nMr

Nuclear Magnetic Resonance (NMR) is a spectroscopic technique based on the interaction of nuclear spins with an external magnetic field. Nuclei with nonzero spin, such as 1H, 13C, 15N, and 31P, have magnetic moments that align with or against B0. Absorption of radiofrequency radiation at the Larmor frequency induces transitions between spin states, producing detectable signals. In liquids, rapid molecular tumbling averages many interactions, yielding narrow resonances whose chemical shifts report on the electronic environment, while spin-spin coupling splits signals into multiplets that reveal bonding and proximity. Relaxation times T1 and T2 reflect molecular dynamics. Fourier transform NMR converts time-domain data into frequency-domain spectra.

Instrumentation includes a superconducting magnet providing B0, a probe with RF coils, a spectrometer console, and shim coils to optimize field uniformity; for imaging, gradient coils enable spatial encoding. Samples are typically dissolved in deuterated solvents and referenced to internal or external standards.

Common applications are determination of molecular structure and dynamics in chemistry and biochemistry. NMR also underpins Magnetic Resonance Imaging (MRI), which maps proton density and relaxation properties to produce medical images.

History and limitations: NMR was developed independently in 1945 by Felix Bloch and Edward Mills Purcell, who shared the Nobel Prize in Physics in 1952. Limitations include relatively low sensitivity, high equipment cost, and the need for strong magnetic fields and suitable isotopes. Ongoing advances include multidimensional spectroscopy, improved sensitivity, and in vivo imaging techniques.