Spectrum acquisition spinning

Figure 5.42. The 2D C, H COSY NMR spectrum of erythromycin in CDCIj as a contour plot beneath the corresponding ID C spin-echo NMR spectrum. The spin-echo spectrum (pulse sequence 90°-(r-I80°-x) -data acquisition) was acquired with broadband proton decoupling during the second period and data accumulation. With x = 8 ms CH2 and C resonances are inverted relative to CHj and CH resonances. The small triangles (A) indicate the two outer lines of the solvent triplet. The contour plot levels are higher than the cross-peaks due to methylene moieties.

SECSY (spin-echo correlated spectroscopy) is a modified form of the COSY experiment. The difference in the pulse sequence of the SECSY experiment is that the acquisition is delayed by time mixing pulse, while the mixing pulse in the SECSY sequence is placed in the middle of the period. The information content of the resulting SECSY spectrum is essentially the same as that in COSY, but the mode... 

For example, for a Fobs of 1 pL and an F of 6 pL/min, the residence time for a pulsed spin is 10 seconds. By increasing the flow rate to 12 pL/min, the residence time is decreased to 5 seconds. This decreases the number of NMR acquisitions that can be recorded for a particular spin, reducing the S/N of the resulting NMR spectrum. 

Employing CRAMPS-type proton spectroscopy of powdered solids by using a 2D acquisition experiment can enhance the resolution by a factor of 2 or 3. A H FSLG CRAMPS-MAS experiment was used to study sample 6. Figure 7.10 shows the H MAS spectrum of sample 6 recorded at a spinning rate of 10 kHz. 

Figure 4. 31.94 MHz 13C NMR data for intact lime cutin (bottom) and the solid residue of a depolymerization treatment (top). Both spectra were obtained with a 1H-13C contact time of 1.0 ms, repetition rate of 1.0 s, spinning rate of 3.0 kHz, a H decoupling field of 60 kHz, and a line broadening of 20 Hz. (For the chosen contact time, peak intensities within each spectrum reflect the approximate numbers of each carbon type.) Only the intact cutin spectrum retained signal intensity near 30 ppm when decoupling was delayed before acquisition (13,14).

Fig. 3. Long range and one-bond carbon-13 satellite spectrum of a 5% w/w solution of ethanediol in D2O at 94°C. 16 transients were measured on a Varian Associates Unity 500 spectrometer using the sequence of fig. 1, with 2.5 s presaturation, a t value of 100 ms, spin lock pulses of 450 ps, no homospoil pulse, and no homodecoupling during acquisition.

Fig. 4. (a) 300 MHz proton spectrum and (b)-(e) selective reverse INEPT spectra of 28% menthone (Aldrich) in acetone-ds, measured using a 5 mm sample in the 10 mm broadband probe of a Varian Associates XL300 spectrometer using the sequence of fig. 1. The sample contains substantial quantities of isomenthone, seen clearly in the methyl region of trace (a). Spectra (b) to (e) used selective excitation of carbon sites 6, 7, 2 and 8, respectively, with delays 2r of 3.85, 3.85, 1.92 and 1.54 ms. 32 transients were used for each trace no spin lock pulses or 180° pulses were used. Traces (b) to (e) have a vertical scale lOOOx that of trace (a). No homodecoupling was used during acquisition. 

Fig. 2. N HSQC spectrum of a 75 mM solution of Pro -cyclosporin in CDCI3 at natural isotope abundance using the pulse sequence of fig. 1 without N decoupling during acquisition, t = 5.7 ms, SL = 2.5 ms. An additional, short spin-lock pulse was used right before signal detection [8]. The projections are shown at the top and on the left. (Reproduced by permission of Academic Press from...

The big disadvantage of 13C NMR spectroscopy is its low sensitivity. Due to the natural abundance of 1.1% of the 13C isotope and due to long spin-lattice relaxation times (Th) of the order of seconds, the acquisition of a routine 13C NMR spectrum of a 0.1 M solution of an organic compound takes at least one minute. 

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