nuclear magnetic resonance

Examples of nuclear magnetic resonance in the following topics:

• Structural Determination

  • Structural determination using isotopes is often performed using nuclear magnetic resonance spectroscopy and mass spectrometry.
  • Structural determination utilizing isotopes is often performed using two analytical techniques: nuclear magnetic resonance spectroscopy (NMR) and mass spectrometry (MS).
  • Mass spectrometry and nuclear magnetic resonance detect the difference in an isotope's mass, while infrared spectroscopy detects the difference in the isotope's vibrational modes.
  • Nuclear magnetic resonance and mass spectrometry are used to investigate the mechanisms of chemical reactions.

• Proton NMR Spectroscopy

  • This important and well-established application of nuclear magnetic resonance will serve to illustrate some of the novel aspects of this method.
  • A solution of the sample in a uniform 5 mm glass tube is oriented between the poles of a powerful magnet, and is spun to average any magnetic field variations, as well as tube imperfections.
  • If the magnetic field is smoothly increased to 2.3488 T, the hydrogen nuclei of the water molecules will at some point absorb rf energy and a resonance signal will appear.
  • Since protons all have the same magnetic moment, we might expect all hydrogen atoms to give resonance signals at the same field / frequency values.
  • This secondary field shields the nucleus from the applied field, so Bo must be increased in order to achieve resonance (absorption of rf energy).

• Introduction

  • Over the past fifty years nuclear magnetic resonance spectroscopy, commonly referred to as nmr, has become the preeminent technique for determining the structure of organic compounds.
  • The resulting spin-magnet has a magnetic moment (μ) proportional to the spin.
  • Strong magnetic fields are necessary for nmr spectroscopy.
  • The international unit for magnetic flux is the tesla (T).
  • These moments are in nuclear magnetons, which are 5.05078•10-27 JT-1.

• Isotopes of Hydrogen

  • The most common use for deuterium is in nuclear resonance spectroscopy.
  • As nuclear magnetic resonance (NMR) requires compounds of interest to be dissolved in solution, the solution signal should not register in the analysis.
  • As NMR analyzes the nuclear spins of hydrogen atoms, the different nuclear spin property of deuterium is not 'seen' by the NMR instrument, making deuterated solvents highly desirable due to the lack of solvent-signal interference.

• Oxidation States of Nitrogen

  • In the example shown at the top of the following diagram it should be noted that resonance delocalization of the unpaired electron contributes to a polar N–O bond.
  • The spin of the nitroxyl unpaired electron may be studied by a technique called electron paramagnetic resonance (epr or esr).
  • Thus, site-directed spin labeling (SDSL) has emerged as a valuable technique for mapping elements of secondary structure, at the level of the backbone fold, in a wide range of proteins, including those not amenable to structural characterization using classical structural techniques, such as nuclear magnetic resonance and X-ray crystallography.

• Isomers

  • 13C Nuclear Magnetic Resonance Spectroscopy: This detects structurally different carbon atoms within a molecule.
  • The numbers next to each different carbon are in a sense magnetic addresses, called chemical shifts, produced in the course of the spectroscopic measurement.
  • Because these chemical shifts are influenced by the full electron and nuclear distribution in each structure, identical values are seldom observed, even for similar kinds of carbon atoms (e.g. 1º, 2º, 3º or 4º).

• Detection and Observation of Radicals

  • The same unpaired or odd electron that renders most radical intermediates unstable and highly reactive may be induced to leave a characteristic "calling card" by a magnetic resonance phenomenon called "electron spin resonance" (esr) or "electron paramagnetic resonance" (epr).
  • Just as a proton (spin = 1/2) will occupy one of two energy states in a strong external magnetic field, giving rise to nmr spectroscopy; an electron (spin = 1/2) may also assume two energy states in an external field.
  • Because the magnetic moment of an electron is roughly a thousand times larger than that of a proton, the energy difference between the spin states falls in the microwave region of the spectrum (assuming a moderate magnetic field strength).
  • The lifetime of electron spin states is much shorter than nuclear spin states, so esr absorptions are much broader than nmr signals.
  • This complexity is the result of hyperfine splitting of the resonance signal by protons and other nuclear spins, an interaction similar to spin-spin splitting in nmr spectroscopy.

• The Chemical Shift

  • Unlike infrared and uv-visible spectroscopy, where absorption peaks are uniquely located by a frequency or wavelength, the location of different nmr resonance signals is dependent on both the external magnetic field strength and the rf frequency.
  • Since no two magnets will have exactly the same field, resonance frequencies will vary accordingly and an alternative method for characterizing and specifying the location of nmr signals is needed.
  • The first feature assures that each compound gives a single sharp resonance signal.
  • Since the deuterium isotope of hydrogen has a different magnetic moment and spin, it is invisible in a spectrometer tuned to protons.
  • The shielding effect in such high electron density cases will therefore be larger, and a higher external field (Bo) will be needed for the rf energy to excite the nuclear spin.

• Spin-Spin Splitting

  • Unlike its 1,2-dichloro-isomer (below left), which displays a single resonance signal from the four structurally equivalent hydrogens, the two signals from the different hydrogens are split into close groupings of two or more resonances.
  • The magnitude of J, usually given in units of Hz, is magnetic field independent.
  • If an atom under examination is perturbed or influenced by a nearby nuclear spin (or set of spins), the observed nucleus responds to such influences, and its response is manifested in its resonance signal.
  • Nuclei separated by three or fewer bonds (e.g. vicinal and geminal nuclei ) will usually be spin-coupled and will show mutual spin-splitting of the resonance signals (same J's), provided they have different chemical shifts.
  • J is the same for both partners in a spin-splitting interaction and is independent of the external magnetic field strength.

• Signal Strength

  • We can take advantage of rapid OH exchange with the deuterium of heavy water to assign hydroxyl proton resonance signals .
  • Hydrogen bonding shifts the resonance signal of a proton to lower field ( higher frequency ).
  • iv) Intramolecular hydrogen bonds, especially those defining a six-membered ring, generally display a very low-field proton resonance.
  • All these anomalous cases seem to involve hydrogens bonded to pi-electron systems, and an explanation may be found in the way these pi-electrons interact with the applied magnetic field.
  • For most of the above resonance signals and solvents the changes are minor, being on the order of ±0.1 ppm.