NMR

1
NMR
2
Obtaining a 13C NMR Spectrum

• 1H Broadband decoupling
• Gives singlet 13C peaks, provided no F, P, or D
  present in the molecule)
• Continuous sequence of pulses at the 1H frequency
  causes a rapid reversal of spin orientation
  relative to the B0, causing coupling to 13C to
  disappear

3
Broadband Decoupling
1H channel
13C channel
4
H3C4-C3HC2H-C1OOH
solvent
C-4
C-2
C-3
C-1
10
180
5
13C Chemical Shifts

• Reference is TMS, sets 0 ppm
• A range of 200 ppm
• Chemical shifts can be predicted
• Empirical correlations
• Ex. Alkanes

di -2.3 9.1na 9.4nb 2.5ng 0.3nd 0.1ne
Sij
2-methylbutane
di -2.3 9.11 9.42 2.51 - 1.1 22.0
(22.3)
6
Signal averaging

• 13C experiment generally take longer than 1H
  experiments because many more FIDs need to be
  acquired and averaged to obtain adequate
  sensitivity.
• NOE effect (enhancement/reduction in signal as a
  result of decoupling)

N4
N4
13C
1H
W2
1H
13C
N2
N2
N3
N3
W1
13C
1H
1H
13C
N1
N1
7
NOE effect

• W2 (Enhancement) dominates in small molecules
• Relevant for all decoupling experiments

8
Other more complex 1D Experiments

• 1H NOE experiment
• Inversion Recovery Experiment Determination of
  T1
• J modulated Spin Echo
• INEPT Experiment
• DEPT Experiment

9
Targeted 1H Spin Decoupling

• Continuous irradiation at a frequency (n2) that
  corresponds to a specific proton in the molecule
  during the 1H NMR experiment
• All coupling associated with the protons
  corresponding to n2 disappears from the spectrum

10
1H targeted decoupling (NOE)
n2 channel
1H channel
11
1
3
2
TMS
n2
12
Inversion Recovery