Nuclear Magnetic Resonance Spectrometry

1
Nuclear Magnetic Resonance Spectrometry
2
Identification of Compound
O
H3C
CH3
H
H
H2
CH3
H
H2
H
CH3
H
3
Spectral Properties, Application and Interactions
of Electromagnetic Radiation
Type Quantum Transition
Type spectroscopy
Type Radiation
Frequency ?
Wavelength ?
Wave Number V
Energy
Hz
cm
cm-1
Kcal/mol
Electron volts, eV
Gamma ray
Gamma ray emission
Nuclear
X-ray absorption, emission
Electronic (inner shell)
X-ray
Ultra violet
Electronic (outer shell)
UV absorption
Visible
Infrared
IR absorption
Molecular vibration
Molecular rotation
Microwave absorption
Micro-wave
Magnetically induced spin states
Nuclear magnetic resonance
Radio
4
Number of signals Position of signals Intensity
of signals Splitting of signals
4
3
14
2
2
4
1
2
2
2
2
4
5
Nuclear Spins in Absence (a) and Presence (b) of
External Magnetic Field
CH3-CHCH-CH2-CHO
6
Methyl Ester of Fatty Acid
O
C
H
C
C
H
O
R
C
H
C
H
C
H
C
H
C
H
3




2
2



7
Nuclear Magnetic Resonance Principle
Precession orbit of nuclear
mass (Precession angular velocity or precession
frequency W0)
W0 2p v
Spinning proton
Nuclear magnetic dipole moment ? ghI
Ho
Spinning charge in proton generates magnetic
dipole moment
Ho
Proton precess in a magnetic field Ho
8
Principles of NMR
2p radian/sec 1 Hz, 1 sec 3 1010 cm
High energy precession
Precessional orbit
Axis of nuclear rotation

Low energy spin state (1/2)
Nuclear Spin (Dipole moment)
Low energy precession
Nuclear Spin (Dipole moment)
Reference axis
Energy Difference
Oscillator Coil (Radio Frequency)
Ho
Precessional orbit
High energy spin state (-1/2)
CH3-CHCH-CH3
Ho
CH4
Precession -Energy Relationship
Oscillator generates rotating component of
magnetic field
9
Magnetic Properties of Nuclei

• Nuclei of certain atoms posses a mechanical spin
  or angular momentum. The angular momentum
  depends on the nuclear spin or spin number. The
  spin number (I) is related to the mass number and
  atomic number.
• Magnetic nucleus may assume any of (2I1)
  orientation with respect to the direction of the
  applied magnetic field

10
Theory of Nuclear Resonance

• A proton in a external magnetic field assumes
  only two orientations corresponding to /- uH.
  It is possible to induce transitions between two
  orientations.
• The frequency (v) of electromagnetic radiation
  necessary for such transition is given by v2uH/h
  where H is the strength of the external magnetic
  field.
• The precession frequency of the spinning is
  exactly equal to the frequency of electromagnetic
  radiation necessary to induce a transition from
  one nuclear spin state to another.

11
Theory of Nuclear Resonance

• There is slightly excess of nuclei in the low
  spin state compared to high spin state.
  Boltzmans distribution (low spin state / high
  spin state is 1.00001. This very small excess
  of lower energy state gives rise to net
  absorption of energy in the radio frequency.
  Without this small excess, there would be no NMR.
  
• Spin-spin relaxation and spin lattice relaxation
• Lattice is the frame work of molecules containing
  the precessing nuclei

