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