The chemical shift has proved to be the most useful characteristic of the hydrogen resonance spectrum for indicating features of molecular structure. This is because, first, the chemical shift is sensitively and reliably related to major and minor aspects of structure. If solvent, concentration, and temperature are held constant, the chemical shifts measured on a stable spectrometer are reproducible to about one part per billion for samples of high purity. For ordinary work, where concentration and temperature are controlled only approximately, the shifts are usually reproducible to 0.02 to 0.05 ppm. This is sufficient precision to show the small variations in shift caused by differences in steric arrangement or differences in the lengths of short chains. These small variations show up as perturbations on the larger variations caused by differences in electronegativity of substituents, degree of saturation, “resonance interaction” of neighboring groups, or the size of small rings.
KeywordsChemical Shift Shift Range Covalent Radius Compound Type Resonance Interaction
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