12
Types of Bonds
Antibonding
s

p
Antibonding





p
s
p
s




Energy

p
n
s
n


Nonbonding
n
Bonding
p

Bonding
s

13
Magnetic Properties of Nuclei
Nuclei of certain atoms posses an angular
momentum. The total angular momentum depends on
the spin number (I). The spin number ( I ) is
related to the mass number and the atomic number.
Each proton has its own spin and I is a result of
these spins.
14
NMR Equation and Magnetic Field Strength
The energy difference between the high energy
spin state and low energy spin state is V
?H0/ 2?, W0 2?V is precessional frequency As
H0 increases, precessional frequency
increases. ? (Magnetogyric Ratio) a
fundamental nuclear constant 267.512 x 106
radians T-1s-1 V Electromagnetic frequency in
radio frequency H0 An external magnetic
field W0 ?H0 , ?H0 2?V, Therefore W0
2?V W0 Precessional frequency
15
Nuclear Magnetic Dipole Moment Angular
Momentum
Nuclear magnetic dipole moment is from the
rotating nuclear charge. Angular momentum is
from rotating nuclear mass.
? (Magnetogyric Ratio) a fundamental nuclear
constant 267.512 x 106 radians T-1s-1 V
Electromagnetic frequency in radio frequency H0
An external magnetic field
16
Relationship between Radio Frequency and
Magnetic Field Strength for Proton
Radio Frequency (Mega Hertz) Magnetic Field
(Gauss) 60 14,100 100 23,500
300 70,500 500 117,500
17
Energy Difference between Spin States as a
Function of Magnetic Fields Strength
Energy
18
Relationship between Applied Magnetic Field
Radiofrequency
?E hv
4.7 T 200 MHz
1.4 T 60 MHz
2.35 T 100 MHz
7.0 T 300 MHz
19
Schematic Diagram of NMR Spectrometer
Sweep coils
Sample



R-F receiver


R-F transmitter


R-F detector


Recorder
Transmitter coil
Receiver coil


Sweep generator
Magnet
20
Chemical Shift
The difference in the absorption frequency of a
particular proton of the sample from the
absorption frequency of a reference proton.
or The separation of resonances frequencies of
nuclei in different chemical environments of
molecule from some arbitrarily chosen standard.

(Reference frequency - Sample
frequency ) ? 106 ? ppm
Operating instrument frequency
21
Chemical Shift
The protons at the electron rich environments
will feel less external magnetic field strength
because the magnetic field strength generated by
electrons surrounding the proton will counteract
the applied magnetic field strength (Ho)
22
Chemical Shift
The Wo (Precessional angular velocity) of the
protons in the electron rich chemical
environments will be less and require less radio
frequency to be resonance with the applied radio
frequency compared to the protons in the electron
poor chemical environments.
23
Chemical Shift

• If the d(Ha) and d (Hb) differs by 1ppm, the
  amount in 600 MHz instrument correspond to an
  energy difference of 600MHz or 6x10-8 cal/mole.
• To measure the small difference between Ha and Hb
  as separate states, they would have lifetimes in
  each conformation of at least ?t 1 / 2 ? ?v
  ?t 1 / (2 ? ?E) 1 / (2 ? x 600 )0.00027 sec.
• The average lifetime of a given conformation is
  only 10-11 sec in a energy barrier of only
  3kcal/mole between one conformer to another.

24
Chemical Shift

• The combination of Heisenberg uncertainty
  principle and the small energy change
  characteristic of NMR spectroscopy is that two
  hydrogen states are convertible.
• If separate lifetimes gt 1sec, NMR can be seen as
  two sharp peaks, lt 1 msec as a combined single
  sharp peak. The two hydrogen states are
  magnetically equivalent. If the lifetimes are in
  an intermediate region, a broad peak results.

25
Chemical Exchange (Proton Transfer)

• Chemical Exchange describes the fact that in a
  given period of time, a single -OH proton may
  attached to a number of different ethyl alcohol
  molecules.
• The rate of chemical exchange (proton transfer)
  in pure alcohol ethyl alcohol is slow, this rate
  is very markedly increased in acidic or basic
  impurities. If the rate of chemical exchange is
  very slow, the expected multiplicity of hydroxyl
  group is observed.
• If the rate of chemical exchange is rapid, a
  single sharp signal is observed. An
  intermediate rates of proton transfer, the
  observation my occur as a broad peak

26
Chemical Exchange (Proton Transfer)

• The rapid chemical exchange causes spin
  decoupling (no multiplicity)
• Heisenberg uncertainty principle quantum
  mechanics
• ?v ?t 1 / 2 ? where ?v and ?t are the
  uncertainties in energy and time in units of
  Hertz and seconds. That is, we can not know
  precisely both the energy and the life time of a
  given state. The longer time the state, the more
  precisely can its energy content be evaluated.

27
Shielding Mechanism

• Ordinary proton magnetic resonance absorption
  frequencies are spread over 7000 cps at 600MHz
  NMR. The magnitude of the separation of the
  position of absorption of a proton from the
  reference is called the chemical shift.
• The shielding that a proton experiences is a
  combination of at least three types of electronic
  circulations
• Local diamagnetic effects
• Diamagnetic and paramagnetic effects from
  neighboring atoms
• Effects from inter atomic currents.
• When the nucleus experiences a smaller magnetic
  field than that applied externally, It is said
  to be shielded.

