Magnetic resonance systems shielding

A. C. de Dios and E. Oldfield, Chem, Phys, Lett., 20S, 108 (1993). Methods for Computing Nuclear Magnetic Resonance Chemical Shielding in Large Systems. Multiple Cluster and Charge Field Effects. 

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... 

Nuclear Magnetic Resonance. A definitive review of interproton allylic spin-spin coupling has been made which contains much of relevance to alicyclic systems. Study of the n.m.r. spectra of 36 axially and equatorially substituted cyclo-hexanols has identified some unexpected chemical shift effects. Substituent-dependent additive shielding increments were used successfully to calculate the chemical shift of methine H atoms present, so confirming the observation of some axial H atom shifts at lower fields than the equatorial H atom resonances, i.e. an inversion of the accepted behaviour. 

Nuclear magnetic resonance (NMR) is a technique of considerable versatility in polymer science. It is used universally as a probe of chemical configurations, it provides information on the dynamics and relaxation times of a polymer system and it offers a route to the determination of orientation parameters, the exact route depending on the particular nuclei employed. In principle quadrupolar, dipolar, shielding tensor and indirect spin coupling interactions can all be employed " however, in practice only the first two have any universal appeal. Dipolar coupling using proton NMR offers the simplest approach in terms of material preparation and will be considered first. 

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