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Revisiting the mean-field picture of dipolar effects in solution NMR
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Image of FIG. 1.
FIG. 1.

Sequences which show limitations of the traditional mean-field picture of dipolar effects. In each case the vertical lines are rf pulses, and shaded boxes are gradient pulses along a specific direction s or s′, with areas GsT or G sT. (a) and (b): Here the first pulse flip angle ɛ is small, so almost all of the magnetization lies along the z axis. For a spherical sample, dipolar effects would then be expected to be unobservable. Instead, these sequences show a frequency shift determined by gradient direction, but independent of ɛ. Both experiments have identical dipolar effects; the second one suppresses inhomogeneous broadening and radiation damping. (c) MAXCRAZED sequence, which alters the CRAZED sequence by imposing strong gradients in a different direction during the interval d2. Certain gradient pair directions dramatically alter the observed peaks. (d) GRACE sequence, which modifies a traditional COSY experiment by adding matched gradient pulse pairs in different directions in the two time intervals. Certain gradient pair directions produce double-quantum signals, as detected by phase cycling the first pulse.

Image of FIG. 2.
FIG. 2.

The n-quantum CRAZED experiment produces signals in the indirectly detected dimension from intermolecular n-quantum coherences. (a) The traditional prototype sequence; (b) a sequence more reflective of common practice, since splitting the gradient reduces the bulk signal during d 2 and helps suppress radiation damping.

Image of FIG. 3.
FIG. 3.

Double-quantum CRAZED pulse sequence, which produces signals from intermolecular multiple-quantum coherences as described in text. The two gradient pulses (shaded boxes) are configured in a 1:2 area ratio, which suppresses all conventional signals. This experimental data comes from a tube with equal volumes of acetone and water, with resonance offsets 630 Hz and 1500 Hz, respectively; all gradient pulses were 1.5 ms duration.

Image of FIG. 4.
FIG. 4.

Experimental signals from the sequence in Figure 1(b), for magic angle (left) or z gradient (right). The magic angle gradients generated an unshifted signal.

Image of FIG. 5.
FIG. 5.

Ratios of the heteromolecular (f 1 at the sum of the acetone and water frequencies) to the homomolecular peaks for various MAXCRAZED sequences, as a function of the delay between the gradient pulses. In the conventional treatment, this should affect nothing except a possible diffusion weighting. In fact, for the XZ-MAXCRAZED it drastically alters peak intensity, and it modestly alters peak intensity for the 1+1 X-CRAZED.

Image of FIG. 6.
FIG. 6.

Comparison of the observed signal in the MAXCRAZED experiments from the predictions by simple diffusion weighting (dashed lines). The XZ MAXCRAZED deviates dramatically from those predictions, but only for the homomolecular peaks (left).

Image of FIG. 7.
FIG. 7.

Experimental data for the sequences in Figure 5, applied to a spherical sample of acetone in water. The sample was an 8 mm sphere with equal volumes of acetone and water, studied in a 360 MHz NMR spectrometer with a 25 mm microimaging coil (giving a small filling factor and the modest radiation damping effects). The phase cycled COSY only gives residual t1 noise, as expected, as does the grad-COSY combination and the XM GRACE; the XZ GRACE and COSY-grad experiments show double-quantum peaks.

Image of FIG. 8.
FIG. 8.

Comparison of observed spectra as a function of the delay between the two strong gradients d 2, which is the interval when the transverse magnetization is modulated. The XZ-GRACE, and to a lesser extent the COSY-grad experiment, show growth of DQ peaks; the XM-GRACE does not.

Image of FIG. 9.
FIG. 9.

Comparison of the homomolecular and heteromolecular peak intensities for the GRACE and COSY-grad experiments, as a function of the delay between the gradient pulses. Homomolecular peak intensities grow with increasing delay in the XZ-GRACE and COSY-GRAD experiments, heteromolecular intensities do not.


Generic image for table
Table I.

Comparison of the four peak intensities in the CRAZED and MAX-CRAZED experiments, here with d 2 = 60 ms. The “absolute” numbers give intensities relative to the strongest X-CRAZED; the “normalized” numbers in parentheses are intensities with the homomolecular water peak given intensity 1 in each spectrum.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Revisiting the mean-field picture of dipolar effects in solution NMR