The delay between the 90º pulse and the beginning of the acquisition is referred to as the "rephrasing time". The trains of pulses are interrupted by a 180º pulse on the X channel that allows the refocussing of the X magnetisation for the X-detection (spin echo). Then two trains of 180º pulses synchronised with the rotor half period are applied on the Y channel to reintroduce X-Y heteronuclear dipolar interactions. The first excitation step (90º pulse or CP step) puts the magnetisation in the transverse plane. Rotational Echo DOuble Resonance (REDOR) pulse sequence. Advanced solid-state NMR spectroscopy Rotational Echo DOuble Resonance (REDOR) Ultra fast MAS (from 60 kHz up to above 111 kHz) is an efficient way to average all dipolar interactions, including 1H- 1H homonuclear dipolar interactions, which extends the resolution of 1H spectra and enables the usage of pulse sequences used in solution state NMR. Dipolar interactions can also be removed with magic angle spinning. Homonuclear decoupling is achieved with multiple-pulse sequences (WAHUHA, MREV-8, BR-24, BLEW-12, FSLG ), or continuous wave modulation (DUMBO, eDUMBO ). The irradiation can be continuous (continuous wave decoupling ), or a series of pulses that extend the performance and the bandwidth of the decoupling (TPPM, SPINAL-64, SWf-TPPM ) Heteronuclear decoupling is achieved by radio-frequency irradiation on at the frequency of the nucleus to be decoupled, which is often 1H. Spin interactions can be removed ( decoupled) to increase the resolution of NMR spectra during the detection, or to extend the lifetime of the nuclear magnetization. The power of one contact pulse is typically ramped to achieve a more broadband and efficient magnetisation transfer. In practice, the pulse power, as well as the length of the contact pulse are experimentally optimised. The maximum dipolar coupling is given by the dipolar coupling constant d, d = ℏ ( μ 0 4 π ) 1 r 3 γ 1 γ 2 is an integer. The magnitude of the interaction is dependent on the gyromagnetic ratio of the spin species, the internuclear distance r, and the orientation, with respect to the external magnetic field B, of the vector connecting the two nuclear spins (see figure). Nuclear spins exhibit a magnetic dipole moment, which generates a magnetic field that interacts with the dipole moments of other nuclei ( dipolar coupling). θ is the angle between the vector connecting I 1 and I 2, and the magnetic field B. Vectors important for dipolar coupling between nuclear spins I 1 and I 2. crystalline powders, glasses, large membrane vesicles, molecular aggregates), anisotropic local fields or interactions have substantial influence on the behaviour of nuclear spins, which results in the line broadening of the NMR spectra. However, in media with no or little mobility (e.g. In a classical liquid-state NMR experiment, molecular tumbling coming from Brownian motion averages anisotropic interactions to zero and they are therefore not reflected in the NMR spectrum. chemical shift anisotropy, dipolar interactions. chemical shift, isotropic J- coupling) and anisotropic interactions ( e.g. The resonance frequency of a nuclear spin depends on the strength of the magnetic field at the nucleus, which can be modified by isotropic (e.g. 4.3 MAS-Dynamic Nuclear Polarisation (MAS-DNP).
4.1 Rotational Echo DOuble Resonance (REDOR).4 Advanced solid-state NMR spectroscopy.
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