Sensitivity-enhanced heteronuclear methods

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-... 

Other strategies that show great promise in reducing NMR acquisition time utilise methods to obtain multiple sets of data from one experiment through a concept known as time-shared evolution. An example of this process that should find utility in natural products elucidation was demonstrated by a pulse sequence called CN-HMBC.93 Traditionally, a separate 13C-HMBC and 15N-HMBC were acquired independently. However, the CN-HMBC allows both 13C- and 15N-HMBC spectra to be obtained simultaneously. By acquiring both data sets simultaneously, an effective 50% time reduction can be achieved.93 This approach has also been demonstrated for a sensitivity-enhanced 2D HSQC-TOCSY (heteronuclear multiple bond correlation total correlation spectroscopy) and HSQMBC (heteronuclear single quantum... 

For Zn, the heteronuclear dipole—dipole interaction is important for the sensitivity-enhancement/spectral editing techniques such as cross polarization (CP) and the methods designed for measuring the intemuclear distance involving zinc (see discussions later). 

The development of carbon-13 NMR during the last eight years has been characterized by a continual increase in the sensitivity and quality of spectra. A reduction in measuring time - equivalent to an enhancement in sensitivity has been achieved mainly by cryomagnet technology. The efficiency with which NMR information can be obtained has been substantially improved by new computer-controllable pulse sequences for one-and two-dimensional NMR experiments. A selection of these new methods, in particular, those used for multiplicity analysis and homo- or heteronuclear shift correlations, is presented in chapter 2 of this edition. 

An alternative method, termed DNP (Dynamic Nuclear Polarisation), relies on the transfer of polarisation from unpaired electrons onto nuclear spins in the solid state but has been further developed to significantly enhance the sensitivity of solution-state NMR, as described briefly below. The technique has been applied to boost the polarisation levels for low-7 heteronuclear spins in particular and shows most promise to date for and N observation the initial report on the technique described a gain in signal-to-noise in excess of 10,000-fold for both these elements when compared to data collected at thermal equilibrium [115]. The hyperpolarisation of P and Si has also been demonstrated. 

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