Resonance condition integer spins

The rotational resonance (R ) determination of the distance between members of a homonuclear spin pair is based on rotation-enhanced transfer of Zeeman magnetization. This transfer occurs when the MAS frequency is adjusted to satisfy the R condition 8 = tKo, where 6 is the isotropic chemical shift difference between the two resonances of the spin pair [110], n is a small integer, and is the spinning speed. For carbon-carbon distances, appropriate samples may be engineered by selective pairwise C labeling. 

Rotational resonance (RR) is an intriguing phenomenon which is observed when Vr is equal to a small integer multiple of the difference in the isotropic chemical shift frequencies of two resonances in the spectram [66, 67]. The most apparent effect of RR is that the normally narrow spectral peaks acquire sphttings and broadenings, the nature of which depend on the dipolar coupHng between the two spins. As a specific example, experimental spectra (together with best-fit simulations) corresponding to the = 1, 2, and 3 RR conditions for aU-E-[ll,20- C2]-retinal are shown in Fig. 9.15 [68]. In this case, it was possible to determine that the intemudear distance between the two labels is 0.301 + 0.008 run. 

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