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

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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