Protons magnetic shielding

VigUone, R. G. Zanasi, R. Lazzeretti, P. Are ring currents still useful to rationalize the benzene proton magnetic shielding , Org. Lett. 2004, 6, 2265-2267. 

Meyer and others the difference in the proton magnetic shielding between benzene and noncyclic olefins observed ... 

JG Oldenziel, NJ Trappeniers. High resolution nuclear magnetic resonance spectroscopy in liquids and gases at pressures up to 2500 bar, HI. Density dependence of the proton magnetic shielding constants in the polar liquids methanol and ethanol. Physica A 83 161-172, 1976. 

In order to establish the identity of the trimethylcarbonium ion, the t-butyl fluoride-antimony pentafluoride system was investigated. It was found that when the vapour of t-butyl fluoride was passed over the surface of purifled liquid antimony pentafluoride (with exclusion of moisture and oxygen) a stable complex layer is formed on the top of the antimony pentafluoride. When this layer was separated and its proton magnetic resonance investigated (see subsequent discussion) the spectrum was found to be identical with that of the least-shielded species formed by decarbonylation of the t-butyloxocarbonium salt... 

In organic molecules the different protons are shielded differently from the external magnetic field by their environments, i.e. by the other atoms in the molecule, by the surrounding chemical bonds, and by the solvent. The resulting resonance condition for the ith proton is commonly given by... 

Compton wavelength of neutron Compton wavelength of proton Diamagnetic shielding factor, spherical H20 molecule Electron g factor Electron magnetic moment Electron radius (classical) Electron rest mass... 

In practice, it is easier to fix the radio wave frequency and vary the applied magnetic field than it is to vary the radio wave frequency. The magnetic field felt by a hydrogen atom is composed of both applied and induced fields. The induced Held is a field created by the electrons in the bond to the hydrogen and the electrons in nearby n bonds. When the two fields reinforce each other, a smaller applied field is required to flip the proton. In this situation, a proton is said to be deshielded. Mien the applied and induced fields oppose each other, a stronger field must be applied to flip the proton. In this state, the proton is shielded. 

Scheme 2.9. Magnetic shielding of protons by antiperiplanar lone electron pairs.

Fig. 12. Experimental setup for observation A. (a) - channel scheme p - internal proton beam, Target - target mechanism, Col - collimator, MS - magnetic shield (b) - magnet and detectors M - poles of spectrometer magnet, VC — vacuum chamber, DC - drift chambers, H - scintillation hodoscopes, S,SM - scintillation counters, C -gas Cherenkov counters, Absorber - cast-iron absorber, MC - monitor counters...

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... 

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