Carbon—hydrogen bonds electron spin resonance

Resonance theory can also account for the stability of the allyl radical. For example, to form an ethylene radical from ethylene requites a bond dissociation energy of 410 kj/mol (98 kcal/mol), whereas the bond dissociation energy to form an allyl radical from propylene requites 368 kj/mol (88 kcal/mol). This difference results entirely from resonance stabilization. The electron spin resonance spectmm of the allyl radical shows three, not four, types of hydrogen signals. The infrared spectmm shows one type, not two, of carbon—carbon bonds. These data imply the existence, at least on the time scale probed, of a symmetric molecule. The two equivalent resonance stmctures for the allyl radical are as follows ... 

Resonance structure b is commonly used to describe delocalization of tt electrons in butadiene. Resonance structures c and d describe hyperconjugation. The structures represent electron transfer between anti hydrogens by ct-ct interactions. Note that the hydrogen atoms act as both electron donors and acceptors. As the two hydrogens are in very similar chemical environments, we expect littie net transfer of charge, but the delocalization affects such properties as NMR spin coupling constants. We will see in Topic 1.1 that this kind of delocalization is also important for hydrogens bonded to sp carbon atoms. 

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