Insensitive nuclei enhanced sensitivity enhancement

The INEPT (Insensitive Nuclei Enhanced by Polarization Transfer) experiment [6, 7] was the first broadband pulsed experiment for polarization transfer between heteronuclei, and has been extensively used for sensitivity enhancement and for spectral editing. For spectral editing purposes in carbon-13 NMR, more recent experiments such as DEPT, SEMUT [8] and their various enhancements [9] are usually preferable, but because of its brevity and simplicity INEPT remains the method of choice for many applications in sensitivity enhancement, and as a building block in complex pulse sequences with multiple polarization transfer steps. The potential utility of INEPT in inverse mode experiments, in which polarization is transferred from a low magnetogyric ratio nucleus to protons, was recognized quite early [10]. The principal advantage of polarization transfer over methods such as heteronuclear spin echo difference spectroscopy is the scope it offers for presaturation of the unwanted proton signals, which allows clean spec-... 

Some important nuclei, including C and N, have low natural abundances and sensitivities. Pulse sequences have been devised to improve the observability of these nuclei when they are coupled to another nucleus of high receptivity, usually a proton. Pulses are applied in such a way that the favorable population of the sensitive nucleus S is transferred to the insensitive nucleus I. A common sequence developed by Freeman for this purpose is called INEPT, for Insensitive Nuclei Enhanced by Polariz.ation Transfer, as follows ... 

A second example of coherence selection using phase cycling is the suppression of the quaternary carbon signals in a polarization transfer spectrum. DEPT and INEPT type pulse sequences use a polarization transfer step to enhance the signal of an NMR insensitive nucleus such as which exhibits scalar coupling to a NMR sensitive nucleus such as IR. 

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. 

We now want to consider observing the high resolution NMR spectrum of a dilute and/or insensitive nucleus (usually 13C) in a solid with enhanced sensitivity by taking advantage of the dipolar reservoir of an abundant nucleus in the same system. These experiments are an outgrowth of two lines of development. One of these is the attempt to suppress dipolar coupling in a solid sample. The second resulted from the experiments of Hartmann and Hahn (1962) in which the detection of a dilute or insensitive spin was made possible through... 

Ag and Ag both have 7=1/2 but ° Ag is more suitable for NMR measurements . The ° Ag NMR spectra, using the INEPT (insensitive nuclei enhanced by polarization transfer) sequence [proton polarization transfer sequence between two / coupled spins /( Ag) and s( H), 7( ° ° Ag- H-imine) = 9.3 Hz (5 8.87) and 6.3 Hz (S 8.69)], have been applied to gain insight into the electronic environment and the structural and dynamic behaviour of Ag complexes in solutions . The chemical shift of the ° Ag nucleus is highly sensitive towards changes in its environment . The... 

There are other two-dimensional techniques, more sensitive than HETCOR, that make use of polarization transfer (Section 4.12.2). Even greater enhancement can be obtained if the magnetization is generated at the insensitive nucleus and then transferred back to the sensitive nucleus for detection. Procedures making use of this principle are called inverse techniques and lead to a great reduction of sample concentration or measurement time. Typical experiments involve recording spectra for insensitive nuclei such as C, Si and N, which are recorded in inverse, proton-detected procedures. The information given by such experiments is the same as that from the HETCOR experiments, but the experiments are much more sensitive and are quicker to perform. 

Determination of magnetically equivalent sensitive nuclei which are scalar coupled to an insensitive, non-abundant nucleus like Dc or 15N, extended spin echo sequences for signal enhancement by nuclear Overhauser effect (APT, SEMUT) or polarization transfer (DEPT, INEPT, etc). 

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