Transcript No Slide Title

Spectroscopic Interpretation
NMR
Dr. Richard W. McCabe
Department of Forensic and
Investigative Science
M57
[email protected]
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Spectroscopy
Spectroscopy is concerned with the absorption of
light of specific wavelengths or frequencies by
molecules (e.g. in the visible, ultra violet, infra red,
radio frequency and other ranges).
 The pattern of absorption versus frequency gives us
a spectrum which can tell us a great deal about the
structure of the molecule.

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Common Types of Spectroscopy
Type
Frequency (or
Wavelength)
Range
Ultra Violet / (200-850 nm)
Visible
Infra Red
4000-50 cm-1
Nuclear
Magnetic
Resonance
10-750 MHz
(depends on
magnet)
N.B. Mass Spectrometry is
not strictly a Spectroscopy!
What is being
observed?
Electronic
Transitions
(colour!)
Vibrations of
bonds (heat!)
Changes in
direction of
nuclear spin (oddly
enough, tells us
about electronic
effects!!!).
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Nuclear Magnetic Resonance
Spectroscopy



One of the most powerful techniques for helping determine
the structure of a molecule.
As the name suggests, the nuclei absorb light of the correct
energy (in the radio frequency range, MHz) when they are
placed in a magnetic field.
The NMR effect is now used in the modern magnetic
resonance body scanners which form their image by
looking at the variation of the concentration of water, (or
other hydrogen-containing molecules), through a “slice” of
tissue - consecutive “slices” are built-up to give a 3Dimage of the inside of a body.
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Information from an NMR Spectrum

Major structural information can be derived from the
following:
1.CHEMICAL SHIFT, d, gives information about the
ELECTRONIC environment of an atom; i.e. hybridisation,
electronic effects (Inductive and Resonance), through space
magnetic effects, etc.
2.With some nuclei quantitation, INTEGRAL.
3.Nuclear magnets interact (COUPLING) and give
information about the number of adjacent nmr active
nuclei, MULTIPLICITIES. This in turn can also give
stereochemical information.
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Origin of the NMR Effect 1


Certain nuclei (isotopes) have a property called spin.
(N.B. Only 2 or 3 elements do not have a spin (NMR)
active isotope.)
Nuclear spin arises from unpaired nucleons (protons or
neutrons) in nuclear energy levels - nucleons spin-pair in
an analogous manner to electrons in electronic energy
levels! Examples of nuclear spins (spin quantum number
is 1/2) include:
–
–
–
–
spin 0 : 4He, 12C, 16O (no spin - so no NMR)
spin 1/2 : 1H, 3H, 13C, 15N, 19F, 31P (spin - so NMR)
spin 1 : 2H, 14N (spin - so NMR)
spin 3/2 , etc : 79Br (spin - so NMR) - and many more!
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Origin of the NMR Effect 2



A spinning (moving) particle (in this case a nucleus)
generates a magnetic field: consider the magnetic field
generated when an electric current flows in a wire!
If the spinning particle is placed in a large magnetic field
the “nuclear magnet” will line up with the external
magnetic field.
When the nucleus is given energy, i.e. in the form of a radio
beam of the correct frequency, then the direction of the
nuclear magnetic field can be forced to point in the opposite
direction. This absorption of light gives the NMR
spectrum. N.B. Each isotope has its own individual
frequency range (MHz).
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Energy Level Diagram for a NMR
Experiment
Bo
Bo
E=h
Rf
E=h
If a Rf pulse of the correct
Before the molecules
When the molecules are
are placed in the
placed in the Magnetic Field frequency "hits" one of the nuclear
spins in the lower energy level,
Magnetic Field, Bo, most of the spins will line up
then the spin can "flip" from
the nuclear spins point with Bo, but some have
pointing with the field to pointing
in random directions
enough energy to point
against - giving the NMR absorption.
against the field
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Simplified Diagram of an NMR
Spectrometer

An NMR spectrometer needs a strong, uniform magnetic
field and usually uses a cryomagnet.
Tube
Spins
Bo
N
Receiver
Coil to
Detector
S
Transmitter
Coil Gives
Rf Pulse
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Chemical Shifts, d
The “frequency” scale in NMR is expressed as the
difference in parts per million that the nucleus
resonates from the frequency () that the nuclei of a
reference nucleus resonate in the particular magnetic
field used.
i.e. d = ( of nucleus -  of standard) Hz
ppm
spectrometer frequency in MHz
 Different chemical environments in a molecule
produce peaks at different frequencies - Thus the
name chemical shift!!

