(Instrument part) Thanundon Kongnok M5610532

OUTLINE Introduction : Type of NMR Spectroscopy What is NMR ? Nuclear spin Resonance frequency Type of NMR Spectroscopy CW NMR Spectroscopy FT NMR Spectroscopy Pulsed Fourier Transform NMR Spectroscopy NMR sequential steps Instrument Conclusion

What is NMR ? Nuclear magnetic resonance (NMR) is a physical phenomenon in which nuclei in a magnetic field absorb and re-emit electromagnetic radiation at resonance frequency.

Net spin of a nucleus Nucleus has NO spin. 12C (0), 18O (0), 32S (0) Nucleus has a half-integer spin. 1H (½), 13C (½), 31P (½) NMR can detect Nucleus has an integer spin 2H (1), 14N (1) B5 o8

Splitting of nuclei spin states Gamma is gyromagnetic ratio constant α spin state ΔE = hγB0/2π

larmor frequency hυ=ΔE = hγB0/2π The magnetic moment of each nucleus precesses around B0. The frequency of this precession (larmor frequency ) is equivalent to the resonance frequency (υ) of the nucleus and the energy difference between the two levels (larmor frequency about 60–1000 MHz) Depends on the strength of the magnetic field and the magnetic properties of the isotope of the atoms hυ=ΔE = hγB0/2π

Type of NMR Spectroscopy Continuous wave NMR Spectroscopy Varying the frequency of radiation at constant magnetic field and measuring the absorption of radiation by the different nucle FT NMR Spectroscopy (better resolution and sensitivity) In FT-NMR all nuclei are excited at the same time by a radio frequency pulse. Radio frequency pulse : if the radiation is emitted as a very short pulse. A short radio frequency pulse contains many frequencies in a broad band around υ0 .

FT-NMR sequential steps Steps 1 : The alignment (polarization) of the magnetic nuclear spins in an applied, constant magnetic field H Gamma is gyromagnetic ratio constant Nβ/Nα = exp(ΔE/kT)

FT-NMR sequential steps Step 2 : The perturbation of the alignment by employing radio frequency (RF) pulse “90° pulse”. Transversal magnetization (dark blue) can now be created by applying an additional magnetic field B1 (red) which is perpendicular to B0. This B1 field is the radiofrequency pulse mentioned above. If the radiation frequency is equal to the larmor frequency of the nuclei the field causes a rotation of the equillibrium magnetization Mz (light blue) around the x axis (cross product, Beff=B1). You can completely transform the z magnetization to y magnetization if the duration of the pulse is sufficiently long. In this case the pulse is called an 'excitation pulse' or '90° pulse' (obviously because the rotation angle of the magnetization is 90°). dM/dt = gamma* [M × Beff] Beff = (B0 + (w0/ gamma)) + B1 = B1

FT-NMR sequential steps Step 3 : After 90° pulse, each spin precesses with its own larmor frequency around the z axis and induces a signal in the receiver coil. The signal decays due to relaxation and is therefore called free induction decay (FID).

FT-NMR sequential steps Step 3 : Fourier Transformation FT

Instrument

Instrument • An intense, homogeneous and stable magnetic field (magnet + shim) • A “probe” which enables the coils used to excite and detect the signal to be placed close to the sample • High-power RF transmitter/s capable of delivering short pulses (RF source + RF Amplifier) • A sensitive receiver to amplify the NMR signals (RF Detector) • A Digitizer to convert the NMR signals into a form which can be stored in computer memory • A “pulse programmer” to produce precisely timed pulses and delays • A computer to control everything and to process the data To optimize the sensitivity this coil needs to be (1) as close as possible to the sample; (2) tuned to resonant at the Larmor frequency of the nuclei being detected and (3) matched to maximize power transfer between the probe and the transmitter and receiver.

Instrument Superconducting Magnet

Instrument The NMR sample The sample should be dissolved in a solvent The NMR sample is prepared in a thin-walled glass tube - an NMR tube.

Sensitivity & Price $ USD 7 - 9.4 - 11.7 - 14 - 18.7 - 21 Tesla 300 - 400 - 500 - 600 - 800 - 900 MHz

A 900MHz NMR instrument with a 21 A 900MHz NMR instrument with a 21.1 T magnet at HWB-NMR, Birmingham, UK

Conclusion Can provide detailed information about the structure, dynamics, chemical environment and identical functional groups of molecules . Most frequently, NMR spectroscopy is used by chemists and biochemists to investigate the properties of organic molecules NMR spectrometers have expensive liquid helium-cooled superconducting magnet

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