Inversion recovery sequence

The Davies-ENDOR teclmique is based on an inversion recovery sequence (see figure B1.15.13(A ). The experiment starts by interchanging the populations of levels 1 and 3 of one of the EPR transitions of the. S-/=i... 

Figure 2.27. Sequence of measurements to determine the C spin-lattice relaxation times of 2-octanol (42) [(CD3)2C0, 75% v/v, 25 °C, 20 MHz, inversion-recovery sequence, stacked plot]. The times at which the signals pass through zero, xo, have been used to calculate, by equation 10, the T values shown above for the nuclei of 2-octanol...

It is important to avoid saturation of the signal during pulse width calibration. The Bloch equations predict that a delay of 5 1] will be required for complete restoration to the equilibrium state. It is therefore advisable to determine the 1] values an approximate determination may be made quickly by using the inversion-recovery sequence (see next paragraph). The protons of the sample on which the pulse widths are being determined should have relaxation times of less than a second, to avoid unnecessary delays in pulse width calibration. If the sample has protons with longer relaxation times, then it may be advisable to add a small quantity of a relaxation reagent, such as Cr(acac) or Gkl(FOD)3, to induce the nuclei to relax more quickly. 

The 50.31 MHz 13C NMR spectra of the chlorinated alkanes were recorded on a Varian XL-200 NMR spectrometer. The temperature for all measurements was 50 ° C. It was necessary to record 10 scans at each sampling point as the reduction proceeded. A delay of 30 s was employed between each scan. In order to verify the quantitative nature of the NMR data, carbon-13 Tj data were recorded for all materials using the standard 1800 - r -90 ° inversion-recovery sequence. Relaxation data were obtained on (n-Bu)3SnH, (n-Bu)3SnCl, DCP, TCH, pentane, and heptane under the same solvent and temperature conditions used in the reduction experiments. In addition, relaxation measurements were carried out on partially reduced (70%) samples of DCP and TCH in order to obtain T data on 2-chloropentane, 2,4-dichloroheptane, 2,6-dichloroheptane, 4-chloroheptane, and 2-chloroheptane. The results of these measurements are presented in Table II. In the NMR analysis of the chloroalkane reductions, we measured the intensity of carbon nuclei with T values such that a delay time of 30 s represents at least 3 Tj. The only exception to this is heptane where the shortest T[ is 12.3 s (delay = 2.5 ). However, the error generated would be less than 10%, and, in addition, heptane concentration can also be obtained by product difference measurements in the TCH reduction. Measurements of the nuclear Overhauser enhancement (NOE) for carbon nuclei in the model compounds indicate uniform and full enhancements for those nuclei used in the quantitative measurements. Table II also contains the chemical... 

There are important FFC sequences in which the preparatory sub-sequence sections includes RF pulses. A typical example is the FFC version of the classical inversion recovery sequence (IR). 

Fig. 28. FFC Inversion Recovery sequence. In the upper case the sample is first prepolarized in a filed Bp, then switched to the acquisition field Ba where the first RF pulse of 180° is applied and the sample magnetization is inverted. The field is then switched to B,. and the sample is allowed to relax for the variable time t. Finally, the field is switched again to the acquisition value and the magnetization is sampled by any of the sample-detection methods (here, a simple FID following a 90° RF pulse). Notice that, as shown in the lower diagram, in the special case when Bp = Ba it is possible to neatly avoid the extra switching interval prior to the inversion pulse.

Polymer Dynamics. 13C spin-lattice relaxation times (Ti) were determined with either an inversion-recovery sequence (16) (for carbons observed by direct polarization) or with a modified cross-polarization experiment (17). 13C rotating-frame relaxation times (Tip(C)) were derived from measurements of the carbon signal that remained after a Tjp(C) hold time of... 

Fig. 1. Pulse sequences for determining spin-lattice relaxation time constants. Thin bars represent tt/2 pulses and thick bars represent tt pulses, (a) The inversion-recovery sequence, (b) the INEPT-enhanced inversion recovery, (c) a two-dimensional proton-detected INEPT-enhanced sequence and (d) the CREPE sequence. T is the waiting period between individual scans. In (b) and (c), A is set to (1 /4) Jm and A is set to (1 /4) Jm to maximize the intensity of IH heteronuclei and to (1/8) Jm to maximize the intensity of IH2 spins. The phase cycling in (c) is as follows 4>i = 8(j/),8(-j/) 3 = -y,y A = 2(x),2(-x) Acq = X, 2 —x), X, —X, 2(x), —x, —x, 2(x), —x, x, 2 —x),x. The one-dimensional version of the proton-detected experiment can be obtained by omitting the f delay. In sequence (d), the phase 4> is chosen as increments of 27r/16 in a series of 16 experiments.

In Figure 4.4d, the inversion-recovery sequence is repeated eight times on diethyl phthalate (see Section 4.4... 

