Chapter 13 NMR Spectroscopy NMR - Nuclear Magnetic Resonance NMR is a form of spectroscopy that uses an instrument with a powerful magnet to analyze organic compounds. Invented by physicists (1950’s), then used by chemists (1960’s). – Magnetic Resonance Imaging (1980’s) special form of NMR used in medicine.
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Chapter 13 NMR Spectroscopy NMR - Nuclear Magnetic Resonance NMR is a form of spectroscopy that uses an instrument with a powerful magnet to analyze organic.
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Chapter 13 NMR Spectroscopy
NMR - Nuclear Magnetic Resonance
NMR is a form of spectroscopy that uses an instrument with a powerful magnet to analyze organic compounds.
Invented by physicists (1950’s), then used by chemists (1960’s).
MRI – Magnetic Resonance Imaging (1980’s) A special form of NMR used in medicine.
What is NMR?NMR – a tool to determine the structure of an
organic compound.
magnet
NMRSpectrometer
computer
1H NMRSpectrum
This instrument gives you
information about an organic
compound’s structure.
What is NMR?NMR Instruments
Small, 60 mHz
instrument for
undergraduate
student use.
magnetmagnet
computercomputer
studentstudent
What is NMR?NMR Instruments
Research grade
instrument,
300 mHz magnet,
that we use at
Western.
magnetmagnet
What is NMR?NMR Instruments
Research grade
instrument,
300 mHz magnet,
that we use at
Western.
computercomputer
What is NMR?
NMR Instruments
State-of-the-art
instrument,
950 mHz magnet.
Rather large and
expensive!
magnetmagnet
researcherresearcher
Why is it called NMR?
Nuclear Magnetic ResonanceNuclear – because it looks at the nucleus of an atom,
most commonly a hydrogen atom. A hydrogen atom nucleus consists of one proton with a
+1 charge and “spin” of ½. It acts like a tiny bar magnet.
protonspinning proton generatesmagnetic
field
bar magnet
No External Magnetic FieldNuclear spins are pointedin random directions
NMR – Effect of Magnetic FieldSample in Magnetic Field
Spins align with or against the external magnetic field
No external magnetic field applied to sample
Random orientation of nuclear spins
Sample placed in an external magnetic field H0
NMR – Effect of Magnetic Field
aligned with field(lower energy)
alignedagainst field
(higher energy)
hydrogennuclei
Spins align with or against field (most align with field)
NMR: Absorption of Energy
Initial State – nucleus at low energy level
Scan with RF field – nucleus absorbs energy, giving a signal in the NMR spectrum
radio waves
nucleus absorbs energy
NMR: Information Obtained from a Spectrum
An NMR Spectrum will generally provide three types of information:
Chemical Shift – indicates the electronic environment of the nucleus (shielded or deshielded)
Integration – gives the relative number of nuclei producing a given signal
Spin-Spin Coupling – describes the connectivity
1H NMR Spectrum – H2O
A sample of water is placed in an NMR instrument, and a proton spectrum is recorded (scanned from left to right).
signal fromprotons in H20
An NMR signal appears. This proves thatwater contains hydrogen atoms!
scanning
When does nucleus absorb energy?
Absorption depends on shielding by electron cloud around the nucleus. More electron density = more shielding = signal shifted to the right.
Magnetic Fields: 1. from spinning proton 2. from magnet 3. from electrons
2, External Field (Ho)from magnet
Not allprotons arethe same!
3.
NMR: Simple 1H NMR Spectrum Showing Chemical Shift
Two types of protons (a CH2 and a CH3) give two separate signals at two different chemical shifts.
Chemical Shift:location of the signal
on the spectrum.
Right Side:high electron
densityLeft Side:
low electrondensity
NMR: Chemical Shift Practice
Assign the four groups shown to the four NMR singals, based on each element’s electronegativity.
Group
-O-CH3
-Si-CH3
-C-CH3
Cl3C-H
-SiCH3
3.5
1.8
2.5
3.0
EN
Left Side:low electron
density(high EN)
-CCH3 -OCH3 Cl3C-H
3 electronegative atoms
NMR: Chemical Shift Reference
Chemical shift measured in ppm. For 1H: roughly 0 to 10 ppm.
Chemical shift zero is set to TMS (tetramethylsilane).
TMS =
(silicon – low electroneg.)
Si CH3
CH3
CH3
CH3
NMR: Chemical Shift Regions
Alkane region (high electron density) is from about .8 – 2.5 ppm.
-CH2-CH3
NMR: Chemical Shift Regions
Heteroatom region (low electron density) is from about 2.5 to 5.
-O-CH3
NMR: Chemical Shift Regions
Double bond region is on the left, from about 5 – 10 ppm.
C=C H H
NMR: Chemical Equivalence and Number of Signals
How many signals will the following compounds show in their 1H NMR Spectrum? (Hint: check for symmetry)
OMe
Br
O
Cl
Cl
NH2Cl
H
Cl
Cl
H
H
HNH2
Cl
HH
2 4 5
2 4 7
NMR: Chemical Equivalence and Number of Signals
How many signals should appear in the proton NMR spectrum for these compounds?
In theory: 9 4
O
octane
Signals actually resolved: 3-4 2
1
2
3
4
NMR: Overlapping Proton Signals
Protons b, c, and d are in roughly the same environment, and their chemical shifts are also about the same.
The -CH2- groups allappear in the same spot
(not resolved)
octane
Review: How Many NMR Signals?
