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

The cyclopropylcyclopropylidenemethyl cation is generated by protonati on of bis(cyclopropylidene)methane. The NMR spectroscopic data, below -135 degrees C, demonstrate the charge delocalization due to C,C-hyper conjugation. Quantum chemical ab initio calculations at the correlated level (MP2/6-31G) support the interpretation of the experimental res ults and reveal the geometrical consequences of sigma-C,C-hyperconjuga tion. C-13 NMR chemical shifts have been calculated by using the GIAO- SCF and GIAO-MP2 method. The experimental shifts are satisfactorily re produced only at the correlated GIAO-MP2 level, thus showing the impor tance of electron correlation for a correct theoretical description of vinyl cations. The cyclopropylcyclopropylidenemethyl cation is the fi rst vinyl cation prepared in solution which is stabilized only by hype rconjugative interaction with C,C-sigma-bonds.