Ab initio 13C NMR and structural studies of the (trifluoromethyl)cyclopentyl carbocations

Ab initio calculations at the MP2/cc-pVTZ level of theory show that the classical 1-(trifluoromethyl)cyclopentyl cation (11) is substantially destabilized (by 9.8 kcal/mol) as compared to its isomeric mu-hydrido-bridged global minimum structure 16. However, the classical carbocation 11 is relatively more stable than the nonclassical cyclobutylmethyl cations 6 and 7, by 7.9 and 3.6 kcal/mol, respectively. We have also shown involvement of the noncovalent H center dot center dot center dot F bonding interactions in the mu-hydrido bridged structures, 13, 16, 19, and 20. The latter H center dot center dot center dot F bond distances are dependent on the number of fluorines in the fluoroalkyl group, and decrease in the order: 16 (CF3) > 20 (CHF2) > 19 (CH2F), in accordance with the decreased basicity of the fluorine atom across this series.