Nuclear magnetic resonance

Nuclear magnetic resonance (NMR) is a physical phenomenon in which nuclei in a magnetic field absorb and re-emit electromagnetic radiation. This energy is at a specific resonance frequency which depends on the strength of the magnetic field and the magnetic properties of the isotope of the atoms; in practical applications, the frequency is similar to VHF and UHF television broadcasts (60–1000 MHz). NMR allows the observation of specific quantum mechanical magnetic properties of the atomic nucleus. Many scientific techniques exploit NMR phenomena to study molecular physics, crystals, and non-crystalline materials through NMR spectroscopy.

Quotes

• Nuclear magnetic resonance spectroscopy depends on the absorption of energy when the nucleus of an atom is excited from its lowest energy spin state to the next higher one. The nuclei of several elements can be studied by NMR. The two elements that are the most common in organic molecules (carbon and hydrogen) have isotopes (¹H and ¹³C) capable of giving NMR spectra that are rich in structural information. A proton nuclear magnetic resonance (¹H NMR) spectrum tells us about the environments of the various hydrogens in a molecule; a carbon-13 nuclear magnetic resonance (¹³C NMR) spectrum does the same for the carbon atoms. Separately and together ¹H and ¹³C NMR take us a long way toward determining a substance’s molecular structure. We’ll develop most of the general principles of NMR by discussing ¹H NMR, then extend them to ¹³C NMR. The ¹³C NMR discussion is shorter, not because it is less important than ¹H NMR, but because many of the same principles apply to both techniques.
  • Francis A. Carey, Robert M. Giuliano, Organic Chemistry 8th ed. (2011), Ch. 13 : Spectroscopy

• In subsequent chapters, discussions regarding a number of nuclear magnetic resonance (NMR) techniques that could not be implemented when nuclear magnetic resonance was first discovered are presented. Their advent required, for example, strong magnetic fields and/or cryoprobes to accommodate limited sample availability. Pulsed field gradients (PFGs) have improved solvent suppression, have enabled efficient selective excitation, and have made accessible a different time range to diffusion coefficient measurement. Such developments have, of course, been made in parallel with increasing access to powerful computers and sophisticated software, permitting speedy processing and analysis of the various types and sizes of acquired data sets. Instrumental and software developments in the past 30 to 40 years have meant that NMR spectroscopy is now used in a wide range of scenarios. Synthetic chemists use NMR to elucidate structures of small molecules. It is employed in pharmaceutical industries for structure elucidation and drug development and screening (Chapter 3, Section 7.1). Biochemistry and biotechnology sectors utilise NMR to probe solution structures and functions of biological polymers (Chapter 7), and it is increasingly used in biomedicine (in particular, biomarker discovery; Chapter 6) for the analysis of complex matrices. Materials science (both soft and hard matters) is another application area in which solution and solid-state NMR has proved extremely valuable. While not an exhaustive list of applications, this is an illustration of the breadth of science that has benefitted from this analytical technique.
  • Julie Fisher, Ch. 1. "The Basics" in Modern NMR Techniques for Synthetic Chemistry (2014) edited by Julie Fischer

• The electron paramagnetic resonance discovered by Evgenii Konstantinovich is undoubtedly a first-class thing. It is a pity that nuclear magnetic resonance 'floated away'. Clearly, if Evgenii Konstantinovich had worked in better conditions, he would have done much more.
  • Vitaly L. Ginzburg, "To the Memory of Evgenii Konstantinovich Zavoiskii", in The Physics of a Lifetime: Reflections on the Problems and Personalities of 20th Century Physics (2001),

• In the absence of an external magnetic field, the spins of magnetic nuclei are oriented randomly. When a sample containing these nuclei is placed between the poles of a strong magnet, however, the nuclei adopt specific orientations, much as a compass needle orients in the earth’s magnetic field. A spinning ¹H or ¹³C nucleus can orient so that its own tiny magnetic field is aligned either with (parallel to) or against (antiparallel to) the external field. The two orientations don’t have the same energy, however, and aren’t equally likely. The parallel orientation is slightly lower in energy by an amount that depends on the strength of the external field, making this spin state very slightly favored over the antiparallel orientation. ... If the oriented nuclei are irradiated with electromagnetic radiation of the proper frequency, energy absorption occurs and the lower-energy state “spinflips” to the higher-energy state. When this spin-flip occurs, the magnetic nuclei are said to be in resonance with the applied radiation—hence the name nuclear magnetic resonance.
  • John McMurry, Organic Chemistry, 8th ed. (2012), Ch. 13 : Structure Determination: Nuclear Magnetic Resonance Spectroscopy

• For magnetic fields that can be routinely produced in the laboratory, the transitions between energy levels for nuclear magnetic dipoles occur in the radio-frequency range, and the transitions between energy levels for unpaired electron spins occur in the microwave range. Nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) yield such valuable structural information that they have become indispensable in chemistry.
  • Robert J. Silbey, Robert A. Alberty, Moungi G. Bawendi Physical Chemistry, 4th ed. (2005) Ch. 15 : Magnetic Resonance Spectroscopy

• The nuclei of certain elements, including ¹H nuclei (protons) and ¹³C(carbon-13) nuclei, behave as though they were magnets spinning about an axis. When a compound containing protons or carbon-13 nuclei is placed in a very strong magnetic field and simultaneously irradiated with electromagnetic energy of the appropriate frequency, nuclei of the compound absorb energy through a process called magnetic resonance. The absorption of energy is quantized. ... We can use NMR spectra to provide valuable information about the structure of any molecule we might be studying. In the following sections we shall explain how four features of a molecule’s proton NMR spectrum can help us arrive at its structure.
  • T. W. Graham Solomons, Craig B. Fryhle and Scott A. Snyder, Organic chemistry, 11th ed. (2014), Ch. 9 : Nuclear Magnetic Resonance and Mass Spectrometry

• Back at Caltech, my research was going strong, and we had four different laboratories busy with experiments and people. In one of these laboratories, we were continuing with our work on coherence; in others, advancing techniques for shorter time resolution and for developing an optical analog for nuclear magnetic resonance (NMR). In NMR, the spin of nuclei with their transitions at radio frequencies is used for a variety of applications, ranging from the studies of molecular structure to magnetic resonance imaging (MRI), which is now commonly used in hospitals throughout the world.
  • Ahmed H. Zewail, Voyage Through Time: Walks of Life to the Nobel Prize (2002), Ch. 5 : The Invisible Atom: Close-up at Caltech