Second-order Shifts in Line Positions

Our analysis thus far has assumed that solution of the spin Hamiltonian to first order in perturbation theory will suffice. This is often adequate, especially for spectra of organic radicals, but when coupling constants are large (greater than about 20 gauss) or when line widths are small (so that line positions can be very accurately measured) second-order effects become important. As we see from 

for example, a spin-1 nucleus with a = 20 G and B0 = 3200 G will have lines at  

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