28
Shielding Mechanism

• Diamagnetic shielding always reduces the apparent
  magnetic field at the proton, and consequently is
  a source of positive shielding.

29
Shielding Mechanism

• Paramagnetic shielding arises from electronic
  circulation within the molecule when they are
  specifically oriented with respect to the
  magnetic field.
• The orientation of the protons relative to the
  induced magnetic currents are called anisotropic
  effects.
• Aromatic nuclei contain large closed loops of p
  electrons in which strong magnetic currents are
  induced by the magnetic field. This effects
  results in a paramagnetic shielding at the
  aromatic proton and is called ring current
  effects.

30
Reference TetraMethylSilane (TMS)
d 0
31
Absorbance Frequency
60 MHz----from 59,999,280 Hz to 60,000,000
Hz 14,100 Gauss 300 MHz--- from
299,996,400 Hz to 300,000,000 Hz 70,500
Gauss 600 MHz---from 599,992,800 Hz to
600,000,000 Hz 141,000 Gauss
O
C
H
C
O
C
H
R
C
H
C
H
C
H
C
H
C
H







2
3
2
5.3
d
2.7
3.6
d
d
32
General Regions of Chemical Shifts
Aliphatic alicyclic
?-Substituted aliphatic
Acetylenic
?-Monosubstituted aliphatic
?-Disubstitutid aliphatic
Olefinic
Aromatic and heteroaromatic
Aldehydic
TMS
0 ?
1
3
4
5
6
10
2
7
8
9
33
Spin-Spin Coupling (Spin-Spin Splitting)
Spin-Spin Coupling is the indirect coupling of
proton spins through the intervening bonding
electrons. Spin-Spin Coupling occurs because
there is some tendency for a bonding electron to
pair its spins with the spin of the nearest
protons. The splitting patterns is due to the
magnetic field experienced by the protons of one
group is influenced by the spin arrangements of
the protons in the adjacent group through the
intervening bonding electrons. .
34
Spin-Spin Coupling (Spin-Spin Splitting)
Coupling is ordinarily not important beyond 3
bonds unless there is ring strains as in small
rings or bridged systems, or bond delocalizaion
as in aromatic or unsaturated systems
35
Spin-Spin Splitting
Signal Ha is split into a doublet by coupling
with one proton. Signal Hb is split into a
triplet by two protons. Spacing in both sets is
same (Jab). The number of multiplicity is n1,
n being neighboring protons. The relative
intensities of the peaks of a multiplet also
depend on n. Doublet (n1) peaks are in the
ratio of 11, triplet peaks are 121 and
quartets are 1331.
a
b
b
a
Jab
Jab
Jab
0 ?
10
36
Number of signals Position of signals Intensity
of signals Splitting of signals
4
3
14
2
2
4
1
2
2
2
2
37
a e
e c e
e b
O
C
H
C
H
C
H
C
H
(
C
H
C
H
C
H
)
C
H
(
C
H
)
C
H
C
3
2
2
2
2
2
5
2
CH3
O
d
a
0.97
b
1.33
c
2.80
d
3.67
e
5.38
38
Information from NMR Spectrum

• Number of signals
• Position of signals
• Intensity of signals
• Splitting of signals

39
Summary

• As external applied magnetic filed increases
• Spinning proton magnetic dipole moment
• increases
• spinning proton angular momentum increases
• proton precession frequency increases
• the energy difference between high energy spin
  state and low energy spin state increases

40
Summary
The magnetic nucleus may assume any one of ( 2 I
1) orientations with respect to the directions
of the applied magnetic field. Therefore, a
proton (1/2) will be able to assume only one of
two possible orientations that correspond to
energy levels of or - ? H in an applied
magnetic field, where H is the strength of the
external magnetic field.
41
Summary
If proper v is introduced, the Wo will be
resonance with the properly applied radio
frequency (Hi) and the proton will absorb the
applied frequency and will be raised to the high
energy spin state. Even though the external
magnetic field strength (Ho) applied to the
molecule is the same, the actual magnetic field
strength exerted to the protons of the molecule
are different if the protons are in the different
electronic chemical environment.