Subject pulse sequences

The delay is generally kept at Vi x> The coupling constant Jcc for direcdy attached carbons is usually between 30 and 70 Hz. The first two pulses and delays (90J -t-180 2-t) create a spin echo, which is subjected to a second 90J pulse (i.e., the second pulse in the pulse sequence), which then creates a double-quantum coherence for all directly attached C nuclei. Following this is an incremented evolution period tu during which the double quantum-coherence evolves. The double-quantum coherence is then converted to detectable magnetization by a third pulse 0,, 2, and the resulting FID is collected. The most efficient conversion of double-quantum coherence can... 

Up to now, neither this method nor STARTMAS has been used by researchers other than their authors, especially because they are subjected to many imperfections of the pulse sequence. Still, it may be anticipated that they will open up new possibilities in a variety of applications, including studies on unstable systems, in-situ high-temperature experiments, hyperpolarized solids, or measurements on very slowly relaxing spins. 

Fig. 1. Pulse sequence of the C HSQC experiment with a spin-lock pulse for the suppression of signals from protons not bound to C. Narrow and wide bars denote 90° and 180° pulses, respectively. The spin-lock pulse is labeled SL. r is set to 1/[2J( C, H)]. The detection period is symbolized by a triangle. Phase cycle ] = 8(y) 4>2 = 2 x,x,y,y) 03 = 4 = 4n = 8(x) 05 =4(x,—x) 05 = 4(x),4(—x) acquisition = 2(x,—x,—x,x). The phases of the C pulses before U (03 and 0.5) are subjected to the States-TPPI scheme [38].

What constitutes an advance in any field will always be subjective. However, the combination of the inherent ability of MR methods to probe the internal structure and transport processes from the A- to cm-scale phenomena non-invasively, quantitatively and with chemical resolution, and with the ability to acquire these data sufficiently fast so that unsteady state processes can be studied is undoubtedly going to open up new avenues of research and allow us to investigate many phenomena for the first time. This section summarises five recent developments in the field of MR in chemical engineering. The first four sub-sections (Sections III.A-III.D) report developments of fast MR measurement pulse sequences, which have recently been implemented for application in chemical engineering research. The final sub-section (Section III.E) addresses a new and different field of research, that of gas-phase imaging. 

Figure 3.17 depicts this sequence. After some number of pulse sequences (depending on sample concentration and isotope being studied), the data are subjected to a Fourier transformation, and the S/N of the resulting frequency-domain spectrum is ascertained. If it is not yet adequate, the pulse sequence is resumed until the desired S/N is attained. 

The 13C spectrum of 2-chlorobutane, first encountered in Figure 7.1, consists of signals at 8 60.4 (CH), 33.3 (CH2), 24.8 (CHj), and 11.0 (CH3). In our experiment, we will first collect the i3C DEPT spectrum (using the pulse sequence in Figure 12.15), with the variable y pulse width set to zero degrees. The process is then repeated 18 more times, with ,. incremented by 10° each time. Finally, the data are subjected to Fourier transformation to give a frequency-domain data set with the F2 axis corresponding to 13C chemical shift and the F axis to y. 

In previous chapters we have referred to sequences of pulses that can be used to make particular measurements, such as 7j and T2 (Section 2.9), and more complex sequences that narrow lines in solids (Section 7.8). In this chapter we explore the use of pulse sequences in more detail and investigate the behavior of magnetizations when subjected to arrays of pulses. We follow these spin gymnastics about as far as possible with the classical picture of magnetization vectors and set the stage for invoking the more powerful formalisms described in Chapter 11. 

As an example, we describe one of many similar experiments devised for assigning resonances in proteins, a subject that we take up in more detail in Chapter 13. This particular experiment is designed to correlate the frequencies within the H—15N—13C=0 portion of a peptide group and is appropriately called simply HNCO. The basic pulse sequence for HNCO is shown in Fig. 12.16. To simplify the notation, instead of I, S, and T, we identify the active spins as H, N, and C, and use K to denote the spin of ar-13C. In a peptide chain, one a carbon is bonded and spin coupled to the nitrogen and another a carbon is bonded and coupled to the carbonyl carbon atom. With recombinant DNA methods, the protein is uniformly and highly enriched in both 13C and 15N, so all of these spins need be considered. 

The rf portion of Fig. 14.2 shows a 90°, r, 180° spin echo pulse sequence, rather than a simple 90° pulse. All imaging studies employ either a spin echo sequence or a gradient echo to avoid acquisition of data during the FID, which decays rapidly in the presence of a magnetic field gradient. Instead, data acquisition occurs during the echo, when the rf circuitry is not subject to aberrations... 

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