13.14 13C NMR Spectroscopy

1
13.1413C NMR Spectroscopy
2
1H and 13C NMR compared

• both give us information about the number of
  chemically nonequivalent nuclei (nonequivalent
  hydrogens or nonequivalent carbons)
• both give us information about the environment
  of the nuclei (hybridization state, attached
  atoms, etc.)

3
1H and 13C NMR compared

• the signal for the NMR of a 13 C nucleus is
  10-4 times weaker than the signal for a hydrogen
  nucleus
• a signal for a 13C nucleus is only about 1 as
  intense as that for 1H because of the magnetic
  properties of the nuclei, and
• at the "natural abundance" level only 1.1 of
  all the C atoms in a sample are 13C (most are 12C)

4
1H and 13C NMR compared

• 13C signals are spread over a much wider range
  than 1H signals making it easier to identify and
  count individual nuclei
• Check the spectra on the next slides Figure
  13.20 (a) shows the 1H NMR spectrum of
  1-chloropentane Figure 13.20 (b) shows the 13C
  spectrum. It is much easier to identify the
  compound as 1-chloropentane by its 13C spectrum
  than by its 1H spectrum.

5
Figure 13.20(a) (page 511)
1H
CH3
ClCH2
ClCH2CH2CH2CH2CH3
Chemical shift (d, ppm)
6
Figure 13.20(b) (page 511)Note in these spectra
the peak intensities are not exactly proportional
to the number of carbon atoms.
13C
ClCH2CH2CH2CH2CH3

• a separate, distinct peak appears for each of
  the 5 carbons

CDCl3
Chemical shift (d, ppm)
7
13.1513C Chemical Shifts

• are measured in ppm (d)from the carbons of TMS

8
13C Chemical shifts are most affected by

• hybridization state of carbon
• electronegativity of groups attached to carbon

9
Examples (chemical shifts in ppm from TMS)
23
138

• sp3 hybridized carbon is more shielded than sp2

10
Examples (chemical shifts in ppm from TMS)
61
202

• sp3 hybridized carbon is more shielded than sp2

11
Examples (chemical shifts in ppm from TMS)
23
61

• an electronegative atom deshields the carbon to
  which it is attached

12
Examples (chemical shifts in ppm from TMS)
138
202

• an electronegative atom deshields the carbon to
  which it is attached

13
Table 13.3 (p 513)
Type of carbon
Chemical shift (d),ppm
RCH3
0-35
R2CH2
15-40
R3CH
25-50
R4C
30-40
14
Table 13.3 (p 513)
Type of carbon
Chemical shift (d),ppm
Type of carbon
Chemical shift (d),ppm
RCH3
0-35
65-90
R2CH2
15-40
100-150
R3CH
25-50
110-175
R4C
30-40
15
Table 13.3 (p 513)
Type of carbon
Chemical shift (d),ppm
RCH2Br
20-40
RCH2Cl
25-50
35-50
RCH2NH2
50-65
RCH2OH
RCH2OR
50-65
16
Table 13.3 (p 513)
Type of carbon
Chemical shift (d),ppm
Type of carbon
Chemical shift (d),ppm
RCH2Br
20-40
RCOR
160-185
RCH2Cl
25-50
35-50
RCH2NH2
50-65
RCH2OH
RCR
190-220
RCH2OR
50-65