A-Methylcyclopropylcarbinyl cation

Ab initio MO calculations at the MP2(FC)/6-31G level of theory gave estimates of the activation barrier of 1 -methylcyclobutyl cation to the a-methylcyclopropylcarbinyl cation of about 20.0 kcal mol1. At MP2/6-31G + ZPE, the trans-isomer is 2 kcal mol 1 more stable than the cw-isomer, whereas the bent 1-methylcyclobutyl cation 9 is less stable than the trans-1 -methylcyclopropylcarbinyl cation 18 by 0.8 kcal mol"1 (vide supra). The MP2/6-31G optimized bond lengths (in A) for the cis and trans isomeric cations are as shown ... 

The a-methylcyclopropylcarbinyl cation is an interesting intermediate case. It seems to have the classical structure as its lowest energy geometry, but studies by Sorensen and CO workers on the interconversion of syn and anti isomers suggest that the reaction, far... 

The 1-methylcyclobutyl cation 8, which can also be considered as equilibrating 1-methylcyclopropylcarbinyl cations, was shown to have a nonclassical structure by 13C NMR spectroscopic studies in conjunction with Saunders isotopic perturbation of equilibria techniques. 

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