Factor magnetogyric

Because J arises from the magnetic interactions of nuclei, the simplest factor affecting it is the product yjY of the two nuclear magnetogyric ratios involved. For example, in FI F is 82 FIz, i.e. x yo/yf - This totally predictable factor is sometimes discounted by quoting the reduced coupling constant =... 

Other Nuclei. Although most nmr experiments continue to involve H, or both, many other nuclei may also be utilized Several factors, including the value of I for the nucleus, the magnitude of the quadmpolar moment, the natural abundance and magnetogyric ratio of the isotope, or the possibihty of preparing enriched samples, need to be considered. The product of the isotopic parameters can be compared to the corresponding value for providing a measure of relative sensitivity or receptivity. Table 1 summarizes these factors for a number of isotopes. More complete information may... 

In Eqs. (4)-(7) S is the electron spin quantum number, jh the proton nuclear magnetogyric ratio, g and p the electronic g factor and Bohr magneton, respectively. r//is the distance between the metal ion and the protons of the coordinated water molecules, (Oh and cos the proton and electron Larmor frequencies, respectively, and Xr is the reorientational correlation time. The longitudinal and transverse electron spin relaxation times, Tig and T2g, are frequency dependent according to Eqs. (6) and (7), and characterized by the correlation time of the modulation of the zero-field splitting (x ) and the mean-square zero-field-splitting energy (A. The limits and the approximations inherent to the equations above are discussed in detail in the previous two chapters. 

The angular momentum L has associated with it a magnetic moment /z. Both are vector quantities and they are proportional to each other. The proportionality factor y is a constant for each nuclide (i.e. each isotope of each element) and is called the gyromagnetic ratio, or sometimes the magnetogyric ratio. The detection sensitivity of a nuclide in the NMR experiment depends on y nuclides with a large value of y are said to be sensitive (i.e. easy to observe), while those with a small y are said to be insensitive. 

The terms pc and py correspond to 1/Tic and 1/Tih, respectively, and CTCH is the cross-relaxation rate. It should be stressed that the simplicity of the above equation is a consequence of the rareness of the I spins and of the dominant strength of the dipolar interaction between directly bonded nuclei. The situation for homonuclear proton spin systems is often more complicated, since the protons usually constitute a much larger spin system, and a separation into distinct two-spin systems may be not valid in this case. The broadband irradiation of the protons yields, in a steady state, Mhz = 0 and M z = Mj (1 rj). The factor 1 + 77 is called, as introduced above, the nuclear Overhauser enhancement factor. The NOE factor is related in a simple way to the equilibrium magnetizations of the I- and S-spins (which are proportional to the magnetogyric ratios 71 and 7s), the cross-relaxation rate and the relaxation rate of the I-spin ... 

Magnesium has one NMR-active isotope, Mg, which is only 10.0% naturally abundant. This is a spin 5/2 nucleus with a magnetogyric ratio of —1.639 X 10 rads T , which means it has a Larmor frequency that is 6% of that of protons in the same magnetic field and a receptivity of 72.9% of that of The relatively low Larmor frequency was one of the key factors... 

The factor of 4 corresponds to the (nearly) four times greater population difference for 1H compared to 13C, which is a direct result of the (nearly) four times larger magnetogyric ratio of H. Note that the equilibrium state for this two-spin system is [Pg.473]

An unpaired electron, like a proton, can adopt either of two spin orientations when immersed in a magnetic field. An electron in either orientation will precess at a frequency given by Eq. (11.1). The g factor is similar in some respects to the magnetogyric ratio (y) used in NMR spectroscopy. The value of g, used as the position parameter in EPR spectroscopy, depends on the exact structure of the free radical possessing the unpaired electron. 

Isotope Spin Natural abundance (x/%) Magnetic moment (/x//xn) Magnetogyric ratio (y/lO rads-i Quadrupole moment (Q/fmb Frequency ratio (H %) Reference sample Sample conditions Line width factor )(Z/fm ) Relative receptivity ) )... 

C NMR resulting in short Ti, the Si relaxation times tend to be long. This results from a combination of two factors a lower magnetogyric ratio for Siand a longer Si-H bond length(148 A for a C-H bond), which when incorporated in equation (24) results in a tenfold lowering of the Si DD relaxation rate, / i(DD) compared with In equation (24), which describes intramolecular DD relaxation for spin-j nuclei ... 

An analogous case is present in the 13c 2 j INEPT experiment (24). Here, only deuterated carbons are observed. While the magnetogyric ratio factor should give only a theoretical 3/5 enhancement, the short 2H T allows very rapid accumulation. The technique should be useful for determination of deuterated X nuclei spectra without interferences from protonated X signals. The sensitivity gain should be very dramatic for fully deuterated X nuclei where normal X nucleus T s can be extremely long because of inefficient dipolar relaxation from 2H or other mechanisms. This experiment offers the capability of detection of deuterated sites at small concentration in, for example. 

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