Nuclear magnetic resonance radiofrequency pulses

Nuclear magnetic resonance spectroscopy Interaction magnetic fields - nuclei Resonance of radiation quanta, h v Radiofrequency pulses Spectrum in time or frequency (FT) domain ... 

The newer instruments (Figure 2.4c) utilize a radiofrequency pulse in place of the scan. The pulse brings all of the cycloidal frequencies into resonance simultaneously to yield a signal as an interferogram (a time-domain spectrum). This is converted by Fourier Transform to a frequency-domain spectrum, which then yields the conventional m/z spectrum. Pulsed Fourier transform spectrometry applied to nuclear magnetic resonance spectrometry is explained in Chapters 4 and 5. 

M. Carravetta, 1. Eden, X. Zhao, A. Brinkmann and 1. H. Levitt, Symmetry principles for the design of radiofrequency pulse sequences in the nuclear magnetic resonance of rotating solids. Chem. Phys. Lett., 2000, 321, 205-215. 

Nuclear magnetic resonance (NMR) is the responses of nuclear spins to external radiofrequency (RF) stimulations, or the absorption and reemission of RF pulses by nuclear spins in a magnetic field. NMR techniques are developed to utilize this NMR phenomenon for the characterization of structures and dynamic properties of molecular systems, and for the identification and visualization of molecules and distributions. In the field of PEMFCs, the NMR techniques are frequently applied to the development and improvement of essential materials, proton exchange membrane and electrocatalysts, and the water management of PEMFCs. 

Pulsed ENDOR. In both the inversion recovery (Fig. 5b) and stimulated echo experiment (Fig. 5c), the echo amplitude is influenced by a radiofrequency pulse applied during the interpulse delay of length T, if this pulse is on-resonance with a nuclear transition. In the former experiment, such a pulse exchanges magnetization between inverted and noninverted transitions, so that echo recovery is enhanced (Davies ENDOR) (32). In the latter experiment the on-resonance radiofrequency pulse induces artificial spectral diffusion, so that the echo amplitude decreases (Mims ENDOR) (33). These pulsed ENDOR experiments exhibit less baseline artifacts and are easier to set up compared with CW ENDOR experiments, as the required mean radiofrequency power is smaller and the ENDOR effect does not depend on a certain balance of relaxation times. Davies ENDOR is better suited for couplings exceeding 1-2 MHz, while Mims ENDOR is better suited for small couplings, for instance matrix ENDOR measurements. 

In Fourier transform spectrometry, the source supplies a wide range of frequencies. For infrared spectrometry, a polychromatic source is used, while for nuclear magnetic resonance (NMR) a powerful microsecond pulse of radiofrequency energy provides the range of frequencies required. 

Electron-nuclear double resonance (ENDOR) spectroscopy A magnetic resonance spectroscopic technique for the determination of hyperfine interactions between electrons and nuclear spins. There are two principal techniques. In continuous-wave ENDOR the intensity of an electron paramagnetic resonance signal, partially saturated with microwave power, is measured as radio frequency is applied. In pulsed ENDOR the radio frequency is applied as pulses and the EPR signal is detected as a spin-echo. In each case an enhancement of the EPR signal is observed when the radiofrequency is in resonance with the coupled nuclei. 

Figure 9a defines the parameters of the SORC sequence. Here a train of radiofrequency pulses of duration ty, and spacing t is applied,Af away from exact resonance, to a pure nuclear quadrupole system in zero external magnetic field. 

---