Insensitive nuclei enhanced experiments

Coherent transfer experiments can roughly be divided into two classes pulse-interrupted free-precession experiments and Hartmann-Hahn-type experiments (Ernst et al., 1987). Examples of homo- and heteronuclear pulse-interrupted free-precession coherence transfer are COSY (correlation spectroscopy Aue et al., 1976), RELAY (relayed correlation spectroscopy Wagner, 1983), and INEPT (insensitive nucleus enhancement by polarization transfer) transfer steps (Morris and Freeman, 1979 Burum... 

Heteronuclear multiple-quantum correlation Experiment for tailored correlation spectroscopy of H and H resonances in peptides and proteins Homonuclear Hartmann-Hahn spectroscopy Heteronuclear quadruple-quantum coherence Heteronuelear triple-quantum coherence Heteronuclear single-quantum coherence TOCSY sequences developed at the Indian Institute of Chemical Technology Insensitive nucleus enhancement by polarization transfer... 

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-... 

The INEPT experiment [26] (Insensitive Nuclei Enhanced by Polarisation Transfer) was one of the forerunners of many of the pulse NMR experiments developed over subsequent years and still constitutes a feature of some of the most widely used multidimensional experiments in modem pulse NMR. Its purpose is to enable non-selective polarisation transfer between spins, and its operation may be readily understood with reference to the vector model. Most often it is the proton that is used as the source nucleus and these discussion will relate to XH spin systems throughout, although it should be remembered that any high-y spin- /2 nucleus constitutes a suitable source. 

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... 

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. 

The Insensitive Nuclei Enhanced by Polarization Transfer (INEPT) experiment is useful to discuss for two reasons. First is its utility in enhancing the S/N level in X-nucleus-detected spectra such as NMR. Second is because it is the basis for building many 2D- and 3D-NMR experiments described below. 

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