Karteikarten
basic NMR
Karten in diesem Stapel (10)
- 1When placed in an external magnetic field, what are the two stable orientations of the magnetic dipole moment of an NMR active nucleus?- Pointing in the same direction as the external magnetic field (parallel)
- Pointing in the opposite direction to the external magnetic field (anti-parallel) - 2What’s the energetic arrangement of the two stable arrangements of the magnetic dipole moment of an NMR active nucleus when it’s placed in an external magnetic field?The parallel (α) state is lower than the antiparallel state (β).
- 3What’s the equation for the energy gap between the parallel and antiparallel states?∆E = hν = h(γB0/2π)
- 4What’s the size of the energy gap between the parallel and antiparallel states and what does this mean for the EM radiation?Very small energy gap = the EM radiation is in the low frequency, radio wave region of the EM spectrum – typically 50-900 MHz (depending on nucleus).
- 5What happens when a photon of EM radiation is absorbed by a nucleus?The orientation of the nucleus switches from parallel to antiparallel by resonance excitation of the Larmor precession frequency, and an NMR signal is detected.
- 6What effect does electron density have on the frequency at which nuclei absorb?Electrons partially shield the nuclei from the external magnetic field, which reduces the absorption frequency. HENCE, the precise frequency at which nuclei absorb depends on the electron denisty:
Higher electron denisty = lower frequency
Lower electron denisty = higher frequency - 7What is the equation for converting absorption frequency to chemical shift?Ꟙ = (νsample – νref)/νres
νsample – νref is in Hz and νres is in MHz, giving 1/106 or ppm as the units. - 8What is the chemical shift value of tetramethylsilane (TMS)?0 ppm
- 9Describe the basic features of an NMR spectrometer:- A superconducting magnet is used to provide a strong, homogeneous magnetic field – an Nb alloy coil immersed in liquid He.
- The thin NMR tube containing the sample dissolved in deuterated solvent is lowered into the bore of the magnet.
- Radio frequency is applied as a broadband pulse aka ‘chirp’ which excites all the 1H or 13C environments at once (depending on frequency range).
- The pulse is switched off and the residual signal is recorded over time to produce a free induction decay (FID).
- Fourier transform is applied to the signal which converts it from the time to the frequency domain and pulls out all the individual absorption frequencies within the composite signal – generating the NMR spectrum.
- To complete the process, the frequency is converted to chemical shift scaled by reference to TMS. - 10draw a mass spectrometer
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- Chemistry (sas) Karteikarten am basic NMR, erstellt von Casey Champion am 23/04/2023.
- Erstellt von
- Casey Champion · Apr 2023
- Aktualisiert
- 23 Apr 2023
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