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NMR

NMR spectroscopy
Definition:Nuclear magnetic resonance spectroscopy, kernmaganetische Resonanzspektroskopie.
Explanation:NMR spectroscopy is based on manipulating nuclear spins by radio frequency (RF) pulses in an external magnetic field (B0). More than 80% of all elements possess at least one isotope that is NMR-active (I ≥ ½). Precession of the spins about B0 occurs in the magnetic field at the lamor frequency (ωL), which is proportional to B0. The polarization of nuclear spins by B0 leads to a macroscopic magnetization of the spins. A π/2 RF pulse creates phase coherence of the spin precession, resulting in a time dependent electric current into a detector coil. This is recorded as a function of time (Free Inductions Decay, FID) as long as the phase coherence is present. A Fourier transformation (FT) converts the FID into an intensity vs. frequency (ω), which results in a NMR spectrum.

There are various internal couplings influencing the local magnetic field (Bloc) and therefore, the NMR signal for different nuclear sites.

Motion of the electrons near the nucleus induces a magnetic field, influences the magnetic field at the nuclear site, and causes a shift of ω. Since the electronic structure of the compound is determined by adjacent atoms, different chemical environments can be distinguished by their signal shift. Similarly, the Knight shift of metals generates a local magnetic field caused by the polarization of conduction electrons.

Quadrupolar coupling is based on the interaction of the nuclear quadrupole moment (Q), for nuclei with I > ½, with the electric field gradient (EFG). The latter being dominated by the charge distribution of the electrons, e.g. the chemical bonding. This gives insights into the anisotropy of the charge distribution around the observed nucleus and thus the bonding situation.

Due to the sensitivity of the signal shift to changes of Bloc and the EFG to changes of the charge distribution, NMR spectroscopy is regarded as a valuable local probe. It allows the investigation of local ordering phenomena like voids, atom exchanges, and preferred site occupancies.

Another measurable quantity is the longitudinal relaxation time constant (T1), which is the time for realignment of the macroscopic magnetization. Since T1 is sensitive to alteration of Bloc by ferromagnetism or antiferromagnetism, the magnetic state can be analyzed by temperature dependent analysis of T1.
SFB-Link:Within the SFB 761, NMR spectroscopy is used to verify predictions of the crystal structure and physical properties, such as preferred site occupancies and magnetism, from quantum mechanical calculations. Thus, it provides better insight into correlations of structure and properties of steel.
References:M. J. Duer, Introduction to Solid-Sate NMR Spectroscopy, Blackwell Science 2004.
F. Haarmann, Quadrupolar NMR of Intermetallic Compounds. In R. K. Harris, R. E. Wasylishen, Editors, Enzyclopedia of Magnetic Resonance. John Wiley & Sons, Ltd, Chichester (2011).
F. Haarmann, K. Koch, P. Jeglič, O. Pecher, H. Rosner, Yu. Grin. NMR spectroscopy of Intermetallic Compounds: An experimental and Theoretical Approach to Local Atomic Arrangements in Binary Gallides. Chem. Eur. J., 2011, 17 (27), 7560 – 7568.