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NMR
Nuclear Magnetic Resonance
1H, 13C, 15N, 19F, 31P
One Level is more populated than the other
Therefore a Resulting Net Magnetization will be Macroscopically Observable
CW Continous Wave
The Exact Resonance Frequency Varies with the Chemical and Electronical Environment of the Nucleus – This is Called the Chemical Shift (measured in ppm)
The Chemical Shift is Measured Relative to a Reference or ‘Standard’ ppm ‘parts per million’ =(-ref)/ref * 106
The most common standard for 1H and 13C NMR is
TMS Tetramethylsilan
CH3
CH3 – Si - CH3
CH3
NMR is ‘quantitative’
The intensity is measured as the integral below the resonance signal. The intensities are proportional to the number of protons, that contribute to the signal.
A – B
B – A – B
C – B – A
Each Particular Arrangement of Nuclei has a Characteristic
Coupling Pattern n Neighbors – (n+1) Lines
CH3CH2
No spin-spin coupling is observed if:
1. protons are separated by four or more single bonds, i.e.,
2. H-C-C-C-H
3. protons are equivalent, i.e.,within a CH3 or CH2 group
example
Pick the molecule that gives rise to the following 1H NMR spectrum!
Assign the Signals to the Correct Hydrogens
(almost) pure D Glucose
~50% - 50% D Glucose
FT NMR
‘Pulse Echo’ NMR
‘Broadband Excitation’Record ‘Transient Response’FID Free Induction Decay
Extract Frequencies by Fourier Transform to get NMR Spectrum
More Complex Experiments
Measurement of Relaxation Times (‘Lifetimes’)
Inversion Recovery
?
?
13C NMR Spectroscopy
Sensitivity low: only 1.1% natural abundance
Complicated 1H coupling pattern,
no couplings between 13Cs (due to low abundance)
S/N can be improved by 1H ‘broadband decoupling’ and FT averaging
3JHH coupling constants contain
Geometrical Information on Torsion Angles
‘Karplus Relation’
COSY Correlated SpectroscopY
Two Dimensional NMR SpectroscopyA Signal as a function of two times becomes after 2D FTSpectrum as a function of two frequencies Usually presented as ‘contour plot’
Cyclosporin A
HN H H CH3/ etc
‘complex’ spectra can be resolved using 2dimensional NMR methods