Introduction to NMR Spectroscopy: A Powerful Tool for Structural Analysis
N
N
UCLEAR
UCLEAR
M
M
AGNETIC
AGNETIC
R
R
ESONANCE
ESONANCE
spectroscopy
spectroscopy
NMR
NMR
spectroscopy
spectroscopy
Alkhair Adam Khalil,
B. Pharm., M. Pharm.
Department of Pharmaceutical Chemistry
College of Pharmacy – Karary University
•
We’ve seen up to this point that
IR
spectroscopy provides
information about a molecule’s
functional
groups
and
that UV spectroscopy provides information about a
molecule’s
conjugated
electron system.
•
Nuclear magnetic resonance(
NMR
) spectroscopy
complements these techniques by providing a “
map
” of
the
carbon–hydrogen framework
in an organic molecule.
•
Taken together, IR, UV, and NMR spectroscopies often
make it possible to find the structures of even very
complex molecules.
•
In this lecture we are going to know about a new spectroscopic
In this lecture we are going to know about a new spectroscopic
technique:
technique:
•
NMR Spectroscopy
•
The physics behind NMR:
–
Nuclear spin
–
Charges and
magnetic
field
–
Effect of
Radio
waves
on nuclear spin
–
Effect of electron’s
local
magnetic field
–
“
shielded
” and “
deshielded
” nuclei
Two common types
of NMR spectroscopy are used
to characterize organic structure:
•
1
H
N
M
R
(
p
r
o
t
o
n
N
M
R
)
i
s
u
s
e
d
t
o
d
e
t
e
r
m
i
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t
h
e
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m
b
e
r
a
n
d
t
y
p
e
o
f
h
y
d
r
o
g
e
n
a
t
o
m
s
i
n
a
m
o
l
e
c
u
l
e
;
a
n
d
•
1
3
C
N
M
R
(
c
a
r
b
o
n
N
M
R
)
i
s
u
s
e
d
t
o
d
e
t
e
r
m
i
n
e
t
h
e
t
y
p
e
o
f
c
a
r
b
o
n
a
t
o
m
s
i
n
a
m
o
l
e
c
u
l
e
.
•
Before you can learn how to use NMR
spectroscopy to determine the structure of a
compound,
•
you need to understand a bit about the physics
behind it. Keep in mind, though, that
NMR
stems
from the
same basic principle
as all other forms
of spectroscopy.
•
Energy interacts with a molecule, and
absorptions occur only when the incident
energy matches the energy difference between
two states.
Magnetic
Nuclear
Resonance
Basic principles of
Basic principles of
NMR-Spectroscopy
NMR-Spectroscopy
Nuclear spin
Nuclear spin
Elements (
isotopes
) with either
odd
mass
or
odd
atomic
number
have the property of
nuclear “
spin
”
•
Only
nuclei that contain
odd
mass numbers
(such as
1
H,
13
C,
19
F,
and
31
P
) or
odd
atomic
numbers (such as
2
H
and
14
N
) give rise to NMR
signals.
•
Because both
1
H
and
13
C
, are NMR active, NMR
allows us to map the carbon and hydrogen
framework of an organic molecule.
Nuclear spin
Nuclear spin
•
The spin quantum number (
I
) is related to the
atomic number and mass number of the nucleus.
•
Because of its
charge
and
spin
; a nucleus can
behave like a tiny
magnet
.
NMR active
Not
NMR active
•
When a charged particle such as a proton
spins on its axis, it creates a
magnetic
field
.
•
For the purpose of this discussion, therefore, a
nucleus is a tiny bar
magnet
, symbolized by
Normally these nuclear magnets are
randomly
oriented
in space,
•
But in the
presence
of an
external magnetic field
,
(symbolized by
B
0
)
, they are oriented
with
or
against
this applied field.
•
More
nuclei are oriented
with
the applied field
because this arrangement is
lower
in energy, but the
energy difference between these two states is very
small
(
< 0.4 J/mole
)
In a magnetic field, there are now two different
energy states for a proton:
•
A lower energy state
with the nucleus aligned in
the
same
direction
as
B
0
•
A higher energy state
with the nucleus aligned
opposed
to
B
0
•
When an
external
energy source (
hν
) that
matches
the energy difference (
∆E
) between
these two states is applied,
energy
is
absorbed
,
causing the nucleus to “
spin flip
” from one
orientation to another (Transition).
•
The source
of energy
in NMR is
radio waves
.
Radiation in the radiofrequency region of the
electromagnetic spectrum (so-called
RF radiation
)
has very long wavelengths, so its corresponding
frequency and energy are both
low
.
•
When these low-energy radio waves interact with a
molecule, they can change the nuclear spins of some
elements, including
1
H
and
13
C
.
