Homonuclear double-quantum coherence

This general process is illustrated for a homonuclear spin system in the scheme of Fig. 5.37 in which only double-quantum coherence existing prior to the pulse is to be retained. Thus, prior to the rf pulse p = 2 coherence exists which experiences a gradient of strength Bgi and duration Ti and thus obtains a spatially dependent phase ... 

The generation of double-quantum coherence requires an antiphase disposition of coupling vectors, which here develop during a period A = l/2Jcc- This is provided in the form of a homonuclear spin-echo to make the excitation independent of chemical shifts. The ti period represents a genuine double-quantum evolution period in which these coherences evolve at the sums of the rotating-ffame frequencies of the two coupled spins, that is, at the sums of their offsets... 

D INADEQUATE experiments are also possible and these type of experiments are unique in that the fl dimension does not correspond to single quantum coherence frequency but to a double quantum coherence frequency. These double quantum coherences are observed because homonuclear coupling and chemical shift evolve simultaneously during the dO period and can not be suppressed. In Check its 5.4.3.3 and... 

Relayed transfer of two-dimensional double-quantum coherence (2QT) has recently been described in homonuclear and heteronuclear correlated spectral system. The pulse sequence used in the relayed 2QT experiment is shown in Figure 5.88. The first three mixing pulses (90°, 180°, 90°) serve to mix double-... 

Figure 1.45 Coherence transfer pathways in 2D NMR experiments. (A) Pathways in homonuclear 2D correlation spectroscopy. The first 90° pulse excites singlequantum coherence of order p= . The second mixing pulse of angle /3 converts the coherence into detectable magnetization (p= —1). (Bra) Coherence transfer pathways in NOESY/2D exchange spectroscopy (B b) relayed COSY (B c) doublequantum spectroscopy (B d) 2D COSY with double-quantum filter (t = 0). The pathways shown in (B a,b, and d) involve a fixed mixing interval (t ). (Reprinted from G. Bodenhausen et al, J. Magn. Resonance, 58, 370, copyright 1984, Rights and Permission Department, Academic Press Inc., 6277 Sea Harbor Drive, Orlando, Florida 32887.)...

Double-Quantum. (DQ) Proton Spectroscopy. Multiple-quantum (MQ) coherences are routinely used in high-resolution solution-state NMR spectroscopy. The MQ coherences cannot be observed directly and the application of two-dimensional methodology is required. The Double-Quantum (DQ) methodology is particularly useful to derive the information on the spatial proximity of different spins on the basis of the homonuclear dipolar couplings. ... 

Homonuclear correlation experiments are not just restricted to the standard COSY experiment, but also include the TOCSY and INADEQUATE experiments. The separation of TOCSY and INADEQUATE from the homonuclear COSY experiment is based on the different coherence evolution and transfer processes involved. Thus the TOCSY experiment is based on cross-polarization in contrast to the polarization transfer used in the homonuclear COSY experiments. INADEQUATE experiments are characterized by the double quantum state of two scalar-coupled nuclei during the tl period such that the second dimension (fl) is scaled into a double quantum frequency. Nevertheless these experiments can all be considered together because they are based on homonuclear scalar coupling and the fl and f2 dimension of the corresponding 2D spectra are related to the same nucleus. 

Homonuclear correlation of nuclei of with low natural abundance, spin-echo sequence for scalar coupling evolution, DQF to suppress unwanted coherences, a double quantum frequency scaled dimension in 2D spectra... 

The double quantum filter In homonuclear COSY the DQF plays an important role for phase sensitive spectra filtering out the dispersive coherence contribution to diagonal peaks. 

NMR is the tool most widely used to identify the structure of triterpenes. Different one-dimension and two-dimension techniques are usually used to study the structures of new compounds. Correlation via H-H coupling with square symmetry ( H- H COSY), homonuclear Hartmann-Hahn spectroscopy (HOHAHA), heteronuclear multiple quantum coherence (HMQC), heteronuclear multiple bond correlation (HMBC), distortionless enhancement by polarisation transfer (DEPT), incredible natural abundance double quantum transfer experiment (INADEQUATE) and nuclear Overhauser effect spectroscopy (NOESY) allow us to examine the proton and carbon chemical shift, carbon types, coupling constants, carbon-carbon and proton-carbon connectivities, and establish the relative stereochemistry of the chiral centres. 

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