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Nondegenerative Equilibrium in Isopropylcycloalkyl Cations; Comparison of the Isopropylcyclohexyl and isopropylcyclopentyl Cations

In order to find out how does the ring size and its conformation influence the rearrangement processes that take place in isopropylcycloalkyl cation, the temperature dependent 13C NMR spectra of cations 1A/1B and 2A/2B were recorded, respectively. Geometries of all species involved in the rearrangement wer optimized on the B3LYP/6-31G(d) level, both in vacuum and in SO2Cl2 (epsilon=10) using the IPCM model at B3LYP/6-31G(d) level of theory. Experimental and theoretical results revealed that equilibrium favors the isomer 1B which has the charge located on endocyclic carbon atom. On the other hand, when the cyclopentyl ring is replaced with cyclohxyl, the direction of the equilibrium is switched, and the more stableisomer is found to be 2A, which has the chargeon exocyclic carbon atom. It was demonstrated that hyperconjugative interactions, angle strain, torsional strain and solvattaion effects contribute in a way to stabilize the structure 1B. In contrast to this, the relative energy differenceof cyclohexyl ring,in which the torsional strain is almost competely avioded and the result is that the structure 2A is more stable. Even though the equilibrium is shifted in opposie direction for the cations compared, the overall hydride shift mechanism with both cations are the same, i.e. the hydride shift occurs through the nonsymmetric hydrido-bridged structures 1TS and 2TS

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