Practical aspects of NMR
High resolution NMR spectra are routinely obtained today, but there are various requirements which must be fulfilled to produce high quality spectra from which a great deal of useful data can be obtained.
Today, most NMR is of the pulse variety, in which all the nuclei in the sample are placed in a static magnetic field and are then excited by a pulse of electromagnetic radiation. The emissions as they relax back to their equilibrium state are then monitored.
The magnetic fields used in NMR are generally very strong (2 – 12 Tesla or more), and the stronger the field the better quality the spectrum obtained. There are several reasons for this:
Firstly, the patterns of multiplets (the components into which a resonance peak may be split by spin-spin coupling) can be resolved much more clearly. This is because the difference between the chemical shifts at which nuclei resonate is proportional to the field strength, but the spin-spin coupling constant is independent of the field strength. Thus higher strength fields reduce overlap between different multiplets.
Secondly, higher field strength will increase signal intensity. It is generally the case that in NMR spectroscopy signal intensities are very low, as they are proportional to the difference in population between the two states. This difference is usually small compared to the thermal energy kT, and thus promotion from the lower to the upper level occurs readily at room temperature. The use of strong magnetic fields increases the energy separation between the two states, as the energies of the states are proportional to the field strength. This makes the population imbalance larger and the signal stronger .
(This is also one reason why the proton is such a favoured nucleus in NMR. The energy levels are also proportional to the magnetogyric ratio of the nucleus being studied, and thus the larger the magnetogyric ratio, the larger the gap between energy levels and the larger the population imbalance, so the stronger the signal. The magnetogyric ratio is a fixed property of a given nucleus, but the proton happens to have the second largest value of γ of any nucleus. This makes it ideal for NMR studies.
Another useful property of the proton as an NMR nucleus is its high natural abundance, which contributes to the strong NMR signal and also means that coupling between 1H nuclei can be observed in the spectrum. This increases the amount of structural data that interpretation of the spectrum can yield.)
It is also important that the magnetic field be homogeneous (of equal strength at all points throughout the sample), or otherwise signals would become blurred as equivalent nuclei throughout the sample would resonate at slightly different frequencies. Homogeneity is achieved by constructing the magnets used for the apparatus in such a way that they generate opposing field gradients which cancel each other out. The sample is usually also spun rapidly. This means that the nuclei pass rapidly throughout any remaining field gradients, with the effect that they resonate at an average frequency in the spectrum.
In practice, the technique of two-dimensional NMR is a very important refinement of the basic NMR procedure. The details of this need not concern us; suffice it to say that it is possible to separate the effects of spin-spin coupling and the chemical shift in a spectrum, and the results can be displayed along two different axes. This can greatly simplify the appearance and interpretation of the spectrum.
The general appearance of such a spectrum is as follows:
The numbers on the axes give the chemical shift, δ, in ppm.
The contours that lie on the diagonal represent the normal peaks in a 1D spectrum. Thus for the spectrum shown above, there would be three different resonant peaks in the one dimensional spectrum, that correspond to three different magnetic environments (A, B and C). Some very useful information is given by the off-diagonal peaks, as they indicate that the protons lying on the same horizontal and vertical lines are spin coupled. Thus in the above diagram, we can immediately state that A and B nuclei are coupled together, as are the A and C nuclei. The B and C nuclei are not spin-coupled.
Although this example is quite a trivial one, it should be apparent how useful this technique might be in the interpretation of more complex molecules.