Gradient selected

A wide variety of ID and wD NMR techniques are available. In many applications of ID NMR spectroscopy, the modification of the spin Hamiltonian plays an essential role. Standard techniques are double resonance for spin decoupling, multipulse techniques, pulsed-field gradients, selective pulsing, sample spinning, etc. Manipulation of the Hamiltonian requires an external perturbation of the system, which may either be time-independent or time-dependent. Time-independent... 

Figure 25 H2BC spectrum recorded on cyclosporine (left), compared with a standard gradient-selected HMBC (right).

ID-transient NOE experiments employing gradient selection are more robust and therefore are more reliable for measuring dipolar... 

S = 0.0 ppm)) of oximes 44-51 measured for solutions in DMSO-d based on -gradient selected H, N HMBC experiments"... 

Gawinecki and co-workers reported the structural determination of isomeric products from a similar reaction, the condensation of 1,2-diaminobenzenes 197 with 1,2-dicaibonyl compound 196 <99T8475>. The two different regioisomeric quinoxalines 198, which were produced in nearly equal amounts, were distinguished through the use of advanced NMR techniques including 2D z-gradient selected H, N HMBC. 

Fig. 2. Pulse scheme for the gradient-selected, sensitivity-enhanced X/Y se-HSQC experiment as employed for 31P/15N correlation spectroscopy in Ref. 25. 90° and 180° hard pulses are denoted by solid and open bars, respectively. 2 are delays of length 1/(4 /x,y)> and is a short delay of the same length as the gradient pulse (typically rj 1 ms). Pulse phases are x, unless specified. The ratio of gradient pulse strengths is set to G2/Gi = Yy/Yx, and quadrature detection in Fi is achieved by recording every transient twice and changing the sign of G2 in the second scan.

Fig. 5. 500 MHz HRMAS H NMR spectra of FMOC-isoleucine on Wang resin swollen in DMF- 7 and spun at 4 kHz. Spectrum A shows the presence of dissolved phenylalanine peaks, particularly around 3.0 ppm from its methylene protons, and residual protons of the solvent. Application of a diffusion filter using gradients selectively removes these signals in spectrum B. Reproduced with permission from Ref. 50. Copyright 1999 Elsevier.

Because the sum is not equal to zero, we end up with twisted coherence and no signal in the receiver. We call this a gradient-selected experiment because the gradients are being used to specifically refocus coherence in the desired coherence transfer pathway (XH SQC -> 13C SQC) and to reject all others. In Chapter 10, we will develop the idea of coherence order in a more precise manner, and we will see that coherence order can be either positive or negative. 

Disadvantages of Phase Cycling Compared to Gradient Selection... 

Loss of Sensitivity Due to Overselectivity. Gradient selection means that only a single coherence level can be present at the time of each gradient. With phase cycling we apply a mask with holes at regular intervals, so that more than one coherence level... 

Gradient-Selected HSQC with Phase-Sensitive Data Presentation... 

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