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NMR spectra arise from a property that some nuclei have, usually called spin. This is a quantum effect; a model of it imagines the nucleus spinning on its axis, this spin being quantised, i.e. having only two directions, up or down. The nuclear spin is a characteristic property of the nucleus.
The spinning nucleus generates a magnetic field, since it is a moving charge. If an external magnetic field is applied to the nucleus, then the spin state where the nuclear magnetic field is aligned with the external field has a different, lower, energy from that spin state that gives rise to an opposing field. The fields are shown by arrows on the diagram below.
Since the two states have different energies, it is possible to convert a nucleus from the lower-energy state E1 to the higher state E2 by the input of suitable energy. The energy difference DE is such that radio frequency waves will perform this switch. The NMR spectrum arises because nuclei in different parts of the molecule experience different local magnetic fields according to the molecular structure, and so have different frequencies at which they absorb. This difference is called the chemical shift, dealt with on the next page.The radio. frequency used depends on the strength of the magnetic field; early machines used magnetic field strengths of around 1.5 Tesla (15000 Gauss), the proton magnetic resonances being around 60MHz. The ability to achieve higher magnetic fields of around 5 T (50 kG) using superconducting magnets has moved the resonances to 200 MHz. In practice the radio frequency is kept constant, and the magnetic field is then swept over a narrow range of field strength. The absorbtions are plotted on a graph where the frequency differences (in reality magnetic field differences) are plotted relative to some standard compound which defines the zero; for proton magnetic resonance this compound is TMS, tetramethylsilane, Si(CH3)4.
Chemistry contents NMR introduction The chemical shift Proton environment and NMR spectra
