Selective proton inversion pulse sequence

XCOR Spectra. A recent improvement made in 2D-heteronu-clear correlated spectroscopy is the use of a selective proton inversion pulse sequence which increases the sensitivity of the experiment by removing proton homonuclear coupling and allows easier correlation between and chemical shifts. The XCOR pulse sequence used for the purpose is shown in... 

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. 

A very efficient suppression of parent resonances can be achieved using the T filter. This, however, requires a rather careful tuning of the relaxation delay T (see Figure 8). If the jump and return inversion pulse is employed, the pulse sequence can be regarded as a selective version of the BIRD experiment [57-59]. Obviously, multiple-frequency selective inversion pulses may be necessary in the case of complex proton spectra. Usually the /-BIRD HMQC experiment gives cleaner spectra as compared with equivalent heteronuclear singlequantum coherence (HSQC) experiments, presumably because of fewer 180° pulses which are frequently a source of various artefacts. 

Fig. 6.6.5. Pulse sequences for measuring proton and fluorine relaxation times selectively for different domains in a semicrystalline polymer, (a) The Torchia method as modified for Tf. (b) Sequence for Tfp. (c) (See next page) Pre-CP proton spin inversion for T . (d) (See next page) Pre-CP proton spin-locking for T . For (c) acquisitions from the two component sequences add (upper sequence) and subtract (lower sequence) to give a Freeman-Hill type of measurement. For measurement of Tf by sequence 5(d) the time t is varied. However, when this sequence is used to discriminate in favour of crystalline (relatively rigid) domains, it is fixed at a value between those of Tf for the crystalline and rigid domains. [CT = contact time SL = spin-lock time DD = high-power (dipolar) decoupling ACQ = acquisition.]...

Another useful technique to increase X receptivity is to use a Selective Population Inversion (SPI) pulse sequence (15) on one proton transition. A long (150 to 250 ms), soft pulse is applied via the decoupler to excite and invert one transition of the coupled proton spectrum selectively. The pulse sequence is visualized in Figure 16 together with a SPI experiment... 

The HMQC sequence aims to detect only those protons that are bond to a spin- A heteronucleus, or in other words only the satellites of the conventional proton spectrum. In the case of C, this means that only 1 in every 100 proton spins contribute to the 2D spectrum (the other 99 being attached to NMR inactive C) whilst for N with a natural abundance of a mere 0.37%, only 1 in 300 contribute. When the HMQC FID is recorded, all protons will induce a signal in the receiver on each scan and the unwanted resonances, which clearly represent the vast majority, must be removed with a suitable phase cycle if the correlation peaks are to be revealed (the notable exception to this is when pulsed field gradients are employed for signal selection, see Section 6.3.3 below). By inverting the first C pulse on alternate scans, the phase of the C satellites are themselves inverted whereas the C-bound protons remain unaffected (Fig. 6.5). Simultaneous inversion of the receiver will lead to cancellation of the unwanted resonances with corresponding addition of the desired satellites. This two step procedure is the fundamental phase-cycle of the HMQC experiment, as indicated in Fig. 6.3 above. 

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