Ground-state, destabilisation stabilisation

One 11 Bond. It hardly needs saying that a n bond is not usually free to rotate. The n energy 2En that we saw in Fig 1.26 (280 kJ mol ) would be lost at the transition structure for rotation about the C—C bond, which would have the two p orbitals orthogonal. This value is higher than the energy normally available for a chemical reaction. For rotation about a n bond to become easy in the ground state, either the transition structures like diradical 2.101 or the zwitterion 2.102 must be stabilised or the planar structure 2.100 must be destabilised. 

E.s.r. spectra of this cation indicated that it has a triplet ground state [261,263]. Electrochemical determination of the pKp+ value were hampered by a number of uncertainties but indicate that it is exceedingly low, and much lower than the value for the allyl cation [264]. Calculations of the ionic heats of formation of the cyclopentyl, cyclopentenyl and cyclopentadienyl cations from the measured ionisation potentials of the corresponding radicals and the heats of formation of these radicals show that whereas introduction of one double bond stabilises the cation, introduction of the second double bond destabilises the system [265]. E.s.r. spectra of the pentaphenyl [258,259] and pentachloro [259,266] derivatives show that the former has a ground state singlet structure but with a triplet state less than 1 kcal.mol" higher, while the latter has a triplet ground state [252,259]. 

---