Spin system designation

Selective experiments can also be performed by the tailored excitation method of Tomlinson and Hill. The selective pulse is frequency-modulated with a function designed to yield zero effective field at the resonance offset of the neighboring nuclei. Although this technique is especially promising for studies of more-complex spin systems, its use is as yet very limited, in part because the instrumentation needed is not yet commercially available. 

Phase quadrature is an over-arching concept for all interferometric detection. However, achieving a common-path configuration that locks in a stable quadrature condition puts constraints on possible system designs. This section reviews the several configurations of common-path quadrature that have been demonstrated so far in spinning-disc systems. These are the micro-diffraction class, the adaptive optical class, the phase contrast and the in-line class. At the end of this section, we show that the phase-contrast and in-line classes are conjugate quadratures of each other. 

The preparation period consists of the creation of a non-equilibrium state and, possibly, of the frequency labeling in 2D experiments. Usually, the preparation period should be designed in such a way that in the created non-equilibrium state, the population differences or coherences under consideration deviate as much as possible from the equilibrium values. During the relaxation period, the coherences or populations evolve towards an equilibrium (or a steady-state) condition. The behavior of the spin system during this period can be manipulated in order to isolate one specific type of process. The detection period can contain also the mixing period of the 2D experiments. The purpose of the detection period is to create a signal which truthfully reflects the state of the spin system at the end of the relaxation period. As always in NMR, sensitivity is a matter of prime concern. 

Hence, originally designed to eliminate dipolar interactions, MAS has the main effect on the removal of the chemical shift anisotropy, which is rather pronounced in 13C NMR spectroscopy. Since heteronuclear interactions could be viewed as a perturbation of abundant spins (I) on the energy states of rare spin system (S), a more convenient way of reducing HD to zero is high power decoupling of the I nuclei < u = 0). 

Protons that are chemically equivalent but magnetically nonequivalent are indicated by, for example, A A. The examples of such systems given below illustrate the medtod. This system for designating spin systems is merely a labeling device. The appearance of actual spectra will depend on die magnitude of die various J values. Nevertheless this is a convenient and common way of categorizing coupled proton systems. 

While the original CP experiments were designed for the proton-to-rare spin polarization transfer, cross-polarisation in the spin system which do not involve protons (i.e. 31P —> 27A1 or even 27A1 —> 31P) have also been described.22,23 These experiments have been widely applied for characterization of the structure of molecular sieves as well as the catalytic activity of their acidic sites.5,22... 

Finally, the application of optimal control theory to DNP studies needs to be discussed. Optimal control theory is a means to systematically design and optimize pulse sequences to maximize the efficiency of transfer between spin states. While this method was initially introduced to benefit high-resolution NMR studies, it has recently been adapted to improve the electron-nuclear polarization transfer in DNP applications by considering simple two- or three-spin systems. " While no experimental implementation of DNP pulse sequences designed by optimal control methods has been reported, these methods have the great potential to enhance DNP performance at X-band, due to the powerful pulsed ESR hardware that is commercially available at these frequencies. 

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