2. Fundamentals of NMR
Nuclear Magnetic Resonance (NMR) spectroscopy currently is one of the most important and most widely used methods for structure analysis in chemistry. The method exploits the magnetic properties of the atomic nuclei (mostly 1H and 13C in organic chemistry). When being placed in a very strong static magnetic field, nuclei with a spin quantum number I > 0 behave like tiny magnetic gyroscopes and precess about the external magnetic field axis, much as real gyroscopes "torque" about the gravitational field axis. The frequency of the nuclear spin precession usually is in the range of several hundreds of MHz, being dependent on the strength of the magnetic field and the type of the nucleus itself.
Imbedded into a molecule, atomic nuclei are not "naked" but experience a surrounding of electrons. According to the different locations in a given molecule, the electron cloud density around a particular nucleus may be different at various places. Functional groups like C=O or halides may lead to a decrease of electron density as compared to alkanes. These variations manifest in slightly different precession frequencies of the involved nuclei - the phenomenon of "chemical shift".
An additional feature of NMR is the mutual interaction of nuclei mediated by the bonding electrons ("scalar coupling", "J-coupling"). A nucleus "sees" its neighborhood nuclei, and according to the number of the coupled "partners" there will be a splitting of the corresponding line in the spectrum. Usually, these splittings may be interpreted by very simple rules and permit even more detailed insight into the structure of a molecule.
In its early years, NMR spectroscopy employed the "continuous wave" (CW) method: at a given fixed magnetic field an RF field generated by an oscillator was varied from high to low frequency (within very small limits, magnitude of ppm). Thus, during the measurement (ca. 1...5 min.) all involved nuclei became "resonant": they absorbed energy for the case that their precession frequency exactly matched the frequency of the applied RF field.
A more elegant and now generally employed method was introduced by R. R. Ernst (Nobel Prize 1991): a very short (order of µsec) RF pulse excites all nuclei, irrespective of their chemical shifts and their spin, spin couplings. Macroscopic magnetization is generated within a sample. At the end of the pulse, the RF receiver is turned on. The nuclei which precess with their individual frequencies induce oscillating currents in the receiver coil which are amplified and digitized for further analysis.
Having been excited by the RF pulse, the nuclei turn back ("relax") to their initial state (exact: the initial equilibrium population) within usually several seconds; the induced magnetization decays. Since the precession of the nuclei is free and due to the decay of the induced magnetization the whole process is termed Free Induction Decay (FID).
The observation and further exploitation of FIDs plays the central role in NMR spectroscopy. Usually, several (two....thousands) of FIDs are co-added in order to improve the signal-to-noise ratio of a spectrum.
The summed up FID is then once subjected to a mathematical operation (Fourier transform) which results in an NMR spectrum identical to that obtained by the CW method.
Here is a typical NMR FID and its corresponding Fourier transformed spectrum.
If you want to read more about introductory NMR, click here.
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August 19, 1996
