Dipolar interactions secular terms

For the dilute aH spins surrounded by a large number of the 2H spins, the heteronuclear aH-H dipolar interaction is dominant, lifting the spectral overlap between the 1H packets. H spin diffusion is driven by the flip-flop term of the 1H-1H dipolar interaction, which becomes secular in the presence of spectral overlap. Thus, spin diffusion would be accelerated if it had not been for the 1H-2H dipolar interaction. In order to confirm this prediction, they used another RF channel of the OPENCORE... 

For example, the form of the secular terms in the two-body homonuclear dipolar Hamiltonian in the frame of the Zeeman interaction is... 

For a perfect high-field case, where electron Zeeman interaction dominates ZFS and all other terms in the spin Hamiltonian, the only secular term in the dipolar interaction is given by... 

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