Download Ch. 13: NMR Spectroscopy - Chemical Engineering Resources

NMR Spectroscopy
Abu Yousuf , PhD
Associate Professor
Department of Chemical Engineering & Polymer Science
Shahjalal University of Science & Technology
Sylhet-3114, Bangladesh
[email protected]
NMR 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 1H-NMR spectroscopy.
– carbon atoms using 13C-NMR spectroscopy.
– phosphorus atoms using 31P-NMR
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 has 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.
• 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
2I + 1 spin states; if I = 1/2, there are two
allowed spin states.
– Spin quantum numbers and allowed
nuclear spin states for atoms common to
organic compounds.
Element
Nuclear spin
quantum
number ( I )
Number of
spin states
1
H
2
H
12
C
13
C
14
N
15
N
1/2
1
0
1/2
1
1/2
2
3
1
2
3
2
16
O
19
F
31
P
32
S
0 1/2 1/2
0
1
1
2
2
Nuclear Magnetic Resonance Spectroscopy
Nuclear spins are oriented randomly in the absence (a) of an external
magnetic field but have a specific orientation in the presence (b) of an
external field, B0
• Some nuclear spins are aligned parallel to the external field
– Lower energy orientation
– More likely
• Some nuclear spins are aligned antiparallel to the external field
– Higher energy orientation
– Less likely
Nuclear Spins in B0
Tesla- A particle carrying a charge of 1 coulomb and passing
through a magnetic field of 1 tesla at a speed of 1 meter per
second perpendicular to said field experiences a force of 1
newton,
Units used:
A = ampere
C = coulomb
kg = kilogram
m = meter
N = newton
s = second
T = tesla
V = volt
Wb = weber
Nuclear Magnetic Resonance Spectroscopy
When nuclei that are aligned parallel with an external
magnetic field are irradiated (To expose to radiation) with the
proper frequency of electromagnetic radiation the
energy is absorbed and the nuclei “spin-flips” to the
higher-energy antiparallel alignment
– Nuclei that undergo “spin-flips” in response to
applied radiation are said to be in resonance with the
applied radiation - nuclear magnetic resonance
– Frequency necessary for resonance depends on
strength of external field and the identity of the nuclei
Nuclear Magnetic Resonance Spectroscopy
The energy difference DE between nuclear spin states
depends on the strength of the applied magnetic field
– Absorption of energy with frequency n converts a nucleus from
a lower to a higher spin state
 DE = 8.0 x 10-5 kJ/mol for magnetic field strength of 4.7 T a
– For field strength of 4.7 T a radiofrequency (rf) of n = 200
MHz is required to bring 1H nuclei into resonance
– For a field strength of 4.7 T a radiofrequency (rf) of n = 50
MHz is required to bring 13C nuclei into resonance
Nuclear Spin in B0
– The energy difference between allowed spin
states increases linearly with applied field
strength.
– Values shown here are for 1H nuclei.
Which Nuclear exhibit NMR
Many nuclei exhibit NMR phenomenon
• All nuclei with odd number of protons
• All nuclei with odd number of neutrons
• Nuclei with even numbers of both protons and neutrons
do not exhibit NMR phenomenon
What happen with absorption of Radio frequency
Excited state = High energy
N
N
S
S
Add
Energy
N
S
N
Energy
Released
S
Aligned = Low Energy
N
S
N
S
Back to low energy ground state
• When the spin falls back into line with the magnetic field it
releases energy. We detect this energy and it provides
information on:
• The environment of the hydrogen in the molecule
• How many hydrogen atoms are in that environment.
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.
– The nucleus begins to precess and traces out a
cone-shaped surface, in much the same way a
spinning top or gyroscope traces out a coneshaped surface as it precesses in the earth’s
gravitational field.
Nuclear Magnetic Resonance
• If the precessing (the motion of a spinning body)
nucleus is irradiated with electromagnetic
radiation of the same frequency as the
rate of precession,
– the two frequencies couple
– energy is absorbed
– the nuclear spin is flipped from spin state +1/2
(with the applied field) to -1/2 (against the
applied field).
Nuclear Magnetic Resonance
– (a) Precession and (b) after absorption of
electromagnetic radiation.
Nuclear Magnetic Resonance
• Resonance: In NMR spectroscopy, resonance is
the absorption of energy by a precessing nucleus
and the resulting “flip” of its nuclear spin from a
lower energy state to a higher energy state.
• The precessing spins induce an oscillating
magnetic field that is recorded as a signal by the
instrument.
– Signal: A recording in an NMR spectrum of a nuclear
magnetic resonance.
