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Chapter 13
NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY
Chapter 13: Nuclear Magnetic Resonance Spectroscopy
N
S+
bar magnet loop of current spinning proton
H H H
current
H0
S
N
H0
S
N
NS H0
S
N
N
S
lower energy,MORE STABLE!
higher energy,LESS STABLE!
N
S
SOME BASICS OF ELECTROMAGNETIC INTERACTIONS
Chapter 13: Nuclear Magnetic Resonance Spectroscopy
H0
lower energy,MORE STABLE!
higher energy,LESS STABLE!
H0
-spin state -spin state
H-NUCLEI (PROTONS) IN AN EXTERNAL MAGNETIC FIELD
H0
placed in an external field
NO FIELD MAGNETIC FIELD
E
lower energy
higher energy
Chapter 13: Nuclear Magnetic Resonance Spectroscopy
MAGNETIC SHIELDING
H0
S
N
induced current
Hinduced
H0
Hinduced
induced curculation of electrons
H0 = 14,092 gauss, proton absorbs
Hinduced = 0.3 gauss
effective field is 14,091.7 gauss
H0 = 14,092 gauss, shielded proton does not absorb
Hinduced = 0.3 gauss
H0 = 14,092.3 gauss, shielded proton absorbs
60 MHz60 MHz 60 MHzH0 H0
Chapter 13: Nuclear Magnetic Resonance Spectroscopy
SIGNALS OF METHANOL
0100200300400500600
290 Hz
205 Hz
H C
H
H
O
H
more shielded(upfield)
less shielded (deshielded) (downfield)
UPFIELDDOWNFIELD
Fig. 13.5
0100200300400500600
290 Hz
205 Hz
050010001500200025003000
1440 Hz
1020 Hz
H C
H
H
O
H
60 MHz Spectrum
300 MHz Spectrum
SAME SPECTRUM WITH CHEMICAL SHIFTS IN PPM
0123456
HC
H
H
O
H
3.4 ppm
4.8 ppm
00.511.522.533.544.55
Compound Electronegativity
CH3I
CH3Br
CH3Cl
CH3F
2.7
3.0
3.2
4.0
012345678
H C
H
H
Cl
H C
H
Cl
Cl
Cl C
H
Cl
Cl
H C
H
H
H
0.2 ppm
3.0 ppm
5.3 ppm
7.3 ppm
NUMBER OF ELECTRONEGATIVE ATOMS (GROUPS)
THE EFFECT OF DISTANCE
00.511.522.533.544.55
CH3CH2CH2CH2Br
CH3
CH2
CH2
CH2
= 0.9 ppm = 3.4 ppm
= 1.7 ppm = 1.3 ppm
00.511.522.533.544.55
00.511.522.533.544.55
CH2CH2CH2CH2Br
300 MHz
60 MHz