College aantekeningen
Lectures Analytical NMR Spectroscopy
- Instelling
- Universiteit Van Amsterdam (UvA)
- Boek
- NMR Spectroscopy
This document contains all the lectures that were given for the course in 2022/2023.
[Meer zien]Voorbeeld 4 van de 58 pagina's
Voorbeeld 4 van de 58 pagina's
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Voorbeeld van de inhoud
Lecture 1Tuesday February 7
Information in a NMR spectrum
Energy Separation
- Difference between those states is ΔE of hv → tiny gap
- Resonance → photon goes from α to β (as long is RF pulse is there)
- You’ll need a bigger resonance for larger magnets
ΔE and Magnet Strength
- Difference between the α and β state of the nuclei
- Energy difference is proportional to the magnetic field strength
- In a 14,092-gauss field, a 60 MHz photon is required to flip a proton
- Low energy, radio frequency
Sensitivity of NMR signal
- 16 nuclei will go from α to β (1 million) (these are access nuclei)
- With 400 MHz magnetic field →64 nuclei will go from the α to the β state → more sensitive
- Stronger magnetic fields increase the population ratio and the sensitivity
,Increase sensitivity of NMR signal
- Enhancing the signal-to-noise ratio in the lower energy state relative to the upper energy state
- In addition to increasing magnetic field strength, this can be achieved by increasing the number of
nuclei in the sample (e.g., raising the concentration or increasing the volume of the sample detected)
Magnetic shielding
- Electrons shield proton from the external field → circulation electrons
- More nuclei are aligned with the field, then against the field → M0 will be up
- Circulating electrons create an induced magnetic field that opposes the external magnetic field
Resonance phenomenon
- µ is aligned with the field and is rotating around the z-axis
- Resonance phenomenon → B0 is original
- Left hand rule!!
- There is a shift in the magnetic moment whenever B1 matches the Larmor frequency
Overview of NMR experiment
- RF-pulse (B1) → will shut the magnetization
- Experimental details will affect the NMR spectra and the corresponding interpretation
- B0 →no MF → α aligned with the field
- B1 → radio-frequency
- FID → magnitude on the magnetic moment on x and y orientation →exponential decay
Summary
- External magnetic field causes loss of nuclear spin degeneracy and allows radio pulse to excite
nuclei which results in a signal
- The NMR experiment allows us to record the resonance signals of magnetic nuclei
,Lecture 2
Friday February 10
Hypothetical spectrum
- Frequency is different (Larmor equation)
- NMR spectroscopy does not detect magnetically different nuclei in a compound
Chemical shift
- Low frequency/upfield is on the right (0)
- High frequency/downfield is on the left
- The chemical shift is measured in hertz and the frequency scale increases from right to left
Why are there different chemical shifts?
This is due to the electrons of the C-H bond in which the proton is involved.
Shielding of nuclei
- Magnetic moment in the nucleus goes against the field (B0)
- B0 induces circulations in the electron cloud surrounding the nucleus → a magnetic moment µ
(opposed to B0) is produced (following Lenz’s law)
- The local field at the nucleus is smaller than the applied field
- The more electron density, the more shielding
- You can use this for proton NMR
- The shielding constant, sigma, is proportional to the electron density of the 1s orbital of the
hydrogen atom and sigma B0 in the magnitude of the secondary field induced at the proton
- Electron currents around a nucleus are induced by placing the molecule in a magnetic field B0
- These electron currents, in turn, induce much smaller magnetic field opposed to the applied
magnetic field
, - Effects at nucleus A caused by the secondary magnetic field arising from induced electronic currents
at nucleus B
- Additional electronic currents induced in the molecule than just those directly around the nucleus
● Some of those currents increase Blocal → deshielding effect
Chemical shift measurements
- Use a standard (TMS = o ppm or CDCl3 = 7.26 ppm)
- The absorbed frequencies are in Hz and the spectrometer frequency is in MHz → millionfold
difference in frequencies 🡺 parts per million
Sigma scale of 1H Chemical Shifts
Spin-spin coupling
- Difference in the position of the resonance signals AND also a difference in the multiplicity of the
signals
- Fine structure is caused by spin-spin coupling
- Methyl group more upfield
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