Nuclear Magnetic Resonance
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Yale Chemistry 800 MHz
Supercooled Magnet
Bo
Atomic nuclei in
The absence of
a magnetic field
Atomic nuclei in the presence
of a magnetic field
 spin - with the field
 spin - opposed to the field
The Precessing Nucleus
resonance
 spin
 spin
Ho
resonance
no resonance
Rf
no resonance
The Precessing Nucleus Again
The Continuous Wave Spectrometer
The Fourier Transform Spectrometer
Observable Nuclei
•Odd At. Wt.; s = ±1/2
1H
13C
Nuclei
1
6
Abundance (%) 99.98 1.1
•Odd At. No.; s = ±1
Nuclei
2H
1
14N
7
…
•Unobserved Nuclei
12C
6
16O
8
32S …
16
15N
7
19F
9
0.385 100
31P
15
100
….
h = Planck’s constant:1.58x10-37kcal-sec
•E = h
•E = h/2(Bo)
•Thus:
= Gyromagnetic ratio: sensitivity
of nucleus to the magnetic field.
1H = 2.67x104 rad sec-1 gauss-1
= /2(Bo)
•For a proton,
if Bo = 14,092 gauss (1.41 tesla, 1.41 T),
= 60x106 cycles/sec = 60 MHz
and
•EN = Nh= 0.006 cal/mole
Rf Field vs. Magnetic Field for a Proton
E
60 MHz
100 MHz
1.41 T
2.35 T
E = h/2(Bo)
500 MHz
11.74 T
Bo
Rf Field /Magnetic Field for Some Nuclei
Nuclei
Rf (MHz)
Bo (T)
/2 (MHz/T)
500.00
11.74
13C
125.74
11.74
10.71
2H
76.78
11.74
6.54
19F
470.54
11.74
40.08
31P
202.51
11.74
17.25
1H
42.58
Fortunately, all protons are not created equally!
at 1.41 T
60 MHz
60,000,000 Hz
60,000,600 Hz
upfield
downfield
240 Hz
}
deshielded
10
9
8
7
6
5
Me4Si
shielded
60 Hz
4
3
2
1
0  scale (ppm)
at 500 MHz
500 Hz
 = (obs - TMS)/inst(MHz) = (240.00 - 0)/60 = 4.00
chemical
shift
Where Nuclei Resonate at 11.74T
12 ppm
2H
RCO2H
12 ppm
1H (TMS)
~380 ppm
31P
76.78
(H3PO4)
~220 ppm
220 ppm
19F
13C
PX3
CPH2
100
125.74
CH4
500.00
-SO2F
-CF2-CF2470.54
202.51
R2C=O
(CFCl3)
RCH3
200
300
400
500 MHz
Chemical Shifts of Protons
The Effect of Electronegativity on Proton Chemical Shifts
TMS
10
10
9
9
8
8
7
7
CHCl3
CH2Cl2
CH3Cl
CH4
=7.26
=5.30
=3.05
=-0.20
6
5
4
3
2
CHBr3
CH2Br2
CH3Br
=6.83
=4.95
=2.68
6
5
4
3
2
1
1
0 
TMS
0

Chemical Shifts and Integrals
 2.2
TMS
area = 3
 4.0
area = 2
10
9
8
homotopic
protons: chemically
and magnetically
equivalent
7
6
5
4
Cl
H3C C CH2Cl
Cl
3
2
1
0

enantiotopic protons:
chemically and
magnetically equivalent
Spin-Spin Splitting
 = 1.2
area = 9
“anticipated” spectrum
 = 6.4
area = 1
10
9
8
7
TMS
 = 4.4
area = 1
6
5
4
3
Br Br CH3
Br C C C CH3
H H CH3
2
1
0

Spin-Spin Splitting
 = 1.2
area = 9
observed spectrum
 = 6.4
area = 1
 = 4.4
area = 1
H
H
H
10
9
8
7
H
6
5
4
3
Br Br CH3
Br C C C CH3
Jcoupling constant = 1.5 Hz
TMS
H H CH3
2
1
0
J is a constant
and independent
of field

Spin-Spin Splitting
 = 6.4
 = 4.4
J = 1.5 Hz
J = 1.5 Hz
H
H
H
local
H
local
observed
observed
8
7
6
applied
5
3

applied
This spectrum is not recorded at ~6 MHz!
 and J are not to scale.
4
Br Br CH3
Br C C C CH3
H H CH3
Multiplicity of Spin-Spin Splitting for s = ±1/2
multiplicity (m) = 2s + 1
# equiv.
neighbors
spin (1/2)
multiplicity
pattern
(a + b)n
symbol
0
0
1
1
singlet (s)
1
1/2
2
1:1
doublet (d)
2
2/2 = 1
3
1:2:1
triplet (t)
3
3/2
4
1:3:3:1
quartet (q)
4
4/2 = 1
5
1:4:6:4:1
quintet (qt)
1H
NMR of Ethyl Bromide (90 MHz)
 = 1.68
area = 3
CH3CH2Br
for H spins


 = 3.43
area = 2

a
90 Hz/46and
pixels
TIFFQuickTime™
(LZW) decompressor
are needed
see this
picture.
= to3J/12
pixels
: J= 7.83 Hz
for H spins


