(or, What is NMR Spectgroscopy?)
Nuclear Magnetic Resonance (NMR) was first experimentally observed in late 1945, nearly simultaneously by the research groups of Felix Bloch, at Stanford University and Edward Purcell at Harvard University. The first NMR spectra were first published in the same issue of Physical Review in January of 1946. Bloch and Purcell were jointly awarded the Nobel Prize in Physics in 1952 for their discovery of Nuclear Magnetic Resonance Spectroscopy.
The NMR phenomenon relies on the interaction of the nuclei of certain atomic isotopes with a static magnetic field. This magnetic field makes the possible spin-states of the nucleus differ in energy, and using NMR techniques the spins can be made to create observable transitions between the spin states. Common NMR active nuclei are 1H, 13C, 31P, 15N, 29Si, and many more. Nearly every element has at least one isotope that is NMR active.
Since then, NMR spectroscopy has become an indespensible tool for the determination of molecular structure, the study of molecular dynamics, and the characterization of materials at the molecular level by chemists, physicists, and molecular biologists. For the first several decades, researchers relied on one-dimensional NMR spectra of NMR active nuclei. These spectra have one frequency axis, and analysis relies upon the relative frequency shifts between chemically inequivalent nuclei, combined with difference in the relative integrated intensities of the peaks. During the 1970s, two-dimensional NMR was discovered and rapidly evolved NMR into the powerful tool that it is today for molecular structural determination. Two-dimensional NMR spectra have two frequency axes, which can correspond to like nuclei (i.e. 1H-1H) or different nuclei (i.e. 1H-13C), and a third dimension of peak intensity. More recently, NMR experiments have been developed that contain information in three, four, and even five dimensions. The power of NMR to elucidate molecular structure seems almost limitless. Illustrating the importance of NMR to the scientific community are subsequent Nobel Prizes awarded to R.R. Ernst in 1991 (chemistry), K. Wütrich in 2002 (chemistry), and P. Lauterbur & P. Mansfield in 2003 (medicine).
The utility of NMR stems from the fact that chemically distinct nuclei differ in resonance frequency in the same magnetic field. This phenomenon is known as the chemical shift. In addition, the resonance frequencies are purturbed by the existance of neighboring NMR active nuclei, in a manner dependent on the bonding electrons that connect the nuclei. This is knows as spin-spin, or "J" coupling. Spin-spin coupling allows one to identify connections between atoms on a molecule, through the bonds that connect them. Combined with the ability to use quantitative information from peak intensities, one can very accurately determine how the atoms combine to form a unique molecular structure.