Proton pulse, simultaneous with

The scheme of Fig. 6.31b has been widely used to produce absolute-value shift correlation spectra, and is often referred to as HETCOR or hetero-COSY. Conversion to the preferred phase-sensitive equivalent (of which various forms have been investigated [55]) requires the reintroduction of the simultaneous 180°( H, C) pulses into the midpoints of both Ai and A2 to remove chemical shift evolution during these periods, exactly as in the full refocused INEPT. In addition, the incorporation of the States or TPPI phase cycling of the 90° proton pulse of the polarisation transfer step is required. Suppression of axial peaks is through the phase alternation of the final proton pulse together with the receiver... 

One way to handle this problem, within the framework of the CPMG scheme, is to add proton pulses at judiciously chosen points in the train of the carbon 7r-pulses [42, 43]. An example of such a sequence is given in fig. 4(a). An important issue when setting up experiments of this type is the duration 8 between the pulses. On the one hand, it should be small compared to (l/2)Jis [43]. On the other hand, it should be much longer than the relevant pulse widths. It is difficult to simultaneously fulfill both these requirements rigorously with typical high-resolution equipment, and some compromise has to be settled on. 

The pulse sequence for ICP experiments appears simple a 90° proton pulse is followed immediately by a spin lock radio-frequency (rf) field of strength B that is phase shifted by 90° relative to the first pulse. By a spin-lock field is meant a strong rf field B that is on resonance with the given nucleus it keeps magnetization in a spin-locked orientation parallel to the B direction where the decay of magnetization is governed by T p. At present the strong continuous B field is replaced by multipulse sequences that are well known from other spin-lock experiments such as TOCSY, ROESY etc. Simultaneously,... 

Simultaneously with the spin locking of the protons, in the third part of the experiment (Fig. 8.20c), a pulse is applied in the channel, and this pulse is carefully adjusted so that the energy gap for spin flips corresponds exactly to that of the protons. This pulse is maintained for a time, tee, and is the contact time. This contact time allows for the exchange of energy between the abundant proton-spin reservoir and the rare carbon-spin system. This exchange, called cross-polarization. 

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. 

Fig. 10.15. Pulse sequence for the multiplicity-edited gradient HSQC experiment. Heteronuclear single quantum coherence is created by the first INEPT step within the pulse sequence, followed by the evolution period, t . Following evolution, the heteronuclear single quantum coherence is reconverted to observable proton magnetization by the reverse INEPT step. The simultaneous 180° XH and 13C pulses flanked by the delays, A = l/2( 1 edits magnetization inverting signals for methylene resonances, while leaving methine and methyl signals with positive phase (Fig. 16A). Eliminating this pulse sequence element affords a heteronuclear shift correlation experiment in which all resonances have the same phase (Fig. 16B).

Fig. 4. spectrum showing the three quaternary carbons Cl, C2 and C3 of compound 2 dissolved in CDiCN (bottom) and heteronuclear NOE spectra simultaneously acquired with pulse sequence II. Three data blocks a, b and c corresponding to three experiments with the number N of proton resonances selected for simultaneous saturation set to 1 (a), 5 (b) and 10 (c) are depicted. Five spectra with the initial saturation of protons 3, 1, 1 3" and 2 are presented. The additional five NOE difference spectra acquired in case c, with the irradiation of the sixth methyl and of four NH (NHa) proton resonances are not shown. (Continued on... 

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