Hyperfine Coupling and the a Value

As was tme for the sign and magnitude of coupling constant J (Chapter 9), the sign and magnitude of a given a value depends on the proximity of the unpaired electron to the 

It may surprise you that p is larger than but this fact is consistent with the way the unpaired electron is delocalized through the molecule. In particular, another contributing nobond (hyperconjugative) resonance structure for the propyl radical is written with the unpaired electron directly on Hp  

There is no valid resonance structure that places the electron directly on H or H.  

Just as we did for H NMR, we can neglect hyperfine coupling to the carbons because of the low natural abundance of C (Table 2.1).  

Although the sign of a has no effect on the appearance of an EPR spectrum [just as the sign of J has no effect on an NMR spectrum (Section 9.1)], it does have an effect on CIDNP behavior (Section 11.7). 

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