Spectral width pulse sequence

A number of parameters have to be chosen when recording 2D NMR spectra (a) the pulse sequence to be used, which depends on the experiment required to be conducted, (b) the pulse lengths and the delays in the pulse sequence, (c) the spectral widths SW, and SW2 to be used for Fj and Fi, (d) the number of data points or time increments that define t, and t-i, (e) the number of transients for each value of t, (f) the relaxation delay between each set of pulses that allows an equilibrium state to be reached, and (g) the number of preparatory dummy transients (DS) per FID required for the establishment of the steady state for each FID. Table 3.1 summarizes some important acquisition parameters for 2D NMR experiments. 

Fig. 5. Pulse sequences of NOESY and ROESY with spin-lock purge pulses for water suppression. (A) NOESY pulse sequence. The spin-lock pulses are typically of length 0.5 ms and 2 ms, and r = 1/SW, where SW is the spectral width in the acquisition dimension. Phase cycle (pi = x,—x) 4>2 = 4 x,x,—x,—x) ...

The H-NMR spectra of FCC feeds were recorded on a Bruker DRX 400 MHz NMR spectrometer. The concentration of the samples of 5 wt% in CDCI3 was recommended by Molina, Navarro Uribe, and Murgich [2] to avoid concentration dependence of the chemical shift. A 30° pulse sequence was applied, with 4.089 s acquisition time, 2 s pulse delay [2], 8012.8 Hz spectral width, and 64 scans. Hexamethyldisiloxane (HMDSO) was used as a reference. NMR processing was realized using MestReNova software. The phase and baseline of the resulting spectra were manually adjusted and corrected. The spectra were integrated six times and average values were taken for the purpose of calculations. The spectra were divided... 

During the acquisition, it is possible to reduce the baseline rolling by using suitable pulse sequences. The most commonly used are the RIDE (ring-down elimination) sequence, first proposed by Ellis7 14 (slightly different from a sequence introduced by Canet et al.15), the ACOUSTIC sequence, proposed by Patt,16 and a series of extended spin-echo sequences.14 However, all these sequences result in inefficient excitation over a wide range of frequencies, and therefore they can be applied only to restricted spectral widths. 

Fig. 21. The new five-pulse sequence for recording static 2H exchange spectra.51 The experiment differs from the simple three pulse sequence in Fig. 19 in the addition of r-90°-r (or A — 90° - A) echo sequences before the t and h periods to avoid spectral distortions caused by receiver deadtime and finite pulse width problems. The broader pulses are 90° pulses the narrower ones 54.7° pulses.

Steady-state scans (Section 2-4i) are used before the start of essentially all 2D experiments. They are particularly important in a number of pulse sequences in order to compensate for spin-lock (Sections 7-7b and 7-10b) and decoupler (Section 7-8) heating effects. Larger numbers of steady-state scans are employed in experiments that have either particularly long spin-lock times or X-nucleus decoupling over especially wide spectral widths. 

Terms such as and T-L dd- cs vanish for all sequences in Table 1, since all of them except MREV8 possess internal reflection symmetry [26]. The scale factors of these sequences decrease with increasing number of pulses. Thus, line narrowing must compensate for the scaling of the spectral width in order to increase the resolution on a corrected ppm scale. 

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