Therefore
;
N
N
UCLEAR
UCLEAR
M
M
AGNETIC
AGNETIC
R
R
ESONANCE
ESONANCE
Thus, two variables characterize NMR:
1.
An applied magnetic field
measured in tesla (T).
2.
The frequency of radiation
used for resonance,
measured in Megahertz (MHz).
•
The frequency needed for resonance and the applied
magnetic field strength are proportionally related:
•
Early
NMR spectrometers used a magnetic field
strength of
~1.4 T
, which required RF radiation of
60 MHz
for resonance.
•
Modern
NMR spectrometers use
stronger
magnets
, thus requiring higher frequencies of RF
radiation for resonance.
•
For example, a magnetic field strength of
7.05 T
requires a frequency of
300 MHz
for a proton to
be in resonance.
•
If all protons absorbed at the same frequency
in a given magnetic field, the spectra of all
compounds would consist of a single
absorption, rendering NMR useless for
structure determination.
•
Fortunately, however, this is not the case.
Local field
Local field
•
The frequency at which a particular proton
absorbs is determined by its
electronic
environment.
•
Because
electrons
are moving charged particles,
they create a magnetic field (
local field
)
opposed
to the
applied
field
B
0
, and the size of the local
magnetic field generated by the electrons around
a nucleus determines
where
it absorbs.
•
Modern
NMR spectrometers use a constant
magnetic field strength
B
0
, and then a narrow
range of
frequencies
is applied to achieve the
resonance of all protons.
Effect of local field
Effect of local field
•
Nuclei
that are
not
in
identical
structural
situations
do not
experience the external
magnetic field to the same extent.
•
The nuclei are “
shielded
” or “
deshielded
” due
to small local fields generated by circulating
sigma
,
pi
and
lone
pair
electrons.
Effect of local field
Effect of local field
•
When the electrons circulate, they generate a
small magnetic field that happens to point in the
opposite
direction
to the
external
field Therefore,
the nucleus experiences a
reduced
external
magnetic field and resonate at
lower
frequency
.
•
This is typically known as “
shielding
”, e.g.
Hydrogens like those in
methane
are said to be
“
shielded
”.
Effect of local field
Effect of local field
•
One the other hand
hydrogens
near an
electronegative
atom should require a
higher
frequency
to flip (e.g.
CH
3
Br
) because
bromine
atom pulls away
electron
toward itself, this is
known as “
deshielding
”, and the hydrogens of the
methyl group are said to be “
deshielded
”
therefore they experience (sense)
more
of the
external
applied field making them to
resonate
at
higher
frequency.
Summary
Summary
•
A nucleus is in
resonance
when it absorbs
RF
frequency and the
spin
flips
into
higher
energy state.
•
Only
nuclei that contain
odd
mass numbers or
odd
atomic numbers
give rise to NMR signals (such as
1
H and
13
C)
•
Two variables characterize NMR:
1.
An applied magnetic field measured in tesla (T).
2.
The frequency of radiation used for resonance, measured in Megahertz
(MHz).
•
The frequency at which a particular proton absorbs is determined
by its
electronic environment (
i.e. local field
).
Questions !
1
1
H NMR Spectrum
H NMR Spectrum
•
To be continued next lecture.
Nuclear Magnetic Resonance (NMR) spectroscopy is a vital technique in determining the structure of organic molecules. It complements IR and UV spectroscopies by providing a detailed map of the carbon-hydrogen framework. Understanding the physics behind NMR, such as nuclear spin and the effect of magnetic fields, is crucial for its application. Proton NMR and carbon NMR are common types used for characterizing organic structures. By matching energy differences between states, NMR uncovers valuable information about molecular composition.
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Presentation Transcript
NUCLEAR MAGNETIC RESONANCE spectroscopy NMR spectroscopy Alkhair Adam Khalil, B. Pharm., M. Pharm. Department of Pharmaceutical Chemistry College of Pharmacy Karary University
Weve seen up to this point that IR spectroscopy provides information about a molecule s functional groups and that UV spectroscopy provides information about a molecule s conjugated electron system. Nuclear magnetic resonance(NMR) complements these techniques by providing a map of the carbon hydrogen framework in an organic molecule. Taken together, IR, UV, and NMR spectroscopies often make it possible to find the structures of even very complex molecules. spectroscopy
In this lecture we are going to know about a new spectroscopic technique: NMR Spectroscopy The physics behind NMR: Nuclear spin Charges and magnetic field Effect of Radio waves on nuclear spin Effect of electron s local magnetic field shielded and deshielded nuclei
Two common types of NMR spectroscopy are used to characterize organic structure: 1H NMR (proton NMR) is used to determine the number and type of hydrogen atoms in a molecule; and 13C NMR (carbon NMR) is used to determine the type of carbon atoms in a molecule.
