NMR Spectroscopy

Nuclear magnetic resonance (NMR) spectroscopy is the study of molecules by recording the interaction of radiofrequency (R_f) electromagnetic radiations with the nuclei of molecules placed in a strong magnetic field.

Nuclear Magnetic Resonance (NMR) spectroscopy is an analytical chemistry technique used in quality control and research for determining the content and purity of a sample as well as its molecular structure. For example, NMR can quantitatively analyze mixtures containing known compounds. For unknown compounds, NMR can either be used to match against spectral libraries or to infer the basic structure directly. Once the basic structure is known, NMR can be used to determine molecular conformation in solution as well as studying physical properties at the molecular level such as conformational exchange, phase changes, solubility, and diffusion. In order to achieve the desired results, a variety of NMR techniques are available.  

Two-Dimensional Spectroscopy

In two-dimensional NMR it consists of two additional sections, namely- Evolution time and mixing time apart from preparation and detection. 

The indirect detection of an additional time-domain takes place during the evolution time. After completion of the acquisition of the Fourier induction decay (FID), including the repetitions necessary to perform the phase cycle of the experiment and to achieve sufficient signal-to noise, the experiment is repeated numerous times. A delay in the pulse sequence is incremented systematically from one experiment to the next, and the intensities detected in the FID during the acquisition time are thus modulated according to the length of this delay. The result is a two-dimensional time-domain data set, one dimension from direct detection, one dimension from systematic incrementation of a delay. It is processed using two subsequent Fourier transformations along with both time domains. First the directly detected FIDs are converted into a series of one-dimensional spectra, the interferogram. Then the interferogram is converted into a two-dimensional spectrum that exhibits two independent frequency axes. Both axes will usually contain the chemical shift of nuclei in the sample, the type of chemical shift as well as the appearance of the two-dimensional spectrum results from the design of the mixing period. During the mixing time the magnetization that has been created during the preparation period and has been modulated by a frequency during the evolution time is converted into another type of magnetization that is subsequently detected during the acquisition. This is accomplished using some type of interaction (scalar or J-coupling, dipolar coupling) between one or several spins in sample.

The concept of 2D-FT NMR was proposed by Jean jeener, but this idea was largely developed by Richard Ernst who won the 1991 Nobel Prize in Chemistry for his work in FT NMR, including multi-dimensional FT NMR, and especially 2D-FT NMR of small molecule. This 2D- FT NMR is used for the determination of the structure of biopolymers such as proteins or even small nucleic acids. There is increased resolution in two dimensional spectra when compared with one dimensional spectrum. The major disadvantage in NMR spectroscopy is overlapping in the analysis of NMR spectra of biomolecules, so this obstacle can be overcome by the use of two-dimensional techniques.