Magnetic nuclei natural abundance

Nucleus Natural abundance (%) Nuclear spin / Magnetic moment Frequency Ratio 5/% ... 

Nucleus Natural Abundance [%] Magnetic Moment //[ iN] Magnetogyric Ratio r[10 rad T- s-i] NMR frequency B [MHz] Standard Relative Receptivity Ef Relative Receptivity... 

Nucleus Natural abundance, % Magnetic moment (/z), in Bohr magnetons Spin quantum number (I) Electric quadrupole moment in multiples of e X 10 cm. ... 

Heteronuclei such as 13C (this magnetically active nucleus has 1.1% natural abundant) and 15N (0.3% natural abundance) are routinely measured with modem NMR spectrometers. Proton decoupled 13C NMR spectra in natural abundance exhibit singlets for each specific carbon atom, which are easier to count than overlapping multiplet lines in H NMR. ID 13C NMR can be used to investigate whether a peptide exhibits a single set of lines or a double (or more) set, which indicate conformational or configurational isomers (see Section 7.5.3). However, ID 13C NMR is rather insensitive and if there is not enough material or the solubility is low, more sensitive techniques have to be applied. 

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... 

Boron compounds contain two isotopes B10 and B11 of natural abundances 19% and 81%, respectively. Although both these isotopes possess magnetic moments, the Bn nucleus is better suited to the high resolution experiment because of its (1) greater natural abundance, (2) smaller quadrupole moment, and (3) larger nuclear moment. Because of the broad range of structures possible in boron compounds, particularly the hydrides, there has been considerable NMR work done in this field to confirm previously proposed structures and in a few cases to first establish geometry of a compound. The B11 spectra of tetraborane and a tetraborane derivative arc considered below. 

Some examples to illustrate the use of this spin system notation to distinguish between first- and second-order systems and to explain the concept of magnetic inequivalence will now be discussed. Because is the only spin-1/2 nucleus with 100% natural abundance that forms a wide variety of inorganic ring systems, most of the examples are taken from phosphorus chemistry (for other examples, see Chapter 11). 

All of the heteroatoms possess at least one naturally occurring isotope with a magnetic moment (Table 15). The nuclei 14N, 170 and 33S also possess an electric quadrupole moment which interacts with the electric field gradient at the nucleus, providing a very efficient mechanism for relaxing the nuclear spin. The consequence of this facilitation of relaxation is a broadening of the NMR signals so that line widths may be 50-1000 Hz or even wider. To some extent this problem is offset by the more extensive chemical shifts that are observed. The low natural abundances and/or sensitivities have necessitated the use of accumulation techniques for all of these heteroatoms. The relative availability of 170 and 15N enriched... 

The main difficulty in l3C NMR is the low natural abundance of the carbon-13 nucleus (1.108%) and its low gyromagnetic ratio y, which yields a much smaller Boltzmann exponent 2yp0B0ikT than that of protons. Low natural abundance and small gyro-magnetic ratio are the reasons why 13C NMR is much less sensitive (1.59%) than H NMR (100%). A common measure of sensitivity in NMR is the signal to noise of a reference sample, e.g. 1% ethyl benzene in deuteriochloroform. Several methods are available for improving the signal noise in 13C NMR. 

Carbon NMR has been considered an essential characterization tool in spite of the fact that its major isotope, the 12C nucleus, is NMR inactive. In spite of its low natural abundance (1.1%) and its low magnetic moment, which lead to a significantly low overall sensitivity relative to... 

Isotope Enrichment. - The natural abundance of the magnetic nucleus under study is vitally important to overall sensitivity. Natural-abundance-2H n.m.r., for example, is roughly a million times less sensitive than 1H n.m.r. This factor must therefore be carefully considered before embarking on an n.m.r. study of an insensitive nucleus. One means to improve the sensitivity of low-abundance nuclei is to perform isotope enrichment. This is frequently done for nuclei such as13C,15N, and 2H it remains, however, a fairly expensive option. 

The 31P nucleus has a natural abundance of 100% and a spin number of 1/2 (therefore no electrical quadrupole moment). The multiplicity rules for proton-phosphorus splitting are the same as those for proton-proton splitting. The coupling constants are large (/H—p 200-700Hz, and /Hc p is 0.5-20 Hz) (Appendix F) and are observable through at least four bonds. The 31P nucleus can be observed at the appropriate frequency and magnetic field (Chapter 6). 

The 12C nucleus is not magnetically active (spin number, /, is zero), but the 13C nucleus, like the XH nucleus, has a spin number of 1/2. However, since the natural abundance of I3C is only 1.1% that of I2C and its sensitivity is only about 1.6% that of 1H, the overall sensitivity of 13C compared with H is about l/5700.f... 

The spectrometer is a radio receiver, and we change the frequency to tune in each nucleus at its characteristic frequency, just like the stations on your car radio. Because the resonant frequency is proportional to the external magnetic field strength, all of the resonant frequencies above would be increased by the same factor with a stronger magnetic field. The relative sensitivity is a direct result of the strength of the nuclear magnet, and the effective sensitivity is further reduced for those nuclei that occur at low natural abundance. For example, 13C at natural abundance is 5700 times less sensitive (1/(0.011 x 0.016)) than H when both factors are taken into consideration. 

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