CYCLOPROPYLCYCLOPROPYLIDENEMETHYL CATION - A UNIQUE STABILIZED VINYL CATION CHARACTERIZED BY NMR-SPECTROSCOPY AND QUANTUM-CHEMICAL AB-INITIO CALCULATIONS

The cyclopropylcyclopropylidenemethyl cation is generated by protonation of bis(cyclopropylidene)methane. The NMR spectroscopic data, below -135 degrees C, demonstrate the charge delocalization due to C,C-hyperconjugation. Quantum chemical ab initio calculations at the correlated level (MP2/6-31G*) support the interpretation of the experimental results and reveal the geometrical consequences of sigma-C,C-hyperconjugation. C-13 NMR chemical shifts have been calculated by using the GIAO-SCF and GIAO-MP2 method. The experimental shifts are satisfactorily reproduced only at the correlated GIAO-MP2 level, thus showing the importance of electron correlation for a correct theoretical description of vinyl cations. The cyclopropylcyclopropylidenemethyl cation is the first vinyl cation prepared in solution which is stabilized only by hyperconjugative interaction with C,C-sigma-bonds.