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1
Nuclear Magnetic
Nuclear Magnetic
Resonance
Resonance
Chapter 13
Molecular Spectroscopy
Molecular Spectroscopy
±
Nuclear magnetic resonance (NMR)
spectroscopy
:
a spectroscopic technique that
gives us information about the number and types
of atoms in a molecule, for example, about the
number and types of
•
hydrogen atoms using
1
HNMR spectroscopy
•
carbon atoms using
13
CNMR spectroscopy
•
phosphorus atoms using
31
PNMR spectroscopy
Nuclear Spin States
±
An electron has a spin quantum number of 1/2
with allowed values of +1/2 and 1/2
•
this spinning charge creates an associated magnetic
field
•
in effect, an electron behaves as if it is a tiny bar
magnet and has what is called a magnetic moment
Nuclear Spin States
Nuclear Spin States
±
The same effect holds for certain atomic nuclei
•
any atomic nucleus that has an odd mass number, an
odd atomic number, or both also has a spin and a
resulting nuclear magnetic moment
•
the allowed nuclear spin states are determined by the
spin quantum number,
I
, of the nucleus
Nuclear Spin States
•
a nucleus with spin quantum number
I
has
2
I
+ 1
+ 1 spin
states; if
I
= 1/2, there are two allowed spin states
•
The table below gives the spin quantum numbers and
allowed nuclear spin states for selected isotopes of
elements common to organic compounds
1
H
2
H
12
C
13
C
14
N
16
O
31
P
32
S
Element
Nuclear spin
quantum
number (
I
)
Number of
spin states
1/2
1
0
0
0
1/2
1
2312 31
1/2
21
Nuclear Spins in B
Nuclear Spins in B
0
•
within a collection of
1
H and
13
C atoms, nuclear spins
are completely random in orientation
•
when placed in a strong external magnetic field of
strength B
0
, however,
interaction between nuclear spins and the applied
magnetic field is quantized, with the result that only
certain orientations of nuclear magnetic moments are
allowed
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Nuclear Spins in B
Nuclear Spins in B
0
•
for
1
H and
13
C, only two orientations are allowed
Nuclear Spins in B
Nuclear Spins in B
0
±
In an applied field strength of 7.05T, which is
readily available with presentday
superconducting electromagnets, the difference
in energy between nuclear spin states for
•
1
H is approximately 0.120 J (0.0286 cal)/mol, which
corresponds to electromagnetic radiation of 300 MHz
(300,000,000 Hz)
•
13
C is approximately 0.030 J (0.00715 cal)/mol, which
corresponds to electromagnetic radiation of 75MHz
(75,000,000 Hz)
Nuclear Spin in B
0
•
the energy difference between allowed spin
states increases linearly with applied field
strength
Nuclear Spin in B
Nuclear Spin in B
0
•
values shown here are for
1
H nuclei
Nuclear Magnetic Resonance
•
when nuclei with a spin quantum number of
1/2 are placed in an applied field, a small
majority of nuclear spins are aligned with the
applied field in the lower energy state
Nuclear Magnetic Resonance
Nuclear Magnetic Resonance
•
the nucleus begins to precess and traces out a
coneshaped surface, in much the same way a
spinning top or gyroscope traces out a cone
shaped surface as it precesses in the earth’s
gravitational field
•
we express the rate of precession as a
frequency in hertz
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 Spring '09
 Valentin
 Atom, Mole

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