Recoupling double-quantum

Fig. 2 (a) DRAMA pulse sequence (using % = t/2 = rr/4 in the text) and a representative calculated dipolar recoupled frequency domain spectrum (reproduced from [23] with permission), (b) RFDR pulse sequence inserted as mixing block in a 2D 13C-13C chemical shift correlation experiment, along with an experimental spectrum of 13C-labeled alanine (reproduced from [24] with permission), (c) Rotational resonance inversion sequence along with an n = 3 rotational resonance differential dephasing curve for 13C-labeled alanine (reproduced from [21] with permission), (d) Double-quantum HORROR experiment along with a 2D HORROR nutation spectrum of 13C2-2,3-L-alanine (reproduced from [26] with permission)... 

Fig. 5 Symmetry-based dipolar recoupling illustrated in terms of pulse sequences for the CN (a) and RNvn (b) pulse sequences, a spin-space selection diagram for the Cl symmetry (c) (reproduced from [118] with permission). Application of POST-CVj [31] as an element in a H- H double-quantum vs 13C chemical shift correlation experiment (d) used as elements (B panel) in a study of water binding to polycrystalline proteins (reproduced from [119] with permission)...

A novel tool is a symmetry-based 29Si dipolar recoupling method (SR264n) [123] for small dipolar interactions that has been initially applied in zeolite structural studies by Brouwer et al. [124], One of the advantages of the new method over INADEQUATE is that the latter misses auto-correlations of symmetry-related double-quantum coherences. The SR26411 method provides such information on auto-correlation which allows to identification of all four connectivities of a tetrahedral Si position. 

Fig.4a,b H MAS spectra of PDMS-yCD inclusion complex (cor/2tt=30 kHz) a comparison of the regular one pulse spectrum with a series of double quantum filtered spectra recorded with increasing recoupling time b 2D DQ correlation spectrum with selected slice. The grey bar identifies the cross peak corresponding to the guest-host dipolar proximity ch identifies the signal from the cyclohexane impurity. (Adopted from [27] with permission)... 

NMR studies devoted to this issue exploit two-dimensional zero- or double quantum NMR methods to recouple homonuclear magnetic dipole-dipole interactions in MAS-spectra. The connectivity information then stems from the presence (or absence) of crosspeaks linking resolved resonances attributable to distinct sites. To date, this type of analysis has been predominantly ap-... 

With optimized spectral resolution, and evolution and detection dimensions can be connected by through-space, through-bond or relaxation-mediated transfer pathways along the polypeptide chain (Fig. 6). Amino acid types and intraresidue interactions are, perhaps, most easily obtained from ( C, C) broadband correlation spectra. Such experiments can involve through-bond or through-space interactions. For the latter, ( C, C) interactions may be actively recoupled (see Refs. 23,25,94 for recent reviews) or they may rely upon ( C, C) transfer facilitated by multiple-( H)-spin effects. Intraresidue correlations are also often easily identified by the inspection of double-quantum single-quantum (2Q,1Q) spectra. 

The pulse sequence for a double-quantum filtration experiment utilizing the HORROR sequence is shown in Fig. 29. The weak on-resonance RF field leads to recoupling of the homonuclear dipolar interaction under MAS if the DQ-HORROR condition Wr = 2 is fulfilled (which may be seen as equivalent to a (homonuclear) n = 1/2 7 experiment). This resonance condition leads to a purely dipolar double-quantum average Hamiltonian which has to be calculated by averaging over two rotation periods. Similarly to... 

While the Ur — 2uj DQ-HORROR condition selectively recouples dipolar interactions, the homonuclear rotary resonance condition = leads to single-quantum and double-quantum recoupling, including recoupling of CSA. Accordingly, the homonuclear n = I condition with Wr = has also been called MSD-HORROR (mixed single- and double quantum HORROR) " and has been employed to determine the orientation of the two C chemical shielding tensors in 2,3- C2-labelled L-alanine. However, the homonuclear resonance condition simultaneously represents a heteronuclear n = 1... 

A new adiabatic double quantum polarisation transfer experiment has been proposed, which is an adiabatic variant of the POST-C7 experiment. A continuous variation of the phase increment between pulses in this experiment leads to the introduction of a fictitious Zeeman field that allows for an adiabatic passage through the recoupling condition. This results in a chemical shift offset-compensated adiabatic experiment, which leads to an efficient and broadbanded polarisation transfer or to a double quantum excitation. 

G. Mali, V. Kaucic, F. TauleUe, Measuring distances between half-integer quadrupolar nuclei and detecting relative orientations of quadrupolar and dipolar tensors by double-quantum homonuclear dipolar recoupling nuclear magnetic resonance experiments, J. Chem. Phys. 128 (2008) 204503. 

It has been shown that the 2D double-quantum (DQ) MAS at about 30 kHz, offers new information about the proximity or connectivity of different nuclei. In the DQ NMR spectroscopy, the generation of DQ coherences requires the existence of a dipolar coupling between two protons. Thus, the observation of particular DQ peaks implies the existence of a sufficient dipolar coupling between the respective nuclei. In this way, detailed information about internuclear proximities can be readily obtained from the 2D peaks. Under MAS conditions the excitation of DQ coherences requires the use of a recoupling pulse sequence. The most used is the BABA scheme, whose acronym comes from the presence of very closely spaced pulses (back-to-back) in the time domain. 

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... 

Dipolar recoupling with a windowless multipulse irradiation DRAWS Windowless DRAMA sequence (Gregory DM, Wolfe GM, Jarvie TP, Shells JC and Drobny GP (1996) Double-quantum filtering in magic-angle-spinning NMR spectroscopy applied to DNA oligomers. Molecular Physics 00(0) 1835-1850). 

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