril y reproduced by GIAO-MP2/tzp/dz calculated shifts for the 1-silylbicyc lobutonium structure. The good agreement between theory and experiment supports a fully degenerate set of interconverting 1-(trimethylsilyl) bicyclobutonium ions and excludes contributions from other isomers to the observed equilibrium process. The geometric and electronic propert ies of the 1-(trimethylsilyl)bicyclobutonium cation are intermediate b etween those of the parent bicyclobutonium ion and those of the methyl -substituted analogue.