Chemical shift local magnetic field

We said previously that differences in chemical shifts are caused by the small local magnetic fields of electrons surrounding the different nuclei. Nuclei that are more strongly shielded by electrons require a higher applied field to bring them into resonance and therefore absorb on the right side of the NMR chart. Nuclei that are less strongly shielded need a lower applied field for resonance... 

Each electronically distinct 1H or 13C nucleus in a molecule comes into resonance at a slightly different value of the applied field, thereby producing a unique absorption signal. The exact position of each peak is called the chemical shift. Chemical shifts are caused by electrons setting up tiny local magnetic fields that shield a nearby nucleus from the applied field. 

Chemical shift Electrons of the atoms and molecules surrounding a nucleus interact with B0 and induce an additional local field at the position of the nucleus being probed. The effect of this local magnetic field is to reduce the magnitude of the external magnetic field experienced by local nuclei. This results in a shift in the resonance frequency of nuclei. Chemical shifts are measured in parts per million (ppm). 

The exact local magnetic field acting on a given nucleus is dependent on its electronic environment, i.e., the kind and number of the surrounding atoms determine its chemical shift (<5), and stereochemistry plays an important role22. <5 Values in this section are referenced to commonly agreed standards tetramethylsilane for 2H and 13C, ammonia for 15N, water for l70, and 85% aqueous phosphoric acid for 31P. 

The electrons modify the magnetic field experienced by the nucleus. Chemical shift is caused by simultaneous interactions of a nucleus with surrounding electrons and of the electrons with the static magnetic field B0. The latter induces, via electronic polarization and circulation, a secondary local magnetic field which opposes B0 and therefore shields the nucleus under observation. Considering the nature of distribution of electrons in molecules, particularly in double bonds, it is apparent that this shielding will be spatially anisotropic. This effect is known as chemical shift anisotropy. The chemical shift interaction is described by the Hamiltonian... 

Chemical shift As mentioned before, the field experienced by nuclear spins of the same species, e.g. of protons, is modified due to magnetic shielding by the cloud of electrons around each nucleus. This is accompanied by a small local magnetic field proportional to B0, but opposite to B0. As a result a slightly higher value of Ba is needed to achieve resonance, because... 

Chemical shifts are determined by the local magnetic fields surrounding magnetic nuclei. 

The interpretation of the chemical shift depends on the identity of the nucleus and whether it interacts with unpaired electrons. In general, the immediate chemical environment plays a central role in determining chemical shift. This is a result of the fact that the electrons near the nucleus affect the local magnetic field such that the magnetic field at a nucleus, Beff, is slightly different from the applied magnetic field, Bo. This effect is known as screening and is described by a proportionality constant a ... 

A second important mechanism for fluorine spin-lattice and spin-spin relaxation is produced by the chemical shielding anisotropy (CSA) [13, 14, 21, 71]. The magnetic field experienced by a nuclear spin depends on both the electronic structure of the molecule and how easily the electrons can move in the molecular orientations. In addition, the CSA depends on how the molecule is oriented in the magnetic field. Like spin-spin and dipole-dipole interactions, the CSA of small, rapidly tumbling molecules will be an averaged value (the chemical shift). However, these tumbling motions cause fluctuations of the local magnetic field that lead to relaxation. Also slower reorientation, or an environment that restricts the molecular motion, will result in broader lines due to CSA. 

The resonance frequencies, or chemical shifts, of various atoms are typically used only for assignment of individual atoms so that structural parameters specific to those atoms can be extracted from the NMR data. The chemical shift itself however is a measure of the local magnetic field experienced by the nucleus, which is dependent on... 

As outlined in Section 3.1.1., the mechanism which accounts for the temperature dependence of the proton chemical shift involves changes in both (T and Xv- The local magnetic field at the nucleus (finuc) is given by ... 

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