NMR spectroscopy/Definition

Nuclear magnetic resonance spectroscopy may be defined as the use of electromagnetic radiation, in the presence of a magnetic field, to obtain information regarding transitions between different nuclear spin states; 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, fluid velocity etc.

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 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.