How many signals will the following compounds show in their 1H NMR Spectrum? (Hint: check for symmetry)
ClH
H HHH
H
H CH3
H
H
H
H H
CH3CH2Cl
CC
1 2 5
Fast chairflips at RT
Fast rotation aboutC-C single bond
No rotation aboutdouble bonds
These H’s are different
NMR: Chloroethane
CC
ClH
H
HH
H
Fast rotation around single bonds gives an “averaged” spectrum for the three methyl hydrogens.
An NMR spectrometer is like a camera with a slow shutter speed.
NMR: Chair Cyclohexane
Rapid chair flipping makes all H’s equivalent. Cylcohexane gives one peak in the 1H NMR spectrum.
An NMR spectrometer is like a camera with a slow shutter speed.
NMR: A Second Proton Spectrum
Note: the signal for the nine methyl H’s (red) is larger than the signal for the CH2 group (blue)
bigger (9 H’s)
smaller (2 H’s)
NMR: Information Obtained from a Spectrum
An NMR Spectrum will generally provide three types of information:
Chemical Shift – indicates the electronic environment of the nucleus (shielded or deshielded)
Integration – gives the relative number of nuclei that produces a given signal.
The integral (area under the curve) is drawnon the spectrum by the instrument.
Spin-Spin Coupling – describes the connectivity
NMR: Integration Indicates Relative Number of Nuclei
The height of the integration line (“integral”) gives you the relative number of nuclei producing each signal.
Integral has relative height 9
Relative height 2
NMR: Information Obtained from a Spectrum
An NMR Spectrum will generally provide three types of information:
Chemical Shift – indicates the electronic environment of the nucleus (shielded or deshielded)
Integration – gives the relative number of nuclei producing a given signal
Spin-Spin Coupling: - describes the carbon connectivity - follows the “n+1”rule”
NMR: Splitting into a Doublet
Note that the red signal at 1.6 ppm for the methyl group is split into two peaks.
Remember that this is one signal, composed of two separate peaks.
doublet
NMR: Signal Splitting, n+1 Rule
• A signal is often split into multiple peaks due to interactions with protons on carbons next door. Called spin-spin splitting
• The splitting is into one more peak than the number of H’s on adjacent carbons (“n+1 rule”)
• Splitting of a signal can give doublets (two peaks), triplets (three peaks), quartets (4 peaks), ect.
n+1 Rule: A signal in the proton NMR spectrum will be split into n+1 peaks, where n is the number of protons on adjacent carbons.
For the Methyl Group: There are two protons ‘next door’ (n=2), so the methyl signal will be split into three peaks (2+1), which is called a triplet. Chemical shift will be about 1.5 (alkane region), integration = 3.
For the -CH2- Group: Three protons next door means the CH2 signal will be split into 4 (3+1) peaks, called a quartet. Chemical shift = 3.3 (heteroatom region), integration = 2.
Type of protonType of protonChemical shift (ppm)Chemical shift (ppm)
1-31-3 HH NRNR
0.5-50.5-5 HH OROR
6-86-8 HH OArOAr
10-1310-13 CC
OO
HOHO
Chemical shifts for protons on heteroatoms are variable, Chemical shifts for protons on heteroatoms are variable, and signals are often and signals are often broadbroad (not generally useful). (not generally useful).
may beuseful
farleft
13C NMR Spectroscopy
• Carbon-13: only carbon isotope with a nuclear spin
natural abundance of 13C is only 1.1%
(99% of carbon atoms are 12C, with no NMR signal)
• All signals are obtained simultaneously using a broad pulse of energy. The resulting “mass signal” changed into an NMR spectrum mathematically using the operation of Fourier transform (FT-NMR)
• Frequent repeated pulses give many data sets that are averaged to eliminate noise
13C signals go from 0 to 240 ppm. 13C signals: always sharp singlets.
(wider range than in 1H NMR) (1H signals: broad multiplets)
These two facts mean that in carbon-13 NMR, each separate signal is usually visible, and you can accurately count the number of different carbons in the molecule.
Chemical shift affected by electronegativity of nearby atoms:
alkane-like range: 0 – 40 ppm (R-CH2-R)
heteroatom range: 50 – 100 ppm (O-CH2-R)
double bond range: 100 – 220 ppm (sp2 carbons)
No signal overlap!No signal overlap!
13C NMR Spectroscopy
NMR: Scanning for All Nuclei
An instrument can only examine one area at a time.
To see both proton and C-13 nuclei, a very wide region would have to be scanned.
1H area is small
13C area is much wider
Why does 13C NMR give singlets?13C is only 1.1% natural abundant, so most carbons are 12C,
and give no NMR signal.
No splitting seen with carbon, because carbons next to the 13C are likely to be carbon-12:
Sample of 1-Propanol:
12CH3-12CH2-12CH2-OH 12CH3-12CH2-12CH2-OH
12CH3-13CH2-12CH2-OH 13CH3-12CH2-12CH2-OH
12CH3-12CH2-12CH2-OH 12CH3-12CH2-12CH2-OH
12CH3-13CH2-13CH2-OH 12CH3-12CH2-12CH2-OH
NMR: Number of Signals for 13C NMR
How many signals should appear in the carbon-13 NMR spectrum for these compounds?
In theory: 10 4
Signals actually resolved: 10 4
O
octane
13C NMR ExampleNote the wide spectral width and the sharp singlets in the
spectrum below.
Also note that there is no integration with 13C NMR.