Protonation excitation pulse

The HMQC sequence can be modified by replacing the initial 90° proton excitation pulse, pi, with a 45° pulse (Figures 31... 

The BIRD pulse [14] is in fact a cluster of pulses (Fig. 6.12) used as a tool in NMR to differentiate spins that possess a heteronuclear coupling from those that do not. The effect of the pulse can vary depending on the phases of the pulses within the cluster, so we concentrate here on the selective inversion described above. For illustrative purposes, proton pulse phases of x, y, X will be considered as this provides a clearer picture with the vector model, although equivalent results are achieved with phases x, x, —x, as in the original publication. The scheme (Fig. 6.14) begins with a proton excitation pulse followed by a spin-echo. Since carbon-12 bound protons have no one-bond... 

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 ... 

We have implemented the principle of multiple selective excitation (pulse sequence II in fig. 1) thereby replacing the low-power CW irradiation in the preparation period of the basic ID experiment by a series of selective 180° pulses. The whole series of selective pulses at frequencies /i, /2, , / is applied for several times in the NOE build-up period to achieve sequential saturation of the selected protons. Compared with the basic heteronuclear ID experiment, in this new variant the sensitivity is improved by the combined application of sequential, selective pulses and the more efficient data accumulation scheme. Quantitation of NOEs is no longer straightforward since neither pure steady-state nor pure transient effects are measured and since cross-relaxation in a multi-spin system after perturbation of a single proton (as in the basic experiment) or of several protons (as in the proposed variant) differs. These attributes make this modified experiment most suitable for the qualitative recognition of heteronuclear dipole-dipole interactions rather than for a quantitative evaluation of the corresponding effects. 

Figure 14.11 Basic 2D pulse sequence used for the measurement of residual dipolar local fields. After the excitation pulse the spins are allowed to evolve for some time tj (indirect dimension) under influence of the relevant spin interactions before crosspolarisation takes place. The direct detection during time t2 then takes place on the 13C side typically under proton dipolar decoupling (DD). The basic scheme can be extended by various spin manipulation techniques (not shown) during time tj. For instance, the heteronuclear dipolar contribution can be removed by a decoupling pulse...

In one-dimensional usage, a ir/2 nonselective excitation pulse can be applied before the WATERGATE sequence. In cases where it is desired to observe exchangeable protons, a selective n/2, pulse (selective of the water resonance, for example an EBURP2 pulse ) can precede the nonselective 7t/2 pulse. WATERGATE can be incorporated into two- and higher-dimensional experiments (see Section 5). 

McCoy et a/. have proposed a method termed ERASER based on self-refocused and hard rectangular pulses. The scheme does not generate phase roll. Moy et alP have demonstrated the use of frequency shifted, self-refocused top hat pulses to observe amide resonances, that they termed selective-excitation-corrected spectroscopy (SelECSy). This method, although not providing exceptional water suppression, overcomes the dynamic range problem and produces uniform amide proton excitation... 

Figure 4.4. Carbon-13 spectra of camphor 4.1 acquired (a) without and (b) with the addition of Cr(acac)3 under otherwise identical standard observation conditions (Is relaxation delay, 30 excitation pulse, 0.5 s acquisition time). In (a) the two quaternary resonances display reduced signal intensities as a result of partial saturation. In (b) this difference is largely eliminated by the addition of the relaxation agent, whilst the reduced signal-to-noise ratio of the protonated resonances arises from the suppression of NOE...

Despite the slightly foreboding title, the basic HMQC sequence is rather simple, comprising only four rf pulses (Fig. 6.3), the operation of which is considered here for a simple H- C spin pair. The sequence starts with proton excitation followed by evolution of proton magnetisation under the influence... 

Figure 42. The correlation between the occupancy of a micelle by both emitter and detector, with the maximal amount of protonated detector measured after the excitation pulse. The results are drawn according to equation the HInT = [BCG] [PN] (fi[MiC] + f2)- The reactants concentrations in the experiment were 40 mg/ml Brij -58 (500 p.M, micellar concentration). Bromocresol green 50 (lAf and (J naphthol 100—1000 pJW. pH 7.5.

To improve the sensitivity the selective J-resolved pulse sequence may be combined with a refocused INEPT polarization transfer experiment. In contrast to the original heteronuclear J-resolved experiment the first excitation pulse is executed on the F2 channel with the spin-echo sequence sandwiched between two incremental delay. Coupling evolves during the second incremental delay before the refocused INEPT unit creates in-phase coherence for the nuclei which are coupled to the selected proton nucleus allowing decoupling on the F2 channel during data acquisition. 

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