Multi-spin dipolar interaction

Static spin echo decay spectroscopy also forms the basis for the measurement of magnetic dipole-dipole interactions between two unlike nuclei I and S. While this interaction is refocused by the Hahn spin echo, it can be recoupled by applying a 7i-pulse to the S-spins during the dipolar evolution period [12]. This manipulation inverts the sign of the heterodipolar Hamiltonian, and thereby interferes with the ability of the Hahn spin echo technique to refocus this interaction. The corresponding pulse sequence, termed SEDOR spin echo double resonance) shown in Fig. 4, compares the I-spin echo intensity as a function of dipolar evolution time (a) in the absence and (b) in the presence of the ti(S) pulses. Experiment (a) produces a decay F(2ti)/Fo, which is dominated by homonuclear dipole-dipole interactions, while experiment (b) results in an accelerated decay, reflecting the contribution from the heteronuclear I-S dipole-dipole interaction, which is now re-introduced into the spin Hamiltonian. For multi-spin systems, a Gaussian decay is expected ... 

For revealing the main peculiarities of the effect of the multi-pulse sequence (71) a simplified theoretical treatment was suggested that does not include dipolar and spin-lattice interactions. Then the equation for a density matrix p of the spin-1... 

Bennett et al. have given a thorough discussion of SEDRA/RFDR-type experiments in ref. 104, where they covered influences from finite pulses, insufficient proton decoupling, and multi-spin interactions, and introduced a modified version of the basic RFDR sequence with delays of duration kr incorporated between sets of four tt pulses (see Fig. 2Id). This modification recouples weak dipolar coupling interactions only at the conditions n/kyjJt, which avoids the conditions (o A =znu>r) if n/k is not an... 

In summary, the multi-pulse scheme is an effective method to average out the dipolar interactions, especially it is indispensable for homonuclear decoupling of abundant spins with high gyromagnetic ratios. 

Multi-quantum imaging has first been explored on dipolar coupled protons in adamantane (Fig. 8.4.2) [Gar2]. Together with the anisotropy A of the bilinear spin interaction, the duration tp of the multi-quantum preparation period determines the order to which the... 

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