Before you can learn how to use NMR spectroscopy to determine the structure of a compound, you need to understand a bit about the physics behind it. Keep in mind, though, that NMR stems from the same basic principle as all other forms of spectroscopy. Energy interacts with a molecule, and absorptions occur only when the incident energy matches the energy difference between two states.
Magnetic Resonance Nuclear In the Nucleus Involves Magnets In the Nucleus
Nuclear spin Elements (isotopes) with either odd mass or odd atomic number have the property of nuclear spin
Only nuclei that contain odd mass numbers (such as numbers (such as signals. Because both allows us to map the carbon and hydrogen framework of an organic molecule. 1H, 13C, 19F, and 2H and 31P) or odd atomic 14N) give rise to NMR 1H and 13C, are NMR active, NMR
Nuclear spin The spin quantum number (I) is related to the atomic number and mass number of the nucleus. NMR active Not NMR active Because of its charge and spin; a nucleus can behave like a tiny magnet.
When a charged particle such as a proton spins on its axis, it creates a magnetic field. For the purpose of this discussion, therefore, a nucleus is a tiny bar magnet, symbolized by Normally these nuclear magnets are randomly oriented in space,
But in the presence of an external magnetic field, (symbolized by B0), they are oriented with or against this applied field. More nuclei are oriented with the applied field because this arrangement is lower in energy, but the energy difference between these two states is very small (< 0.4 J/mole)
In a magnetic field, there are now two different energy states for a proton: A lower energy state with the nucleus aligned in the samedirection as B0 A higher energy state with the nucleus aligned opposed to B0 When an external energy source (h ) that matches the energy difference ( E) between these two states is applied, energy is absorbed, causing the nucleus to spin flip from one orientation to another (Transition).
The source of energyin NMR is radio waves. Radiation in the radiofrequency region of the electromagnetic spectrum (so-called RF radiation) has very long wavelengths, so its corresponding frequency and energy are both low. When these low-energy radio waves interact with a molecule, they can change the nuclear spins of some elements, including 1H and 13C.
Therefore; NUCLEAR MAGNETIC RESONANCE
Thus, two variables characterize NMR: 1. An applied magnetic field measured in tesla (T). 2. The frequency of radiation used for resonance, measured in Megahertz (MHz). The frequency needed for resonance and the applied magnetic field strength are proportionally related:
Early NMR spectrometers used a magnetic field strength of ~1.4 T, which required RF radiation of 60 MHz for resonance. Modern NMR spectrometers use stronger magnets, thus requiring higher frequencies of RF radiation for resonance. For example, a magnetic field strength of 7.05 T requires a frequency of 300 MHz for a proton to be in resonance.
If all protons absorbed at the same frequency in a given magnetic field, the spectra of all compounds would consist of a single absorption, rendering NMR useless for structure determination. Fortunately, however, this is not the case.
Local field The frequency at which a particular proton absorbs is determined by its electronic environment. Because electrons are moving charged particles, they create a magnetic field (local field) opposed to the applied field B0, and the size of the local magnetic field generated by the electrons around a nucleus determines where it absorbs.
Modern NMR spectrometers use a constant magnetic field strength B0, and then a narrow range of frequencies is applied to achieve the resonance of all protons.
Effect of local field Nuclei that are not in identical structural situations do not experience the external magnetic field to the same extent. The nuclei are shielded or deshielded due to small local fields generated by circulating sigma, pi and lone pair electrons.
Effect of local field When the electrons circulate, they generate a small magnetic field that happens to point in the opposite direction to the external field Therefore, the nucleus experiences a reduced external magnetic field and resonate at lower frequency. This is typically known as shielding , e.g. Hydrogens like those in methane are said to be shielded .
Effect of local field One the other hand hydrogens near an electronegative atom should require a higher frequency to flip (e.g. CH3Br) because bromine atom pulls away electron toward itself, this is known as deshielding , and the hydrogens of the methyl group are said to be deshielded therefore they experience (sense) more of the external applied field making them to resonate at higher frequency.
Summary A nucleus is in resonance when it absorbs RF frequency and the spin flips into higher energy state. Only nuclei that contain odd mass numbers or odd atomic numbers give rise to NMR signals (such as 1H and 13C) Two variables characterize NMR: 1. An applied magnetic field measured in tesla (T). 2. The frequency of radiation used for resonance, measured in Megahertz (MHz). The frequency at which a particular proton absorbs is determined by its electronic environment (i.e. local field).
1H NMR Spectrum To be continued next lecture.