The Nature of NMR Absorptions
The absorption frequency is not the same for
all 1H and 13C nuclei
– Nuclei in molecules are surrounded by electrons
– Electrons set up tiny local magnetic fields that act in
opposition to the applied field, shielding the nucleus
from the full effect of the external magnetic field
– The effective field actually felt by the nucleus is the
applied field reduced by the local shielding effects
Beffective = Bapplied – Blocal
1) The environment of the Hydrogen atom
• The frequency of energy needed to flip the magnet (Spin) is different
for hydrogen atoms that are in different positions.
H
H
C
H
O
C
H
Needs high frequency energy to flip
here, so high frequency energy released
when it flips back.
Flips quite easily – needs
low frequency energy.
• So the type of energy given off tells us the position of the hydrogen in
the molecule.
2) How many hydrogen atoms are in each position
If one hydrogen releases a set amount of energy when it
falls back in line (eg 2 units). Then 3 hydrogen atoms will
release 3 times that amount when they fall in line (6 units).
This affects the size of the peak in the NMR spectrum.
The more hydrogen atoms, the bigger the peak!
Interpreting NMR Spectra
• Counting Hydrogen environments – One molecule can contain many
hydrogen environments. Each environment will release a different
frequency of energy when it drops down from its excited state to line
up with the magnetic field.
• So for each different hydrogen environment, we will see a different
peak in the NMR spectrum.
H
H
C
H
O
C
H
2 x H environments so 2
peaks in NMR spectrum.
Your Turn!
H
CH3
C
C
H3C
H
H2
C
H3C
OH
C
H2
2 x H environments so 2
peaks in NMR spectrum.
4 x H environments so 4
peaks in NMR spectrum.
3 x H environments so 3
peaks in NMR spectrum.
Equivalent Hydrogens
• Equivalent hydrogens: Hydrogens that have
the same chemical environment.
– A molecule with 1 set of equivalent
hydrogens gives 1 NMR signal.
O
CH3 CCH3
ClCH 2 CH2 Cl
C
H3 C
Propanone
(Acetone)
CH3
H3 C
1,2-Dichloroethane
Cyclopentane
C
CH3
2,3-Dimethyl2-butene
Equivalent Hydrogens
– A molecule with 2 or more sets of
equivalent hydrogens gives a different
NMR signal for each set.
Cl
CH3 CHCl
1,1-D ich loroeth ane
(2 signals )
Cl
O
Cyclop entan on e
(2 s ign als)
CH3
C C
H
H
(Z)-1-Ch loroprop ene
(3 signals)
Cyclohexen e
(3 signals)
Signal Areas
– Relative areas of signals are proportional to the
number of H giving rise to each signal, Modern
NMR spectrometers electronically integrate and
record the relative area of each signal.
Chemical Shifts
Chemical shift
•
•
the chemical shift is the resonant frequency of a nucleus relative to a standard
Position on NMR chart at which a nucleus absorbs
– The chemical shift of TMS is set as zero point
– Other absorptions normally occur downfield
– NMR charts calibrated using delta (d) scale
• 1 d = 1 part per million of operating frequency
– Chemical shift of an NMR absorption in d units is constant, regardless of the
operating frequency of the spectrometer
Since the numerator is usually in hertz, and the denominator in
megahertz, delta is expressed in ppm.
Thus, an NMR signal that absorbs at 300 Hz higher than that of TMS at
an applied frequency of 300 MHz has a chemical shift of:
Chemical Shifts
The NMR Chart
The downfield, deshielded side is on the left, and requires a lower
field strength for resonance
The upfield, shielded side is on the right, and requires a higher
field strength for resonance
The tetramethylsilane (TMS Si(CH3)4) absorption is used as a
reference point
Nuclear Magnetic Resonance
– The difference in resonance frequencies among
the various hydrogen nuclei within a molecule
due to shielding/deshielding is generally very
small.
– The difference in resonance frequencies for
hydrogens in CH3Cl compared to CH3F under an
applied field of 7.05T is only 360 Hz, which is 1.2
parts per million (ppm) compared with the
irradiating frequency.
360 Hz
6
300 x 10 Hz
=
1.2
6
10
= 1.2 p pm
Nuclear Magnetic Resonance
– Signals are measured relative to the signal of the
reference compound tetramethylsilane (TMS).
CH3
CH3
Si CH3
CH3
Tetrameth yls ilane (TMS)
– For a 1H-NMR spectrum, signals are reported by
their shift from the 12 H signal in TMS.
– For a 13C-NMR spectrum, signals are reported by
their shift from the 4 C signal in TMS.
– Chemical shift (d): The shift in ppm of an NMR
signal from the signal of TMS.
NMR Spectrum
• 1H-NMR spectrum of methyl acetate.
– High frequency: The shift of an NMR signal to the
left on the chart paper.
– Low frequency: The shift of an NMR signal to the
right on the chart paper.