 QuickTime™ and a
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1H
NMR of Isopropanol (90 MHz)
(CH3)2CHOH
1H septuplet; no coupling to OH
6H, d, J = 7.5 Hz
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4.25
3.80
1H, s
Proton Exchange of Isopropanol (90 MHz)
(CH3)2CHOH + D2O
(CH3)2CHOD
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Diastereotopic Protons: 2-Bromobutane
homotopic sets
of protons
Br
H
pro-R
H
H
H
H
H
H
H
R
Diastereotopic protons
H
pro-S
Diastereotopic Protons: 2-Bromobutane at 90MHz
Br
H
H
H
H
H
H
H
H
1.03 (3H, t)
QuickTime™ and a
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1.70 (3H, d)
1.82 (1H, m)
1.84 (1H, m)
4.09 (1H, m (sextet?))
1H
NMR of Propionaldehyde: 300 MHz
H
H
O
H
1.13 (3H, t, J = 7.3 Hz)
H
Is this pattern at 2.46
a pentuplet given that
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thereand
area two different
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9.79 (1H, t, J = 1.4 Hz)are needed to see
values
for J?
this picture.
H
H
No!
1H
NMR of Propionaldehyde: 300 MHz
2.46
H
H
O
H
7.3 Hz
7.3 Hz
7.3 Hz
1.4 Hz
H
7.3 Hz
H
H
7.3 Hz
QuickTime™ and a
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9.79 (1H, t, J = 1.4 Hz)
1.13 (3H, t, J = 7.3 Hz)
quartet of doublets
1H
NMR of Ethyl Vinyl Ether: 300 MHz
H
J
H
3H, t, J = 7.0 Hz
= 14.4 Hz
J
= 1.9 Hz
J
= 6.9 Hz
O
H
H
H
H
H
H
3.96 (1H, dd, J =
QuickTime™ and a
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6.9,
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4.17 (1H, dd, J = 14.4, 1.9 Hz)
6.46 (1H, dd, J = 14.4, 6.9 Hz)
2H, q, J = 7.0 Hz
6.46
ABX Coupling in Ethyl Vinyl Ether
14.4
6.9
H
H
H
O
H
4.17
H
H
H
H
1.9
14.4
3.96 (1H, dd, J = 6.9, 1.9 Hz)
3.96
4.17 (1H, dd, J = 14.4, 1.9 Hz)
6.46 (1H, dd, J = 14.4, 6.9 Hz)
Another example
6.9
1.9
Dependence of J on the Dihedral Angle
The Karplus Equation
1H
NMR (400 MHz): cis-4-t-Butylcyclohexanol
OH
He
He
He
Ha
triplet of triplets
Ha
He
3.0
3.0
2.7
2.7
QuickTime™ and a
He He4.03,
4.03,
J(H
J(H
)=
3.0
3.0
HzHz
a) a=
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picture.
J(H
J(H
) see
=
2.7
2.7
HzHz
e) e=
11.4 Hz
400 Hz
1H
NMR (400 MHz): trans-4-t-Butylcyclohexanol
Ha
He
OH
He
triplet of triplets
Ha
Ha
Ha 3.51, J(HQuickTime™
and a
a) = 11.1 Hz
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J(He) to=see
4.3this
Hzpicture.
are needed
Ha
11.1
11.1
4.3
4.3
30.8 Hz
X
Y
W
Peak Shape as a Function of  vs. J
Z
H
H
10
9
J

 >> J
8
7
6
5
4
3
2
1
0

8
7
6
5
4
3
2
1
0

8
7
6
5
4
3
2
1
0

 ~= J
10
9
 = 0
10
9
Magnetic Anisotropy
H
H
H
H
H
H
Downfield shift for aromatic protons ( 6.0-8.5)
Bo
Magnetic Anisotropy
R
C
C
H
Upfield shift for alkyne protons ( 1.7-3.1)
Bo
13C
Nuclear Magnetic Resonance
13C
Chemical Shifts
Where’s Waldo?
One carbon in 3 molecules of squalene is 13C
What are the odds that two 13C are bonded to one another?
~10,000 to 1
13C
NMR Spectrum of Ethyl Bromide at 62.8 MHz
JCH = 151 Hz
H
H
JCH = 3 Hz
JCH = 126 Hz
C1
H
Br
C2
H
H
JCH = 5 Hz
H
TMS
JCH = 118 Hz
H
H
H
H
H
Si
H
H
H
H
H
H
30
20
26.6
18.3
10
ppm ()
0
13C
NMR Spectrum of Ethyl Bromide at 62.8 MHz
JCH = 151 Hz
Off resonance decoupling
of the 1H region removes
small C-H coupling.
H
JCH = 126 Hz
C1
H
JCH = 3 Hz
H
Broadband decoupling
removes all C-H coupling.
Br
C2
H
H
JCH = 5 Hz
H
TMS
JCH = 118 Hz
H
H
H
H
H
Si
H
H
H
H
H
H
30
20
26.6
18.3
10
ppm ()
0
13C
Spectrum of Methyl Salicylate
(Broadband Decoupled)
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13C
Spectrum of Methyl p-Hydroxybenzoate
(Broadband Decoupled)
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The 13C Spectrum of Camphor
H
H
H
H
H
H
H
H
H
H
H
H
H
O
H
H
H
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The End
F. E. Ziegler, 2004