Massachusetts Institute of Technology
Organic Chemistry 5.13
5
September 15, 2006
Prof. Timothy F. Jamison
Notes for Lecture #5
Nuclear Magnetic Resonance (NMR) Spectroscopy
Isotope
1
H
12
C
13
C
Natural Abundance
99.98%
98.9%
1.1%
Spin
1/2
0
1/2
Isotope
14
N
16
O
31
P
Natural Abundance
99.6%
99.8%
>99.9%
Spin
1
0
1/2
13
C NMR is useful for:
1. Determining the number of chemically non-equivalent (or “different types of”) carbon
atoms in a molecule, based on the number of peaks in the spectrum. (See
substitution test, below.)
2. Identifying the types of functional groups in a molecule based on the chemical shift
of each peak. In contrast to IR spectroscopy, the number of each type of functional
group (e.g. 2 chemically non-equivalent ketones) often can be determined.
1
H NMR is useful for:
1. Determining the number of chemically non-equivalent (or “different types of”)
hydrogen atoms in a molecule, based on the number of peaks in the spectrum. Note:
“number of peaks” in this case does not include the splitting pattern (see below), e.g. a
triplet is considered to be one “peak”.
2. Determining the relative number of chemically non-equivalent hydrogen atoms by
measuring the relative area of each peak (by integration of each curve – not by
measuring the relative peak heights).
3. Identifying neighboring functional groups based on the chemical shift of each peak,
which is a measure of the chemical environment of each proton in the molecule.
4. Determining which carbon atoms are connected to which based on the splitting
pattern or multiplicity of each peak. This information is the most useful of all the
methods we have discussed in determining the connectivity of the molecule, i.e.
assembling all of the functional groups and fragments identified into an actual structure.
The substitution test is used to determine whether two atoms or
groups are chemically non-equivalent:
1. Replace each atom or group in turn with “X”.
2. If these two structures are identical (can be superimposed) or are enantiomers, then
the two atoms or groups are chemically equivalent (homotopic and enantiotopic,
respectively) and thus are indistinguishable by NMR spectroscopy.
3. If the two structures are different (e.g. diastereomers – making the two groups
diastereotopic, alkene isomers, structural isomers), then the two atoms or groups are
chemically non-equivalent and may be distinguishable by NMR spectroscopy.
5-2
Regions of the 13C NMR Spectrum
Aromatic
120-160 ppm
Carbonyls
150-220 ppm
200
180
C-O, N
40-90 ppm
Alkenyl
100-150 ppm
160
140
120
Alkynyl
70-90 ppm
100
80
Alkyl
0-50 ppm
60
40
20
0
Chemical Shift (δ)
Figure by MIT OCW.
Characteristic Functional Group Chemical Shifts in 13C NMR (ppm)
Alkanes
Methyl (RCH3)
Methylene (RCH2R’)
Methine (RCH(R’)(R”))
Quaternary (RC(R’)(R”)(R’’’))
Alkenes
Aromatic
Alkynes
Nitriles
Alcohols, Ethers
Amines
0-30
15-55
25-55
30-40
100-150
120-160
70-90
110-125
50-90
40-60
Organohalogen
C–F
C–Cl
C–Br
C–I
Ketones, Aldehydes
Carboxyl Derivatives
Acids
Esters
Amides
Carbamates
70-80
25-50
10-40
–20-10
185-220
150-185
155-180
150-180
150-160
The Physical Basis of the NMR Phenomenon
direction of
magnetic field
E
∆E = hv = hc/λ
Strength of applied magnetic field H0
H0
v = γH/2π
γ. gyromagnetic ratio
(a constant characteristic
of each type of nucleus)
H: magnetic field strength
For a proton (γ = 26,753 radians sec-1 gauss-1)
using a spectrometer with 70,460 gauss magnet
"spin flip" will occur on irradiation at
ν = 3 x 108 Hz and λ = 100 cm
(∆Ε = 9.5 X 10-6 kcal/mol)
Figure by MIT OCW.
5-3
Images of
13
C NMR Spectra for 2-pentanone and 3-pentanone removed due to copyright restrictions.
Please see: http://www.aist.go.jp/RIODB/SDBS/cgi-bin/direct_frame_top.cgi?lang=eng
5-4
Remarkable Sensitivity of 13C NMR to Chemical Structure
c
H3C
a
b
a
b
c
CH3
35
30
25
20
PPM
15
10
5
0
e
H3C
b
a
d
e f
207.1
o
c
o
CH3
f
a
206.0
b
220 200 180 160 140 120 100
80
c
d
60
40
20
0
PPM
Figure by MIT OCW.
5-5
Remarkable Sensitivity of
a
H3C
C NMR to Chemical Structure (Continued)
13
124.5
a
CH3
16.7
b
b
120
100
80
60
40
20
0
PPM
123.3
a
CH3 b
CH3
10.7
b
a
120
100
80
60
40
20
0
PPM
e
a CH3
d
c
b
b c
a
140
e
d
120
100
80
60
PPM
40
20
0
Figure by MIT OCW.