Download Nuclear Magnetic Resonance Spectroscopy

KOT 222 ORGANIC CHEMISTRY II
CHAPTER 13
NUCLEAR MAGNETIC
RESONANCE SPECTROSCOPY
Part I
1
NMR Spectroscopy
¾ The most powerful tool for organic structure
determination.
¾ Small sample is needed and it does not harm
the sample.
¾ Being used to study a wide variety of nuclei:
™ 1H
™13C
™15N
™19F
™31P
2
Nuclear Spin
¾ A nucleus with an odd atomic number or an odd
mass number has a nuclear spin.
¾ It has spin number (I) ≠ 0 and can absorb/emit
electromagnetic radiation (RF for NMR).
3
Proton:
¾ The simplest nucleus with odd atomic number of
1.
¾ The nuclear spin of a proton will generates a
small magnetic field called the magnetic moment
(B).
4
¾ In the absence of external magnetic field, proton
magnetic moments have random orientations.
¾ When an external magnetic field is applied, each
proton in a sample assumes the alpha or the
beta-spin state.
5
ΔE and Magnet Strength
• Energy difference is proportional to
the magnetic field strength.
• ΔE = hν = γ h B0
2π
• Gyromagnetic ratio, γ, is a constant for each
nucleus (26,753 s-1gauss-1 for H).
• The strength of the magnetic field (B0) is
proportional to the operating frequency (MHz).
• In a 14,092 gauss field, a 60 MHz photon is
required to flip a proton.
• For now, proton resonance frequencies occur in
the radio-frequency (RF) region.
6
NMR Absorption
¾ A nucleus is “in
resonance” when it is
irradiated with RF
photons having energy
equal to the energy
difference between the
spin states.
¾ A photon with the right
amount of energy can be
absorbed and cause the
spinning proton to flip.
The nuclei are said to be
in resonance with the RF
frequency.
7
Magnetic Shielding
¾ If all protons absorbed the same amount of
energy in a given magnetic field, not much
information could be obtained.
⇒ this happens to the naked protons.
¾ In organic compounds, the protons are
surrounded by electrons that shield them from
external magnetic field.
¾ Circulating electrons create an induced
magnetic field that opposes the external
magnetic field.
Beffective = Bexternal - Bshielding
8
Shielded Protons
Magnetic field strength must be increased for a
shielded proton to flip at the same frequency.
9
Protons in a Molecules
¾ Depending on their chemical environment,
protons in a molecule are shielded by different
amounts.
¾ The presence of electronegativity atom will
withdraw electron from the proton, hence it is
deshielded.
10
NMR Spectrometer
Emit a precise
frequency
The magnet with a
sensitive controller
produce a precise
magnetic field.
Measure the sample’s
absorption of RF energy
11
NMR Spectrum
12
NMR Signals
¾ The number of signals shows how many
different kinds of protons are present.
¾ The location of the signals shows how shielded
or deshielded the proton is.
¾ The intensity of the signal shows the number of
protons of that type.
¾ Signal splitting shows the number of protons
on adjacent atoms.
13
The Chemical Shift
¾ Variations of the positions of NMR absorptions
due to the electronic shielding and deshielding.
¾ It is a measure of how far the signal of the
proton (sample) is from the reference signal.
¾ Most common reference compound is
tetramethylsilane (TMS), (CH3)4Si.
14
CH3
H3C
Si CH3
CH3
• Si is less electronegative than C.
• Methyl groups withdraw electrons
and their protons are shielded.
• The TMS protons absorb at higher field compare to
the most hydrogens bonded to other elements and
its signal defined as zero.
• Organic protons absorb downfield (to the left) of the
TMS signal.
15
Chemical Shifts
¾ Measured in parts per million (ppm).
¾ Ratio of shift downfield from TMS (Hz) to total
spectrometer frequency (MHz).
¾ The chemical shift is independent of the operating
frequency of the spectrometer
¾ Same value for 60, 100, or 300 MHz machine.
¾ Common scale used is the delta (δ) scale.
16
Delta Scale
Each δ unit is 1 ppm difference from TMS 60 Hz at
60 MHz and 300 Hz at 300 MHz.
17
Location of Signals
¾ Depends on the shielding and deshielding
effects on the protons.
Methyl
protons in
alkane
absorb at
δ0.9
¾ More electronegative substituent deshield more
and give larger chemical shifts.
18
H
H
δ
H
γ
H
H
β
C
C
C
H
H
H
0.9
1.3 1.7
α
C
Br
H
3.4
¾ Effect of an electronegative group on the
chemical shift depends on its distance from the
protons.
¾ The effect decreases with increasing distance.
¾ The effects can be negligible if the protons are
separated from the electronegative group by
four or more bonds.
19
¾ Deshielding effect
increases with more
electronegative
group.
¾ The deshielding
effecs are nearly
additive.
20
21
Characteristic Values of
Chemical Shifts
22
23
Aromatic Protons, δ7-δ8
Along the ring axis, the
induced field act to oppose
the external field.
Due to the
deshielding
effect at the
edge of the
ring, aromatic
protons absorb
at lower field
24
Spectrum of toluene
It is the protons attached directly to the aromatic ring
that will be the most deshielded.
25
Vinyl Protons, δ5-δ6
¾ Similar deshielding effect to aromatic protons.
¾ The effect is weaker due to lack of large, effective ring
of electrons as in the benzene.
26
Acetylenic Protons, δ2.5
Since the acetylenic protons are in the axis of the generated
field that opposes the applied field, the acetylenic protons are
shielded and will be found at higher fields than vinylic protons.
27
Aldehyde Proton, δ9-δ10
Electronegative
oxygen atom
The aldehyde proton is deshielded by the circulation of
electrons in the pi bond. It is also deshielded by the
electron-withdrawing effect of the carbonyl (C=O) group
28
O-H and N-H Signals
¾ Chemical shift depends on concentration.
¾ Hydrogen bonding in concentrated solutions
deshield the protons, so signal is around δ3.5 for
N-H and δ4.5 for O-H.
¾ Proton exchanges between the molecules
broaden the peak.
¾ The protons pass through a variety of
environments during this exchange, absorbing
over a wider range of frequencies and field
strengths.
29
Carboxylic Acid Proton, δ10+
¾ Hydrogen bonding and oxygen atom strongly
deshield the carboxylic acid proton.
30