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Symmetry and Chemical Shift
Quite often we see less peaks than the number of
carbons in a molecule.
 Carbons that are identical by symmetry have the
same chemical shift and so the only indication that
we have identical carbons is that the peaks are
sometimes larger.
 Example:

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13C
NMR Correlation Chart
CC H 3
CCH 2C
CCHC
O=C C
C=C C
N CH
Hal- C
O CH
CCH
C= C
N C
N= C
Ar CH
Ar C
O C=O
N C=O
C=C C=O
CO
CHO
220
200
180
160
140
120
100
80
60
40
20
0
d ppm
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13C

NMR Chemical Shifts
Typically the ranges of 13C NMR d values can be
divided into “80 ppm’s”:
–
–
–
alkanes
alkenes/aromatics
carbonyls
ca. d 0 to d 80 ppm
ca. d 80 to d 160 ppm
ca. d 160 to d 240 ppm

N.B. 1. dCH3 < d CH2 < d CH < d C

N.B. 2. dAromatics > d Alkene - see ring current
effects later.
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13C

NMR Spectra of Alkenes, Alkynes
and Aromatics 1
Typically resonances for unsaturated Carbon atoms come at
higher chemical shift values (d) than alkanes, i.e.:
– Aromatics > Alkenes >
Alkynes > Alkanes
d160 ppm
d80 ppm
d0 ppm

N.B. Each p-system creates a small magnetic field which
adds to the external field and so increases d.

N.B. d alkynes lower than expected as the sp orbital
extends past, and shields, the C atoms of the CC.
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13C

The chemical shift values for C=O carbons occur in
the range d 160-240 ppm. Their exact position
depends upon a combination of Inductive and
Resonance effects:
–
–

NMR Spectra of Carbonyl
Compounds
Inductive effects give d >200 ppm:
MCO > COCl ~ (CO)2O > HCO > RCO
Resonance effects give d < 200 ppm:
C=CCO > ArCO > OCO > NCO
N.B these effects parallel the reactivity and IR
frequency of the C=O group.
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Shielding and Deshielding in NMR
Spectra 1

Different nuclei give
different chemical shifts
because electrons in
bonds also produce their
own magnetic field
which opposes the
external magnetic field,
SHIELDS the nucleus
from the external
magnetic field and thus
affects the radio
frequency needed for
resonance.
Lines of Force Shielding at Nucleus
Nucleus
Electrons
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Shielding and Deshielding in NMR
Spectra 2
If electronic effects in the molecule (i.e. hybridisation,
inductive or resonance effects) pull electron density
away from the region of the nucleus the chemical shift,
d, increases and the nucleus is said to be
DESHIELDED.
 If the electronic effects push electron density into the
region of the nucleus, d, decreases and the nucleus is
SHIELDED
 N.B. >d+ increases deshielding and gives >d ppm!!
(i.e. all d’s!!!)

