Abstract
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Cyclopentadienyl derivatives of tin(IV) and mercury(II) and alkylcyclopentadienyl derivatives of mercury(II) are photolysed in solution to show the e.s.r. spectra of the appropriate radicals RC
5 H4 �� (R = H, D, Me, Et, Pri, or But). The C5 H5 �� radical is a planar ��-radical with average D54 symmetry, and its spectrum is broadened in the presence of organic bromides, perhaps by a charge-transfer mechanism. The introduction of alkyl groups breaks the degeneracy of the ��A and ��S molecular orbitals of the ��-system by electron release, destabilising the ��S MO, and the e.s.r. spectrum reflects the spin density distribution in the configuration ��A Z ��S 1. Deuterium has a small but detectable perturbing effect : the ��A MO is destabilised by ca. 100 J mol-1, and thermal mixing of the two energy levels results in the configuration ��S 1.515 ��A 1.485. This is compatible with the model of a vibrational perturbation of the resonance integral ��, rather than of the Coulomb integral ��.