Nuclear Magnetic Resonance

Nuclear Magnetic Resonance (NMR) Spectroscopy Introduction Nuclei with an odd number of protons, neutrons, or both, will have an instrinsic nuclear spin. Spin quantum number for various nuclei Number of protons Number of Neutrons Spin Quantum Number Examples 12 Even Even 0 C, 16O, 32S 1 Odd Even 1/2 H, 19F, 31P 11 " " 3/2 B,35Cl, 79Br 13 Even Odd 1/2 C 127 " " 3/2 I 17 " " 5/2 O 2 Odd Odd 1 H, 14N

When a nucleus with a non-zero spin is placed in a magnetic field, the nuclear spin can align in either the same direction or in the opposite direction as the field. These two nuclear spin alignments have different energies and application of a magnetic field lifts the degeneracy of the nuclear spins. A nucleus that has its spin aligned with the field will have a lower energy than when it has its spin aligned in the opposite direction to the field.

Nuclear magnetic resonance (NMR) spectroscopy is the absorption of radiofrequency radiation by a nucleus in a strong magnetic field. Absorption of the radiation causes the nuclear spin to realign or flip in the higher-energy direction. After absorbing energy the nuclei will reemit RF radiation and return to the lower-energy state. The energy of a NMR transition depends on the magnetic-field strength and a proportionality factor for each nucleus called the magnetogyric ratio. The local environment around a given nucleus in a molecule will slightly perturb the local magnetic field exerted on that nucleus and affect its exact transition energy. This dependence of the transition energy on the position of a particular atom in a molecule makes NMR spectroscopy extremely useful for determining the structure of molecules.

Instrumentation There are two NMR spectrometer designs, continuous-wave (cw), and pulsed or Fouriertransform (FT-NMR). CW-NMR spectrometers have largely been replaced with pulsed FT-NMR instruments. However due to the lower maintenance and operating cost of cw instruments, they are still commonly used for routine 1H NMR spectroscopy at 60 MHz. (Low-resolution cw instruments require only water-cooled electromagnets instead of the liquid-He-cooled superconducting magnets found in higher-field FT-NMR spectrometers.) These two spectrometer designs are described in separate CW-NMR and FT-NMR documents.

Continuous-Wave Nuclear Magnetic Resonance (NMR) Spectroscopy

Introduction Continuous-wave NMR spectrometers have largely been replaced with pulsed FT-NMR instruments. However due to the lower maintenance and operating cost of cw instruments, they are still commonly used for routine 1H NMR spectroscopy at 60 MHz. (Low-resolution cw instruments require only water-cooled electromagnets instead of the liquid-He-cooled superconducting magnets found in higher-field FT-NMR spectrometers.)

Instrumentation A cw-NMR spectrometer consists of a control console, magnet, and two orthogonal coils of wire that serve as antennas for radiofrequency (RF) radiation. One coil is attached to an RF generator and serves as a transmitter. The other coil is the RF pick-up coil and is attached to the detection electronics. Since the two coils are orthogonal, the pick-up coil cannot directly recieve any radiation from the generator coil. When a nucleus absorbs RF radiation, it can become reoriented due to its normal movement in solution and re-emit the RF radiation is a direction that can be recieved by the pick-up coil. This orthogonal coil arrangement greatly increases the sensitivity of NMR spectroscopy, similar to optical fluorescence. Spectra are obtained by scanning the magnet and recording the pick-up coil signal on paper at the control console.

Fourier-Transform Nuclear Magnetic Resonance (FT-NMR) Spectroscopy Introduction Fourier-transform NMR spectrometers use a pulse of radiofrequency (RF) radiation to cause nuclei in a magnetic field to flip into the higher-energy alignment. Due to the Heisenberg uncertainty principle, the frequency width of the RF pulse (typically 1-10 µs) is wide enough to simultaneously excite nuclei in all local environments. All of the nuclei will re-emit RF radiation at their respective resonance frequencies, creating an interference pattern in the resulting RF emission versus time, known as a free-induction decay (FID). The frequencies are extracted from the FID by a Fourier transform of the time-based data.

Instrumentation

An FT-NMR spectrometer consists of a control console, magnet, and a coil of wire that serves as the antenna for transmitting and receiving the RF radiation. (Only one coil is necessary because signal reception does not begin until after the end of the excitation pulse.) Because the FID results from the emission due to nuclei in all environments, each pulse contains an interference pattern from which the complete spectrum can be obtained. Because of this multiplex (or Fellgett) advantage, repetitive signals can be summed and averaged to greatly improve the signal-to-noise ratio of the resulting FID.