Cyclopentadienyl radicals, orbital degeneracy

The degeneracy of the /a and is molecular orbitals in the cyclopentadienyl radical can be broken by the electronic effect of the substituent. The /A orbital has a node at Cl, and is unaffected, but the i /s orbital, with a high coefficient at Cl, has its energy raised by an electon-releasing substituent, and lowered by an electron-attracting substituent, placing the unpaired electron preferentially in the i /s or /a orbital, respectively, as shown in Figure 20-1. 

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