Figure 10.5 compares the spin-lattice relaxation time (7)) obtained after inversion-recovery sequences of the APP/PER and the APP/PER-4A systems versus heat treatment temperature (HTT). At every HTT, only one 7) value was obtained and it can be therefore expected that the slow relaxation domains size will be smaller than 10 nm. 

In principle, Ti and T2 can be measured experimentally by the inversion recovery sequence (180°—x—90°) and the Hahn spin-echo sequence, respectively. In practice, these experiments can be easily performed only when the 33S signal is very narrow. If the signal is broad, the difficulties in obtaining 33S spectra with a good S/N make the direct measurements very time-consuming and less precise. The problem can be easily circumvented because T2 (and Ti) can be obtained with good precision directly from line width. 

Table 4 33S 71 relaxation times in some compounds, measured by inversion-recovery sequence... 

FLAIR is an inversion recovery sequence with long repetition and echo times and an inversion time that is tailored to null the signal from CSF. Most pathologic processes including LA show increased signal intensities on T2-weighted images and the conspicuity of lesions located close to interfaces between brain parenchyma and CSF may be poor (Rumboldt and Marotti 2003). Additionally, the... 

Woodcock RJ Jr, Short J, Do HM et al (2001) Imaging of acute subarachnoid hemorrhage with a fluid-attenuated inversion recovery sequence in an animal model comparison with non-contrast-enhanced CT. AJNR Am J Neuroradiol 22 1698-1703... 

There are many tricks to get around the problem, such as sandwich 180° pulses (e.g., 90 -180 -90 ) and broadband shaped pulses. Figure 8.4 (top) shows the inversion profile for a simple 180° pulse at the highest available power (fp = 28.4 p,s, yB l2it — 17.6 kHz). The profile is obtained using an inversion-recovery sequence (180°x — r — 90° ) with recovery time r = 0. The final 90° pulse frequency and the 13C peak (13CH3l) are both at the center of the spectral window, but the frequency of the 180° pulse is moved in 10 ppm (1500 Hz)... 

The above multipulse inversion-recovery sequence is our first example of a 2D NMR spectrum. That is, it is a plot of signal intensity versus both the frequency of the signal (Av,)... 

Some methods take advantage of a difference in a particular property between water and the molecule to be studied. In particular, a macromolecule usually has a shorter value for proton Tt than water and a much lower diffusion coefficient. One of the oldest methods for water signal suppression is WEFT (water elimination Fourier transform), in which an inversion recovery sequence is applied (see Fig. 2.12) with r chosen to be the time that the water signal goes through zero (Tj In 2), just as in the BIRD pulse sequence. Another method makes use of the technique described in Section 9.3 to measure diffusion coefficients. 

If it is desired to emphasize Tt differences, one easy choice is to repeat the pulse sequence with a short repetition time TR, so that magnetizations are unable to relax completely back to equilibrium. Thus signals Corn water molecules with long Tj are reduced in intensity. Alternatively, an inversion recovery sequence (180°, T, 90°) can be applied instead of just a 90° pulse to excite the resonance. With careful selection of the value of r, as indicated in Fig. 14.4, T, differences can be emphasized. 

This is the case in 7, mapping using an inversion recovery sequence and collecting n images at different inversion delays (jc = 77). In this case,... 

Figure 3. Partial 100-MHz 2H NMR spectra of methyl p-D-glucopyranoside, showing the single selective determination of the spin-lattice relaxation rate of H-l using a two-pulse inversion recovery sequence. All spectra were monitored as for those in Figure 2 except for pulse delay — 25 sec. The duration of the selective 180°-pulse was 38 msec (13 Hz bandwidth). The time interval (sec) between the selective 180°- and 90° -pulses are indicated to the right of each...

Fig. 3. Simulated partially relaxed powder spectra, calculated assuming a two-site jump and for an inversion recovery sequence with a solid echo for detection and recovery times of (a) 1.5 ms, (b) 5 ms, (c) 10 ms, (d) 20 ms and (e) 500 ms. Correlation times are 0.35 and 20 ns for the left and right columns, demonstrating the different Ti anisotropy for short and long correlation times. (Adapted with permission from Ref. 456.)...

Three pulse schemes which were proposed for indirect Ti measurements and produce the same results as the inversion-recovery sequence are shown in Fig. 12. The first method is an inversion-recovery experiment on the insensitive "Y spin, with a subsequent DEPT transfer to the observed nucleus "X. Although successfully applied in model studies, the experiment suffers from the necessity of long relaxation delays and was considered to be too insensitive for application to and insensitive metal nuclei. Higher sensitivity can be achieved with double polarization transfer methods which start with "X magnetization. TWo sequences were employed on the basis of double DEPT (Fig. 12(b)) or INEPT transfer (Fig. 12(c)). In order to remove effects of... 

Fig. 2. Radiation damping effects on water longitudinal relaxation versus the t delay in an inversion recovery sequence (i.e. 7r-T-7r/2-Acq.) in a lysozyme sample (10 mM in 10 90 H20 H20, pH 3.7) at room temperature at 300 MHz using a 5mM inverse...

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