Nuclei magnetogyric ratio

Carbon-13 nmr. Carbon-13 [14762-74-4] nmr (1,2,11) has been available routinely since the invention of the pulsed ft/nmr spectrometer in the early 1970s. The difficulties of studying carbon by nmr methods is that the most abundant isotope, has a spin, /, of 0, and thus cannot be observed by nmr. However, has 7 = 1/2 and spin properties similar to H. The natural abundance of is only 1.1% of the total carbon the magnetogyric ratio of is 0.25 that of H. Together, these effects make the nucleus ca 1/5700 times as sensitive as H. The interpretation of experiments involves measurements of chemical shifts, integrations, andy-coupling information however, these last two are harder to determine accurately and are less important to identification of connectivity than in H nmr. 

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... 

A nucleus having spin generates a magnetic moment pi. which is proportional to the angular momentum. Theory is not capable of calculating pi, so it is commonly expressed as Eq. (4-42), where 7 is called the magnetogyric ratio. 

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. 

Gyromagnetic ratio A measure of how strong the response of a nucleus is. The higher the value, the more inherently sensitive will be the nucleus. XH has the highest value. Also known as Magnetogyric ratio. 

The principal interaction experienced is the Zeeman interaction (Hz), which describes the interaction between the magnetic moment of the nucleus and the externally applied magnetic field, B0 (tesla). The nuclear magnetic moment, p (ampere meter2) is proportional to the nuclear spin quantum number (/) and the magnetogyric ratio (y, radian telsa-1 second-1) ... 

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-... 

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 magnetogyric ratio is then the ratio between the magnetic moment and the angular momentum of a rotating particle and has a characteristic value for each type of nucleus ... 

The magnetogyric ratio 7 is a constant for a given nucleus. When a nucleus is subjected to a magnetic field H(), the energy of the nuclear spin system is... 

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