Antiphase magnetization

In the two examples above on the right, observable magnetization (antiphase coherence) is transferred by the 90° 1H pulse from Hb to Ha (top) and from Ha to Hb (bottom). This is a key process in all advanced NMR experiments that depend on / couplings. The role of the two operators is reversed as the operator in the x-y plane (the observable net magnetization) rotates to the z axis and the operator on the z axis (the multiplier that represents microscopic z magnetization) rotates to the x-y plane. After the rotations, we reverse the order of the two operators because we always write the observable operator first in the product. 

Transverse magnetization (single quantum coherence) Transverse / magnetization, antiphase in S Transverse S magnetization, antiphase in /... 

In addition to the selected magnetization component (e.g., 7 ), several terms in the density operator survive the application of trim pulses (or z filters). For example, if a trim pulse is applied along the x axis of the rotating frame, all terms of the density operator that commute with remain unaffected, that is, in addition to the in-phase operators and (x magnetization), antiphase combinations like (lyS - I Sy) or (I SyTy + also survive the trim pulses. In the effective field frame, these terms represent operators with coherence order p = 0. Modified z filters and spin-lock pulses that are able to suppress these zero-quantum-type terms will be discussed in Section XII.B. 

Ikyllz X and y components, respectively, of/c-spin magnetization antiphase with respect to /... 

Clearly the extent to which the H nuclei line up along the x -axis (Ild) will depend on the duration of tu which in turn will determine the extent of the antiphase z-magnetization created by the second 90° H pulse. The... 

The INEPT experiment can be modified to allow the antiphase magnetization to be precessed for a further time period so that it comes into phase before data acquisition. The pulse sequence for the refocused INEPT experiment (Pegg et al., 1981b) is shown in Fig. 2.13. Another delay, A. is introduced and 180° pulses applied at the center of this delay simultaneously to both the H and the C nuclei. Decoupling during data acquisition allows the carbons to be recorded as singlets. The value of Z), is adjusted to enable the desired type of carbon atoms to be recorded. Thus, with D, set at V4J, the CH carbons are recorded at VsJ, the CH2 carbons are recorded and at VeJ, all protonated carbons are recorded. With D3 at %J, the CH and CH ( carbons appear out of phase from the CH2 carbons. 

A 90° Gaussian pulse is employed as an excitation pulse. In the case of a simple AX spin system, the delay t between the first, soft 90° excitation pulse and the final, hard 90° detection pulse is adjusted to correspond to the coupling constant JJ x (Fig- 7.2). If the excitation frequency corresponds to the chemical shift frequency of nucleus A, then the doublet of nucleus A will disappear and the total transfer of magnetization to nucleus X will produce an antiphase doublet (Fig. 7.3). The antiphase structure of the multiplets can be removed by employing a refocused ID COSY experiment (Hore, 1983). 

Figure 7.3 One-dimensional COSYspectram for an AX system, (a) A common ID sjjectrum. (b) Selective excitation of spin A leads to a ID COSY spectrum with antiphase X lines and maximum transfer of magnetization from A to X. (Reprinted from Mag. Reson. Chem. 29, H. Kessler et at, 527, copyright (1991), with permission from John Wiley and Sons Limited, Baffins Lane, Chichester, Sussex P019 lUD, England.)...

SELINQUATE (Berger, 1988) is the selective ID counterpart of the 2D INADEQUATE experiment (Bax et al., 1980). The pulse sequence is shown in Fig. 7.4. Double-quantum coherences (DQC) are first excited in the usual manner, and then a selective pulse is applied to only one nucleus. This converts the DQC related to this nucleus into antiphase magnetization, which is refocused during the detection period. The experiment has not been used widely because of its low sensitivity, but it can be employed to solve a specific problem from the connectivity information. 

The SELINCOR experiment is a selective ID inverse heteronuclear shift-correlation experiment i.e., ID H,C-COSYinverse experiment) (Berger, 1989). The last C pulse of the HMQC experiment is in this case substituted by a selective 90° Gaussian pulse. Thus the soft pulse is used for coherence transfer and not for excitation at the beginning of the sequence, as is usual for other pulse sequences. The BIRD pulse and the A-i delay are optimized to suppress protons bound to nuclei As is adjusted to correspond to the direct H,C couplings. The soft pulse at the end of the pulse sequence (Fig. 7.8) serves to transfer the heteronuclear double-quantum coherence into the antiphase magnetization of the protons attached to the selectively excited C nuclei. 

