Nuclear spin transitions

Pulse A short burst of radio frequency used to bring about some nuclear spin transition. 

ENDOR spectroscopy has proven to be a valuable technique to provide information on both free and protein bound flavin radicals. Since flavin radical ESR spectra can be partially saturated at moderate microwave power, ENDOR spectra may be observed as nuclear spin transitions by detection of changes in the partially saturated ESR signal as a function of nuclear radio frequency. The resonance condition for nuclei (when I = Vz) is described by the following equation ... 

Chemical Shifts in NMR. The first effect is very useful in chemical analysis The nuclear spin transitions are affected by the "hyperfine" I S coupling between electron spin S (for any single electron in the molecule that has density at the nuclear position) and nuclear spin I this is due to the isotropic Fermi contact term ... 

Energy levels with Overhauser effect (a) Relaxation due to a time-dependent isotropic contact electron-spin-nuclear-spin hyperfine interaction a(t)l S which has a zero time-average, but allows processes X and Y and enhances nuclear spin transitions when the electron populations are made equal by saturation, (b) Relaxation is due to all dipole-dipole interactions, which allow processes X, V, and PNi nuclear spin transitions are forced into emission by the Overhauser effect. In (a) the relative Boltzmann populations before saturation are shown. 

This would imply a very simple linear Zeeman effect but, as we show in chapter 8, additional terms describing the nuclear spin rotation interaction and the spin-spin interaction make the system much more interesting. The nuclear spin transitions are induced by an oscillating magnetic field applied perpendicular to the static magnetic field, the perturbation being represented, for example, by the term... 

The spectrum follows rules similar to those described in chapter 8 for the molecular beam magnetic resonance study of 7Li79Br, which also has a + ground state and two nuclear spins of 3/2. In that case the transitions studied were nuclear spin transitions within the J = 1 level, but the effective Hamiltonian is similar in the two cases. The Hamiltonian used by Low, Varberg, Connelly, Auty, Howard and Brown [89] contained rotational, quadrupolar and nuclear spin-rotation terms,... 

Using PMDR methods, the broad excimer emission formed at defect sites in the lattice of hexachlorobenzene crystal at 1.6°K (by doping it with high-energy triplet molecules at 1.6°K) is shown (38) to have zf transitions. This provides strong evidence that it is a triplet excimer. For molecules with atoms having nuclear spin, transitions between hyperfine levels of different zf levels are also observed. 

Thus, the frequency of the nuclear spin transition depends on the strength of the applied magnetic field and magnitude of Y for the nucleus. The transition frequency of a particular nucleus is one of the important parameters in NMR and is related to the chemical shift as explained in the next section. The value of y also determines sensitivity of a given isotope, as AE (equation 4) determines the ratio of the number of spins in the upper (ti ) and lower (ti2) energy levels according to the Boltzmann relationship ... 

A form of ESR experiment involving observation of the spectrum while irradiating nuclear spin transitions (see ESR spectroscopy)... 

Electronic, vibrational, rotational, and nuclear spin transitions... 

Much of the technical development of NMR over the past half century has focused on improving sensitivity. The fundamental problem is the low starting Boltzmann polarization that arises from the low energies of nuclear spin transitions. Several methods have been developed to improve the sensitivity or S/N in NMR. One major approach is through pulse sequence development to optimize the efficiency and information content of NMR spectra through manipulating the spin physics some of the more important experiments for small molecules were described above. 

A comparatively recent new development in ENDOR spectroscopy is electron spin echo ENDOR (ESE-ENDOR), where nuclear spin transitions are detected by their effect on a transient EPR signal (the spin echo) generated by a two- or three-pulse excitation. 

It is widely appreciated that modem NMR spectrometers use a short pulse of radiofrequency energy to excite nuclear resonances over a range of frequencies. This pulse is supplied as monochromatic radiation from the transmitter, yet the nuclear spin transitions giving rise to our spectra vary in energy according to their differing Larmor frequencies and so it would appear that the pulse will be unable to excite all resonances in the spectmm simultaneously. However, Heisenberg s Uncertainty principle tells us that an excitation pulse of duration At has associated with it a frequency uncertainty or spread of around 1/At Hz... 

A refinement of the ENDOR experiment is electron-nuclear-nuclear triple resonance, now commonly denoted TRIPLE. In TRIPLE experiments one monitors the effect of a simultaneous excitation of two nuclear spin transitions on the level of the EPR absorption. Two versions, known as special TRIPLE (ST) and general TRIPLE (GT), are routinely performed on commercially available spectrometers. 

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