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Factors Governing Chemical Shifts, (a)
Hybridisation



Alkanes - four carbon sp3 orbitals around the carbon nucleus form a
“sphere” of electron density. This shields the C nucleus quite
effectively - \ low dC (0-80 ppm) values.
Alkenes - the carbon sp2 hybrid orbitals form a plane - “exposing”
the nucleus to the magnetic field - the p bond is quite diffuse,
resides mainly between the atoms and not around the nuclei so
leaving the nucleus exposed again! - \ high dC (80-160 ppm)
values.
Alkynes - the carbon sp orbitals form a line, effectively “exposing”
the nucleus, but the shorter triple bond “pulls” the nuclei into the
“cylinder” of electron density formed by the 2 p bonds - \ lower dC
(60-100 ppm) values than expected.
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Factors Governing Chemical Shifts,
(b) Electronic Effects

As the electronegativity, or d+ character, of an atom
attached to a particular nucleus increases then the d
value increases:
H3C-Si dC & dH 0 ppm {tetramethylsilane [TMS] (H3C)4Si, used
as standard, i.e. d 0 ppm, for 1H, 13C and 29Si NMR}
H3C-C
dC 15 ppm
H3C-C=O
dC 25 ppm N.B. O “far” away
H3C-N
dC 30 ppm
H3C-Br
dC 33 ppm
H3C-Cl
dC 35 ppm
H3C-O
dC 50 ppm N.B. O near - effect large!
H3C-F
dC 75 ppm
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Examples of Chemical Shifts in
Alkenes + Benzene rings
X
Electronic Effect
H
Ethene
dC dC
123.3 123.3
Me
Weak p- and -donor
133.9 115.4
OMe
p-Donor, -acceptor
152.7 84.4
Weakp-donor, -acceptor
CH=CH2 Simple conjugation
Cl
SiMe3
p-Acceptor, - donor
p-Acceptor, - acceptor
Benzene (aromatics
~5-10
ppm
higher than similar alkenes)
COMe

125.9 117.2
136.9 130.3
138.7 129.6
138.3 129.1
128
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The number of nuclei causing a peak Integration
(semi-)Quantitative estimates of the relative number of
nuclei causing a peak can be obtained by integration (a
measure of the area under the peak). This can be done
automatically by the nmr computer and presented as
either a numerical value or by measuring the height of
an integral trace on the printout.
 Integration is easy for 1H, but difficult for most other
nuclei due to saturation problems.
 Generally 1H integrals accurate to ca. 10% unless
extreme measures taken.

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Spin-Spin Coupling - Tells us how many
NMR active nuclei are adjacent!

When spin-active nuclei are near to one another in a
molecule their magnetic fields interact and the NMR
signal is split. The general pattern for 13C-1H couplings
in the 13C NMR spectrum is:
–
–
–
–

C (with no H’s!) singlet (s) - one line
CH
doublet (d) - two lines
CH2 triplet (t) - three lines
CH3 quartet (q) - four lines
This can be confusing sometimes as many lines can
overlap and small couplings from further nuclei can also
interfere!!!
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Determining the no. of H’s on a Carbon 1 Decoupling & Off Resonance
 13C


NMR spectra are usually simplified so that we see just
one resonance for each different carbon.
The spin-spin splitting of the C by coupling to H can be
eliminated by irradiating the H nuclei with a strong Rf
beam. This removes the coupling and gives a single line for
each different C (C’s which are identical by symmetry give
just one line).
The above process has lost information (i.e. we no longer
know the no. of H’s on the C!!!) so a second experiment can
be run which brings back some of the coupling - the OFF
RESONANCE spectrum - and gives narrow splitting into
the q, t, d & s.
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The Different Spin-Spin Coupling
NMR Experiments

13C
The 13C spectrum may be obtained in each of the
following ways:
Full Coupling
Full Decoupling
Partial Coupling
e.g. consider:
Hx
H
H
C
13C{H}-spectrum
N.B. Short-range coupling
to 2H's gives 1:2:1 triplet,
whilst long-range coupling
to Hx further splits the lines
into narrow doublets - poor
signal/noise!
13C{offres}-spectrum
[Offresonance]
[Proton Noise Decoupled]
All H's are strongly
The H-C decoupling power
irradiated to "scramble"
is reduced so that a narrow
the H-C coupling. The
coupling from only the directly
resultant signal will be
attached H's returns! - moderate
much more intense as all
signal/noise!
of the previously split
components are now added
together! - good signal/noise!
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