Nuclear Spin Energy Diagrams and the Sign of

If asked how many transitions are possible among the four spin states, you might be tempted to list all of the following l- 2, l- 3, l- 4, 2- 3, 2- 4, and 3- 4. But there is a selection rule that controls the probability (and hence intensity) of each transition An allowed transition involves only the flip of one nuclear spin all transitions involving more than one flip are forbidden. From the list of transitions we can therefore delete 1— 4 and 2— 3, because these involve the simultaneous flip of both spins. Transitions l- 2 and 3- 4 result from the flip of only the Hb spin and are thus allowed and responsible for the Hb signal. Likewise, transitions 1— 3 and 2— 4 result from the flip of only the Ha spin and give rise to the Ha signal. Further, notice (from symmetry) that transi- 

FACTORS THAT INFLUENCE THE SIGN AND MAGNITUDE OF J SECOND-ORDER (STRONG) COUPLING EFFECTS 

Theoretical chemists have a relatively complete understanding of the factors that control the sign and magnitude of J. In fact, they have derived some impressive equations that describe the spin-coupling interaction. We will try our best to steer clear of the high-powered math and take a more qualitative approach. 

Although this model is somewhat naive, it does help us rationalize why one- and three-bond H-H, C-H, and C-C coupling constants are usually positive, while two-bond coupling constants are usually negative. 

So much for the sign of J. In general, the magnitude (absolute value) of a coupling constant between nuclei a and b (17ab ) is determined by the generalized equation. 

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