Table and discussion Nuclear magnetic resonance

The commonest use of n.m.r. in chemistry is to decide whether certain groups are present or absent. Hence it is much used in finding the constitution of newly synthesized or isolated substances. It is also valuable in investigating potentially tautomeric molecules. When the tautomeric equilibrium is reached slowly (as usually occurs when the mobile proton is attached to a carbon atom in one tautomeric form), signals from both tautomeric forms are recorded independently, and a kinetic study may be made. The equilibrium is reached almost instantaneously when the mobile proton is attached to a nitrogen or oxygen atom. 

Apart from the shielding effects mentioned above, the external field of the instrument can induce electron currents which strongly shield acetylenic hydrogen atoms (8 2.3) whereas aromatic protons are strongly deshielded (8 7 to 8). 

The use of spin—spin relaxation will be discussed next. Part of the n.m.r. spectrum of an agent undergoes broadening after adsorption has occurred on to a macromolecule. Briefly, the technique is based on the fact that the relaxation 

Particularly convenient for study by this technique is the common type of complex for which the binding constants falls in the range 10 -10 . Such complexes exist in rapid equilibrium with the uncomplexed species, the rate of complex formation in these systems being of the order of 10 -10 litres mol s and the rate of dissociation Arp, by virtue of the relationship = k / 

Actinomycin (. J 5), when examined by H, and T n.m.r. in the presence of deoxyoligonucleotides, revealed intercalation by the chemical shifts, caused by the ring-current fields of the phenoxazone nucleus, in the purine-pyrimidine resonance signals of the nucleotide. This led to detailed understanding of the solution structure of the intercalated components, and established that intercalation occurred at the site of the central GC base-pairs (Patel, 1974). This work was extended, using N-enriched actinomycin (Brown, Shafer and Mirau, 1982). 

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