Jeener pulse sequence

Fig. 6. The generalized Jeener-Broekaert three pulse sequence. Note that FT of the solid echo and the alignment echo starts at times delayed by the pulse separation r, after the second and third pulse, respectively...

Jeener s idea was to introduce an incremented time ti into the basic ID NMR pulse sequence and to record a series of experiments at different values of second dimension to NMR spectroscopy. Jeener described a novel experiment in which a coupled spin system is excited by a sequence of two pulses separated by a variable time interval <]. During these variable intervals, the spin system is allowed to evolve to different extents. This variable time is therefore termed the evolution time. The insertion of a variable time period between two pulses represents the prime feature distinguishing 2D NMR experiments from ID NMR experiments. 

In some sequences, such as the FFC variety of the Jeener-Broekaert sequence, the RF pulses applied during the preparatory sub-sequence need to be coordinated in phase with those applied during the detection subsequence. In such cases the preparatory and detection sub-sequence sections of the FFC sequence are no longer mutually independent. 

The classical Jeener Broekaert sequence (133) is used to determine the dipolar-order relaxation time (in systems of spin 1/2 nuclides) and the Tiq relaxation time (in systems with spin 1 nuclides) of spin 1 nuclides with quadrupolar contributions to 7. Its FFC version is similar to the Inversion Recovery, except that the first 180° pulse is replaced by the sequence 90, — 5 — 45, the detection pulse becomes 45 and a special phase cycle is required. We shall not dwell on the details and purpose of the sequence since they go beyond the scope of this chapter. We wish to underline, however, the fact that sequences of this type require a close coordination of the preparatory sub-sequence with the signal-detection sub-sequence in order to isolate not just a particular magnetization component but a particular relaxation pathway. 

In 1971, Jean Jeener conceived of a new way of applying pulse sequences and displaying the results in terms of two separate frequency scales. The concept was soon developed into the very important method of two-dimensional NMR,... 

In the stimulated echo experiment, also shown in Fig. 6.2.3, the second pulse transfers the system into a mixture of Zeeman and double quantum order (alongandpg). Here, the relevant relaxation times are Ti (longitudinal Zeeman) and T q (double quantum), for which the 45 pulses of the Jeener-Broekaert sequence are replaced by 90v pulses. Again, two echos evolve at T] around the third pulse, and are refocussed by the fourth pulse. The two negative echo amplitudes vary as function of T2, with -[exp(-T2/Tiz) + exp(-T2/Ti3Q)], and both Ti and Tqq can be determined as separate values [14]. 

Fig. 5. Comparison of the frequency dependence of the total longitudinal proton relaxation time Ti and of the dipolar proton relaxation time Tto in the low-temperature nematic liquid crystal MBBA. r (v) was measured by the usual T] field-cycle with one B, r.f. pul%, shown by Rg. 1. Tto was measured by the usual Jeener-Broekaert sequence of three B r.f. pulses, in combination with a Bq field-cycle. which introduce an adjustable relaxation period between the second and third Bj pulse to give Tto(i )- The plots in the upper diagram show model fits according to equations (13a) 13d) with extensions described in the text. From the details at bottom about the experimental errors it can be clearly seen that the ratio T Tto significantly exceeds a value of 3 at medium frequencies, and in accordance with the model plot (frill line) approaches 1 in the low-frequency limit, where Bo is smaller than Bloc.

This is illustrated in Fig. 9. The 2D spectra refer to quadrupole echo sequences and characterize two possible reorientation mechanisms of a methyl group (three-site jumps vs continuous diffusion). Drastic spectral differences are observed. Ajqjarratly, these 2D relaxation spectra sensitively indicate the type of motion. The same is true for the corresponding normalized contour dots (see Fig. 9). We note that similar 2D spectra can be obtained from inversion recovery or Jeener-Broekaert sequences (see Fig. 6) [68]. Thus, by applying this 2D technique to different pulse sequoic, the various motions can be differentiated over an extremely wide dynamic range, extending from the fast-rotational to the ultraslow motional re me. Sin<% the different motions (see Fig. 4) modulate different kinds of molecular order (see Fig 3) these orders can be differentiated, likewise. 

Jeener s unpublished suggestion, which lay dormant for several years until taken up and enlarged upon by Ernst and coworkers,3 49 was to perform a double Fourier transformation on data collected using the following pulse sequence ... 

The simplest experiment is to extend the basic Jeener two-pulse sequence to two nuclei ... 

