Selective proton inversion pulse

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. 

Technically, the inverse experiment used to be very demanding because the excess of protons not coupled to the nucleus of interest (e.g., protons coupled to the almost hundred-fold excess of 12C instead of 13C) needed to be suppressed. Originally, this was achieved by the use of elaborate phase-cycling schemes, but today the coherence pathway selection by gradient pulses facilitates this process. 

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. 

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... 

For use in the laboratory, it is convenient to choose a simple, robust inversion pulse as the element S, and the Gaussian pulse is well suited to routine use. Example excitation profiles for this are illustrated in Fig. 9.20 and offer guidance on selection of pulse duration for a desired excitation window. For proton spectroscopy, a Gaussian pulse of around 40 ms proves suitable for many applications. 

A fundamentally different approach to signal excitation is present in polarization transfer methods. These rely on the existence of a resolvable J coupling between two nuclei, one of which (normally the proton) serves as a polarization source for the other. The earliest of these type of experiments were the SPI (Selective Population Inversion) type (19>) in which low-power selective pulses are applied to a specific X-satellite in the proton spectrum for an X-H system. The resultant population inversion produces an enhanced multiplet in the X spectrum if detection follows the inversion. A basic improvement which removes the need for selective positioning of the proton frequency was the introduction of the INEPT (Insensitive Nucleus Excitation by Polarization Transfer) technique by Morris and Freeman (20). This technique uses strong non-selective pulses and gives general sensitivity enhancement. 

The sensitivity limitations inherent in the observation of proton decoupled N signals become more significant if undecoupled spectra are desired. The method of selective population inversion (M 31i), which is capable of noticecibly enhancing the line intensities of coupled spectra, therefore seems very attractive. In this method a tt pulse is applied selectively to a given component (or an appropriate sub-multiplet) of the satellite proton spectrum pertaining to the molecules which contain the N isotope. Just after this selective inversion, a non selective... 

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... 

III. SELECTIVE POPULATION INVERSION. Selective population inversion (SPI) is a technique which furnishes the same information as but lacks the disadvantages of selective proton decoupling. In a heteronuclear C- H SPI experiment a selective rf 7r-pulse applied at a proton transition inverts the populations of the upper and lower energy levels. A strong nonselective... 

In a coupled spin system, the number of observed lines in a spectrum does not match the number of independent z magnetizations and, fiirthennore, the spectra depend on the flip angle of the pulse used to observe them. Because of the complicated spectroscopy of homonuclear coupled spins, it is only recently that selective inversions in simple coupled spin systems [23] have been studied. This means that slow chemical exchange can be studied using proton spectra without the requirement of single characteristic peaks, such as methyl groups. 

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