The Nature of NMR Absorptions
The absorption frequency is not the same for
all 1H and 13C nuclei
– Each chemically distinct nucleus in a molecule has a
slightly different electronic environment and consequently
a different effective field
– Each chemically distinct 13C or 1H nucleus in a molecule
experiences a different effective field and will exhibit a
distinct 13C or 1H NMR signal
The Nature of NMR Absorptions
(a) 1H NMR spectrum and (b) 13C NMR spectrum of methyl
acetate. Peak labeled “TMS” at far right is for calibration
The Nature of NMR Absorptions
• The two methyl groups of methyl acetate are nonequivalent
– The two sets of hydrogens absorb at different positions
• When the frequency of rf irradiation is held constant and
the applied field strength is varied each nucleus in a
molecule comes into resonance at a slightly different field
strength, mapping the carbon-hydrogen framework of an
organic molecule
The Nature of NMR Absorptions
The 13C spectrum of methyl acetate shows three peaks, one
for each of the three chemically distinct carbon atoms in the
molecule
Chemical Type of
Hydrogen
Shifts
( CH3 ) 4 Si
1H-NMR
Chemical
Shift ( d)
0 (by definition)
RCH3
0.8-1.0
RCH2 R
1.2-1.4
R3 CH
1.4-1.7
R2 C= CRCH R2 1.6-2.6
RC CH
2.0-3.0
A rCH3
2.2-2.5
A rCH2 R
2.3-2.8
ROH
0.5-6.0
RCH2 OH
3.4-4.0
RCH2 OR
3.3-4.0
R2 NH
0.5-5.0
O
RCCH3
2.1-2.3
O
RCCH2 R
2.2-2.6
Type of
Hydrogen
O
RCOCH3
O
RCOCH2 R
RCH2 I
RCH2 Br
RCH2 Cl
RCH2 F
A rOH
R2 C= CH2
R2 C= CHR
A rH
O
RCH
O
RCOH
Chemical
Shift ( d)
3.7-3.9
4.1-4.7
3.1-3.3
3.4-3.6
3.6-3.8
4.4-4.5
4.5-4.7
4.6-5.0
5.0-5.7
6.5-8.5
9.5-10.1
10-13
Chemical Shift
• Chemical shift depends on the
(1) electronegativity of nearby atoms,
(2) hybridization of adjacent atoms, and
(3) diamagnetic effects from adjacent pi bonds.
• Electronegativity
Electron eg- Chemical
CH3 -X
ativity of X
Shift (d)
CH3 F
CH3 OH
CH3 Cl
CH3 Br
CH3 I
4.0
3.5
3.1
2.8
2.5
4.26
3.47
3.05
2.68
2.16
(CH3 ) 4 C
(CH3 ) 4 Si
2.1
1.8
0.86
0.00
Chemical Shift
• Hybridization of adjacent atoms.
Type of Hydrogen
(R = alkyl)
N ame of
Hydrogen
Chemical
Sh ift (d)
RCH3 , R2 CH2 , R3 CH
Alk yl
0.8 - 1.7
R2 C=C(R)CHR2
Allylic
1.6 - 2.6
RC CH
Acetylen ic
2.0 - 3.0
R2 C=CHR, R2 C=CH2
Vin ylic
4.6 - 5.7
RCHO
Ald ehydic
9.5-10.1
Chemical Shift
• Diamagnetic effects of pi bonds
– A carbon-carbon triple bond shields an
acetylenic hydrogen and shifts its signal to lower
frequency (to the right) to a smaller d value.
– A carbon-carbon double bond deshields vinylic
hydrogens and shifts their signal to higher
frequency (to the left) to a larger d value.
Type of H
RCH3
N ame
Alk yl
RC CH
R2 C=CH2
Acetylenic
Vin ylic
Chemical
Shift (d)
0.8- 1.0
2.0 - 3.0
4.6 - 5.7
Chemical Shift
– Magnetic induction in the p bonds of a carboncarbon triple bond shields an acetylenic
hydrogen and shifts its signal lower frequency.
Chemical Shift
– Magnetic induction in the p bond of a carboncarbon double bond deshields vinylic hydrogens
and shifts their signal higher frequency.
Chemical Shift
– The magnetic field induced by circulation of p
electrons in an aromatic ring deshields the
hydrogens on the ring and shifts their signal to
higher frequency.
NMR Spectrometer
• Essentials of an NMR spectrometer are a
powerful magnet, a radio-frequency generator,
and a radio-frequency detector.
• The sample is dissolved in a solvent, most
commonly CDCl3 or D2O, and placed in a sample
tube which is then suspended in the magnetic
field and set spinning.
• Using a Fourier transform NMR (FT-NMR)
spectrometer, a spectrum can be recorded in
about 2 seconds.
NMR Spectrometer
Schematic operation of a basic NMR spectrometer
300 MHz NMR
900 MHz NMR
p. 547