The origin of fine structure

Spin-spin coupling in molecules in solution can be explained in terms of the polarization mechanism, in which the interaction is transmitted through the bonds. 

If the X nucleus is x, a p electron of the bonding pair will tend to be found nearby (because that is energetically favorable for it). The second electron in the bond, which must have a spin if the other is p, will be found mainly at the far end of the bond (because electrons tend to stay apart to reduce their mutual repulsion). Because it is energetically favorable for the spin of Y to be antiparallel to an electron spin, a Y nucleus with p spin has a lower energy than a Y nucleus with a spin  

The opposite is true when X is P, for now the a spin of Y has the lower energy  

To account for the value of Vxy as in H-C-H, we need a mechanism that can transmit the spin alignments through the central C atom (which maybe C, with no nuclear spin of its own). In this case (Fig. 13.16), an X nucleus with a spin polarizes the electrons in its bond, and the a electron is likely to be foimd closer to the C nucleus. The more favorable arrangement of two electrons on the same atom is with their spins parallel (Hund s rule. Section 9.11), so the more favorable arrangement is for the a electron of the neighboring bond to be close to the C nucleus. Consequently, the p electron of that bond is more likely to be found close to the Y nucleus and therefore that nucleus will have a lower energy if it is a  

according to this mechanism, the lower energy of Y will be obtained if its spin is parallel (ttxtty and pxpy) to that of X. That is, /hh is negative, for then hJntxmY is negative when ntx and my have the same sign. 

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