Figure 10. Homonuclear correlation 2D spectrum for viomycin in D2O, obtained using the Jeener two-pulse sequence with the added phase cycling scheme [13] to allow frequency discrimination in both dimensions, recorded on an XL-200 using a data matrix of 256x256 complex points. Solvent saturation was again used to suppress the residual HDD signal. Spurious signals near f2 = 3.1 and 4.5 ppm have been removed in order not to obscure cross peaks.

In another work the MAS NMR technique is compared to the static powder quadrupole echo (QE) and Jeener-Brockaert (JB) pulse sequences for a quantitative investigation of molecular dynamics in solids. The line width of individual spinning sidebands of the ID MAS spectra were found to be characteristic of the correlation time from 10 to s so that the dynamic range is increased by approximately three orders of magnitude when compared to the QE experiment. As a consequence, MAS NMR is found to be more sensitive to the presence of an inhomogeneous distribution of correlation times than the QE and JB experiments which rely upon line shape distortions due to anisotropic T2 and Tiq relaxation, respectively. All these results have been demonstrated experimentally and numerically using the two-site flip motion of dimethyl sulfone and of the nitrobenzene guest in the a-p-tert-butylcalix[4]arene-nitrobenzene inclusion compound. 

Li and Be work has gained from the application of the stimulated-echo spectroscopy to study the ultra-slow dynamics of nuclear spin-3/2 probes. Apart from the dominant first-order quadrupolar interaction, the impact of the homonuclear dipolar interactions was also considered. Explicit analytical expressions describing various aspects of a coupled quadupolar pair subjected to a Jeener-Broekaert pulse sequence have been derived. Extensions to larger spin systems are also briefly discussed. These results are compared with experimental data on a single-crystalline Li ion conductor. 

In the rotating reference frame spin-lock rates Fj p(SL) were measured with locking fields of 10 and 40 G. Below 170 K, decay of quadrupolar order was observed with Jeener-Broekaert pulse sequences. 

The Jeener-Broekaert (J-B) pulse sequence [2.18] shown in Fig. 2.5 allows the creation of spin alignment and the observation of a stimulated echo [2.19] for a spin-1 system. The density matrix at the end of second pulse... 

A useful technique is to use broad-band decoupling in the co dimension. The pulse sequence shown in Figure 5.64 is used to achieve this. The fully coupled spectrum appears on one axis while the fully decoupled spectrum appears on the other axis. This method has the advantage that the chemical shifts of the individual protons can be read from the singlets on one axis and the proton-proton couplings can be easily identified. The Jeener spectrum of a tricyclodecane derivative is shown in Figure 5.65. The spectrum above the... 

Jeener-Broekaert pulse sequence, 128-29 Karplus equation, 57,224 3-Lactam, 138-39... 

In systems with non-zero average interactions, a Jeener echo may be observed after a convenient pulse sequence, whose decay is sensitive to very slow motions (in the range 1 to 100 Mz) . These motions should be related to slow eventual rearrangements of junction points, involving many chain collective motions. An exponential decay, with characteristic rate T- = 3/5 is observed experimentally, up to 0.3 s, which means that... 

Another relaxation technique that is likely to see much utilization for structural studies in the future is the two-dimensional (2D) NOE experiment. The basic experiment has been largely developed in Ernst s lab with applications to proteins illustrated in Wutti ch s k. The pulse sequence [ n/2) -ti- n/2 -ZM ( /2)%-h-] generates a 2D spectrum following the two Fourier transforms. The intenrity of a peak in fte 2D-NOE spectrum arising from interaction of nucleus k with nucleus 1 is (Jeener et al., 1979 Macura and Ernst, 1980)... 

Fig. 1. (a) Time division of all two-dimensional NMR experiments, (b) Jeener two-dimensional NMR pulse sequence. 

A two-dimensional experiment has been described for correlating the chemical shifts of nuclei undergoing chemical exchange (Jeener ef al., 1979 Macura et al., 1981). The pulse sequence for generating a homonuclear correlated exchange spectrum is shown in Fig. 20. At the end of the... 

Fig. 20. Pulse sequence used to produce chemical-exchange correlated two-dimensional spectra. The phases of Pj, Pj, and the receiver may be cycled to suppress axial peaks, J cross-peaks, and allow quadrature detection in both dimensions (Jeener et al., 1979 Macura et al 1981).

The concept of two-dimensional NMR spectroscopy was introduced in 1971 by Jeener [15] and its potential was demonstrated by Aue, Bartholdi and Ernst [16]. Jeener suggested a sequence in which an extra time interval and pulse were inserted into the usual pulse H-NMR experiment to give 90 —/1—90 —<2-... 

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