The dipolar coupling

The dipolar coupling between two nuclei 7,.S is systems containing pairs of... 

In equation (bl. 15.24), r is the vector coimecting the electron spin with the nuclear spin, r is the length of this vector and g and are the g-factor and the Boln- magneton of the nucleus, respectively. The dipolar coupling is purely anisotropic, arising from the spin density of the impaired electron in an orbital of non-... 

The NOESY spectrum of buxatenone shows four cross-peaks, A-D. Cross-peak B represents the dipolar coupling between the most upfield C-19 cyclopropyl proton (8 0.68) with the most downfield olefinic proton (8 6.72). This could be possible only when the double bond is located either between C-1 and C-2 or between C-11 and C-12. The possibility of placing a double bond between C-11 and C-12 can be excluded on the basis of chemical shift considerations, since conjuga-... 

N.m.r. studies of liquid crystal media showed that phenyl sulphonate counter ions induced abrupt changes in the dipolar coupling of methy1phosphonate. 61... 

Each of these couplings is inversely proportional to the cube of the distance between the atoms concerned, and all the dipolar couplings depend on the degree of orientation of the molecules. The ratios of couplings thus depend on the ratios of distances in the molecule, and from them the entire molecular structure, except for the overall size, may be determined. Studies of silyl compounds dissolved in cyanobiphenyl liquid crystals have yielded the following HSiH angles ... 

The dipolar coupling tensor D is defined by Eq. (6D). Again taking the case of an unpaired electron localized in a 2pz atomic... 

Within this framework, the dipolar coupling between two spins I and S may be described in terms of the Hamiltonian... 

As demonstrated by Griffin, Levitt, and coworkers in the late 1980s [21, 93], it is also possible to recouple homonuclear dipolar couplings through interference between isotropic chemical shifts and the rotor revolution. This phenomenon, called rotational resonance, occurs when the spinning frequency is adjusted to a submultiple of the isotropic chemical shift difference, i.e., ncor = ct> so o) °. To understand this experiment, the dipolar coupling Hamiltonian in (10) is transformed... 

Addressing first the heteronuclear case (i.e., the DCP experiment introduced by Schaefer and coworkers in 1984), the dipolar coupling may be expressed as in (10) that upon heteronuclear truncation of the transverse terms may be recast in irreducible tensor operator form (7Z = T[ 0, Sz = Tf 0) as... 

The so-called HORROR experiment by Nielsen and coworkers [26] introduced continuous rf irradiation recoupling to homonuclear spin-pairs and initiated the later very widely used concept of /-encoded recoupling. Using a irreducible spherical approach as described above, the HORROR experiment (Fig. 2d) is readily described as starting out with the dipolar coupling Hamiltonian in (10) and x-phase rf irradiation in the form Hrf = ncor(Ix +SX), also here without initial constraint on n. The dipolar coupling Hamiltonian transforms into tilted frame (rotation n/2 around Iy + Sy)... 

For protons (ADD) = 0, so that the dipolar coupling is determined by the two- and three-center contributions. Recently, Keijzers and Snaathorst1271 have shown that the three-center contributions (ADD)3 should not, in general, be neglected in the computation of anisotropic proton hf coupling constants. In most ENDOR work, however, only the two-center contribution (ADD)2>, ( distant term) has been considered. 

Owing to the large spin density at the central ion M, the dipolar coupling (5.5) of the proton is usually dominated by the term... 

Although (5.6) yields only a rough approximation for the dipolar coupling, it can often be applied to get estimates of R(M) and n(M) and thus to assign the hfs tensors to the corresponding protons. 

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