The polarization patterns are dependent upon the strength of the magnetic field, in which the reactions are carried out. If the reactions are carried out at high fields (i.e., notably within the NMR spectrometer), the resonances appear in antiphase - that is, there is an equal number of absorption and emission lines and no net polarization. At low field however (i.e., when the reaction is carried out at zero or a very low field and then transferred into the high field of the NMR spectrometer for subsequent investigation), the resonances display net polarization, as has been outlined by Pravica and Weitekamp [9]. 

Fig. 10.14. Gradient-enhanced HMQC pulse sequence described in 1991 by Hurd and John derived from the earlier non-gradient experiment of Bax and Subramanian. For 1H-13C heteronuclear shift correlation, the gradient ratio, G1 G2 G3 should be 2 2 1 or a comparable ratio. The pulses sequence creates heteronuclear multiple quantum of orders zero and two with the application of the 90° 13C pulse. The multiple quantum coherence evolves during the first half of ti. The 180° proton pulse midway through the evolution period decouples proton chemical shift evolution and interchanges the zero and double quantum coherence terms. Antiphase proton magnetization is created by the second 90° 13C pulse that is refocused during the interval A prior to detection and the application of broadband X-decoupling.

The last 90° pulse on 13C acts as a purge pulse for the undesired dispersive magnetization.47,48 The function of the pulse is to convert any magnetization remaining antiphase with respect to the 13C spin into unobservable multiple-quantum coherence. This will provide cross peaks with pure lineshapes and with higher resolution, and consequently establishes reliable determination of coupling constants.47,48... 

An alternative way of realizing an isotope filter is shown in Fig. 17.4b, where the 90° phase difference between the two proton magnetizations is exploited [18]. A second 90° j1 ) pulse (of same phase as the excitation pulse) at the end of the period r =l/2j leaves the heteronuclear antiphase magnetization of the X-bound protons unaffected, while the other protons are converted to z magnetization ... 

Fig. 17.4 Common filter elements a X-half filter based on X pulse phase cycling [16, 17], b X-half filter with purge gradient [18], c X-half filter as in a, but with refocusing period for the hetero-nuclear antiphase magnetization [16, 17]. Sequences d [22], e [23] and f [18] show double filters based on single filter elements the delays r and r can be set to slightly different values to cover a broader range of ]J coupling constants (see text for a more detailed description).

In a ID COSY-RELAY experiment [38] (fig. 13(a)) a multistep relay transfer is applied after the filtration. If the filtration is performed on the H-2 proton, the CSSF is incorporated into the first spin-echo. If there is not sufficient chemical shift separation between H-2 protons, the filter is shifted to the second spin-echo. The method is illustrated for the separation of the spin systems of two terminal /3-glucopyranose residues of a modified LPS (5) containing a total of nine saccharide units [76]. The anomeric proton resonances of the two /3-glycopyranoses overlapped almost completely, with a chemical shift difference of only 1.9 Hz, while the corresponding H-2 resonances were separated by 55.0 Hz. The length of the filtration interval, Ti, was adjusted to yield a maximum antiphase magnetization of H-2 pro-... 

The purpose of the C(o i)-half-filter is to start the TOCSY experiment only with the magnetization of protons bound to C. No further C pulses are applied after the start of the evolution time t. For the description of the multiplet fine-structure of the resulting cross-peaks, it is instructive to consider a 3-spin system with the operators H, and C denoting the spins of two protons and one carbon. Starting from antiphase magnetiza-... 

As shown in Fig. 2.48 (e), the vectors of C13 — C13 doublet magnetizations are aligned in opposite directions when irradiated by the 90 pulse. Thus, in the INADEQUATE spectrum the C13 —C13 doublet signals will appear with the corresponding antiphase relationship, as shown in Fig. 2.49, which also demonstrates the effective suppression of the strong 13C — 12C signals of piperidine. Analysis of carbon-carbon coupling constants can be performed easily in this simple case. 

The antiphase relationship of the C13 —C13 doublet signals in the INADEQUATE spectrum can be eliminated by an additional spin-echo sequence (— 1/4Jcc — 180° — 1 /4 Jcc —) before the 90 monitor pulse [58]. The sensitivity of the experiment may be improved by the application of stronger magnetic fields or by using proton polarization transfer techniques [59]. 

---