Proton magnetogyric ratio

Receptivity D is proportional to y NKJ + 1) where y is the magnetogyric ratio, N the natural abundance of the isotope, and / the nuclear spin quantum number Dp is the receptivity relative to that of the proton taken as 1.000. 

Another separation method in using nonselective relaxation-rates is based on deuterium substitution, and utilizes the fact that the efficiency with which a nucleus contributes to the relaxation of a neighboring nucleus is proportional to the square of the magnetogyric ratio of the donor nucleus, that is, Phd/PHH = /3(yo/yn) - Because yo/yn — l/, replacement of a proton by a deuterium nucleus would be expected to contribute to the... 

Transfer of polarization from C nuclei to H nuclei and their subsequent detection leads to a 16-fold increase in sensitivity because the C magnetization is being measured indirectly through detecting it via the nucleus with the higher magnetogyric ratio (i.e., H). Irradiation of the protons between the scans causes a further threefold increase in the population of the C nuclei due to nOe, so an overall 50-fold increase in sensitivity is achievable in contrast to direct C measurements. However, because of... 

Polarization transfer Application of certain pulse sequences causes the transfer of the greater-equilibrium polarization from protons to a coupled nucleus, e.g., C, which has a smaller magnetogyric ratio. 

Fig. 42a-c Spectra of tetramethyltin in CDC13. a Proton decoupled, b proton coupled (2JsncH 54.3 Hz), c proton spectrum. The satellite signals are due to coupling to tin-117 (inner lines) and tin-119 (outer lines). The ratio of the coupling with tin-119 to that with tin-117 is 1.046 1 (the ratio of the magnetogyric ratios of the two nuclei)... 

Because the sensitivity of NMR is the highest for protons compared to other nuclei, all examples of quantitation work described in this chapter are based on proton NMR data. The signals from other NMR active nuclei such as 19F or 13C may also be used for quantitation. The quantification of TFA using 19F NMR is a good example. However, except for 19F, the sensitivities and detection limits are usually compromised in these measurements because nuclei other than H and 19F typically have a lower natural abundance and a lower magnetogyric ratio. 

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 isotope N, with a natural abundance of 99.9%, has nuclear spin 7 = 1 and gives broad signals which are of little use for structural determinations. The N nucleus, with I = 1/2, is therefore preferred. However, the low natural abundance of about 0.4% and the extremely low relative sensitivity (Table 1) make measurements so difficult that N NMR spectroscopy was slow to become an accepted analytical tool. A further peculiarity is the negative magnetogyric ratio since, in proton decoupled spectra, the nuclear Overhauser effect can strongly reduce the signal intensity. DEPT and INEPT pulse techniques are therefore particularly important for N NMR spectroscopy. 

The INEPT (Insensitive Nuclei Enhanced by Polarization Transfer) experiment [6, 7] was the first broadband pulsed experiment for polarization transfer between heteronuclei, and has been extensively used for sensitivity enhancement and for spectral editing. For spectral editing purposes in carbon-13 NMR, more recent experiments such as DEPT, SEMUT [8] and their various enhancements [9] are usually preferable, but because of its brevity and simplicity INEPT remains the method of choice for many applications in sensitivity enhancement, and as a building block in complex pulse sequences with multiple polarization transfer steps. The potential utility of INEPT in inverse mode experiments, in which polarization is transferred from a low magnetogyric ratio nucleus to protons, was recognized quite early [10]. The principal advantage of polarization transfer over methods such as heteronuclear spin echo difference spectroscopy is the scope it offers for presaturation of the unwanted proton signals, which allows clean spec-... 

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... 

Lower magnetogyric ratio of 13C, making the signal for 13C much lower than that of a proton. 

Indeed, paramagnetic broadening is proportional to the square of the nuclear magnetogyric ratio y. Therefore, the inverse detection of heteronucleus (i.e. the transfer of magnetization from the X nucleus to the proton followed by proton detection) may prevent the observation of X signal because of proton 72 relaxation. In fact, the relative values of y for H, 13C and, 5N nuclei are 1 1/4 1/10, and thus the relative contribution to overall relaxation arising from the hyperfine interaction is 1 1/16 1/100, respectively. Therefore, to identify... 

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