Gradient inverse

Figure 5.6 Comparative segments from (a) conventional and (b) phase-sensitive 8-Hz optimized HMBC spectra of DP-2 recorded using a 3-mm sample positioned coaxially in a 5-mm gradient inverse triple resonance Varian Cold-probe . ...

Fig. 11. Eight transient 500 MHz H-NMR spectra of an 11.9 pg (0.027 pmol) sample of the antibiotic clindamycin (7) prepared in 500 pL of CDC13 in a 5 mm NMR tube (top trace) 292 pL in a 4mm tube (middle trace) and 163 pL in a 3 mm tube (bottom trace). All data were acquired using a 500 MHz 5 mm gradient inverse triple resonance Varian Cold-probe . The s/n ratio was measured for each spectrum using a 200 Hz region of representative noise downfield of the anomeric proton resonating at 5.3 ppm. The s/n ratios were 14.4 1, 20.8 1, and 21.5 1 for the 5, 4, and 3mm tubes, respectively.

Fig. 13. Results obtained with 4 mm samples in a 500 MHz gradient inverse triple resonance cryogenic NMR probe, (a) Non-spinning resolution of the -methanol multiplet for a 30 mm solvent column in a 4 mm tube, (b) Non-spinning resolution of the -methanol multiplet for a 22 mm solvent column in a 4 mm tube. As expected from Fig. 12, the resolution is lower with a solvent column of this short (the optimal solvent column for a 4 mm tube is 30 mm) is shown in Panel A. (c) Resolution of the -methanol multiplet for a 22 mm solvent column in a 4 mm tube with the sample spinning at 20 Hz. For very scarce samples when it is necessary to resort to the shortest possible solvent column height to facilitate the acquisition of high-quality 2D-NMR data, it may be beneficial to spin the sample during the acquisition of the proton reference spectra.

Fig. 15. Comparison of HMBC spectra for a 20 gg sample of retrorsine (3) dissolved in 150 pL rf4-metlianol in a sealed 3 mm NMR tube. The data shown in both panels are 8 Hz optimized non-gHMBC spectra. The spectrum shown in Panel A was acquired in 15 h using a 5 mm 500 MHz cryogenic gradient inverse triple resonance. Almost all of the expected resonances are observed when these data are compared to those for a 700 pg sample of 3 shown in Fig. 2. In contrast, the spectrum shown in Panel B, which was acquired with identical conditions using a 3 mm gradient inverse triple resonance probe, shows the most prominent responses in the spectrum and only a relatively small number of the other responses expected. For a sample of this size to yield a useful HMBC spectrum, it would be necessary to acquire data for a weekend when using a conventional 3 mm NMR gradient inverse-detection NMR probe.

Fig. 18. Comparison spectra for a sealed 3 mm NMR sample tube containing 40 pg (120 nmol) of strychnine (5) dissolved in 165 pL CDC13.234 All of the spectra were acquired and processed identically. The non-gHSQC spectrum shown in Panel A was acquired in 90 m using a 3 mm inverse-detection cryogenic NMR probe operating with an rf coil temperature of 12 K. The sealed 3 mm sample was used to acquire the 90 m spectrum shown in Panel B in a conventional 3 mm gradient inverse-detection probe. All parameters were identical. Panel C shows the results obtained for the sealed 3 mm sample in a conventional 3 mm NMR probe with an overnight (17.5 h) acquisition. (Reprinted with permission from J. Nat. Prod., 63, 1049 (2000). Copyright 2000, American Chemical Society and American Society of Pharmacognosy.)...

To illustrate this point, a comparison of the results obtained with a conventional 3 mm gradient inverse triple resonance and 5 mm cryogenic gradient inverse probe technology for the acquisition of long-range data was performed. Using a 2 mg sample of the... 

Cursory examination of the NMR sample, when dissolved in DMSO, allowed the deduction that an 11-cryptolepinyl moiety was contained in the structure based on the intense purple color, which is characteristic of the extended conjugation of cryptolepine. Because of the relatively small size of the sample, 100 pg, all of the NMR spectral data were recorded using a sealed sample of the degradant isolate in 150 pi of d -DMSO in a sealed 3 mm NMR tube using a 5 mm 500 MHz gradient inverse-detection triple resonance cryogenic NMR probe. 

Let interdifiusion in the binary compound A- B lead to the formation of a metastable parent phase (soHd solution or amorphous phase) with a concentration gradient inversely proportional to -v/Dt, where D is the diflfusion coefficient in the parent phase. Let us study the possibility for the nuclei of the stable intermediate phase to appear under the mentioned gradienL We approximate the concentration dependence of Gibbs potential for both phases by a parabola with a minimum at cj = Cq = 1/2 (this approximation is essential only for comparison with the analytical solutions and is optional at MC simulations). The concentration profile of the parent phase near the forming nucleus wiU be approximated by a linear dependence. 

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