NMR spectroscopy/Advanced

The conventional NMR spectroscopy experiment contains three basic components:

• Changing the population of the nuclear spin states (generally by introducing the sample into a magnetic field)
• A time dependent perturbation: generally consisting of a pulse or sequence of radiofrequency pulses; this may be accompanied by changes in the magnetic field usually in the form of time dependent magnetic field gradients.
• Detection: This is usually carried out after turning off the field responsible for the perturbation in the previous stage, however, in many experiments a different time dependent perturbation either in the form of additional pulses of electromagnetic radiation or pulsed magnetic field gradients or a combination of both is applied even during the detection phase.

In general, all of the three steps are carried out in the same primary magnetic field. Magnetic field strengths used in NMR spectroscopy typically range from milli-Tesla to 20 Tesla, and under these conditions radiofrequency electromagnetic radiation is necessary to cause transitions between the nuclear spin states (ν=ΔE/h). However, there are many non-conventional methods of obtaining NMR spectral information - in some of these methods, an external magnetic field may not be required during some steps of the NMR experiment; and in others, radiofrequency electromagnetic radiation is not used in the excitation or detection stages of the NMR experiment. A few examples of such non-conventional methods are: zero-field NMR spectroscopy, optical pumping, optically detected magnetic resonance, magnetic force microscopy/spectroscopy, stochastic-NMR spectroscopy, etc.

The energy difference between the different spin states depends upon the molecular structure as well as the applied magnetic field and hence the frequency/frequencies at which absorption of electromagnetic radiation is absorbed encodes information regarding molecular structure. The rate of transition between the different spin states depends upon the molecular structure as well as the interaction with the environment and can be used to quantify inter- and intra-molecular dynamics and to assess bulk properties such as viscosity.