Homonuclear recoupling

We note that DRAMA has also formed the basis for refined variants such as the chemical-shift-compensated MELODRAMA [85], windowless DRAWS [29], and the DRAMA-XY4 [86] experiments, finding important applications for example for measurement of 13C-13C distances involving carbonyl spins. We should also note the existence of many other important pulsed homonuclear recoupling experiments such as the widely used Radio Frequency Driven Dipolar Recoupling (RFDR) [24] and Back-to-Back (BaBa) [87] experiments. 

The idea of frequency selectivity is certainly not restricted to J-coupling mediated polarization transfer. Furthermore, frequency selective polarization transfer can also be realized by DQ recoupling techniques. For the technique of shift-evolution-assisted selective homonuclear recoupling (SEASHORE), which employs POST-C7 to construct an effective DQ coupling [82, 83], frequency selective polarization can be achieved when the transmitter offset is set to the... 

The simplest technique that enables phosphorus-phosphorus correlations for complex multi-component system and elucidation of structural constraints is the PDSD sequence.48 This experiment is relatively easy to set up however, the distances between spins cannot be established with satisfactory precision. One common way of determining distance type information using spin-7i NMR is based on the acquisition of the DQ filtered spectra as a function of the DQ mixing time.49 The recently introduced DQ homonuclear recoupling sequences based on C and/or R symmetry developed by Levitt and co-workers allows very precise analysis of intemuclear distances.50 The accuracy of distances obtained from these DQ experiments depends on the effective suppression of contributions to the spin Hamiltonian other than the required homonuclear dipolar coupling terms. 

A number of recoupling techniques are utilized for specific purposes, but they all rely on synchronizing measurements, and, in most instances, pulse sequences, with the rotation period. For homonuclear recoupling (e.g., 13C-13C or 31P-31P), rotational resonance (R2) is based on magnetization exchange when the spinning rate wr is adjusted so that a multiple of it equals the chemical shift difference between the two dipolar coupled nuclei, and DRAMA (dipolar recovery at the... 

A theoretical treatment of the DREAM adiabatic homonuclear recoupling experiment has been given using Floquet theory. An effective Hamiltonian has been derived analytically and the time evolution of the density operator in the adiabatic limit has been described. Shape cycles have been proposed and characterized experimentally. Application to spin-pair filtering as a mixing period in a 2D correlation experiment has been explored and the experimental results have been compared to theoretical predictions and exact numerical simulations. 

In another report by the same group, a high-resolution two-dimensional H double-quantum (DQ) homonuclear recoupling experiments, combined with SAM homonuclear decoupling is presented. The experiment affords highly resolved and clean H DQ-SQ 2D spectra at very-fast MAS rates (35 kHz). It is shown that the new method is well suited to probe H- H... 

Two-dimensional homonuclear correlation NMR spectra of solids of biological interest have been reported at high magnetic fields (14.1 and 18.8 T) and MAS rates up to 67 kHz, using a special symmetry-based homonuclear recoupling and CRAMPS decoupling. This method is shown to afford exceptional spectral resolution and is well suited to probe H- H proximities in powdered solids. 

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