Download 13 - University of Miami

Chemistry 201 - Section C
Structure Determination:
Nuclear Magnetic Resonance
Spectroscopy
This presentation was created by
Professor Carl H. Snyder
Chemistry Department
University of Miami
Coral Gables, FL 33124
[email protected]
Copyright 2004 by Carl H. Snyder,
University of Miami. All rights
reserved.
Nuclei in a Magnetic Field
Nuclear spin gives nuclei of some elements the
properties of bar magnets.
In a magnetic field, a small majority of these bar
magnets line up parallel to the field. A minority are
antiparallel.
disorganized
disorganized
organized, mostly parallel
organized, mostly parallel
Nuclear Magnetic Resonance
When the energy of an applied radio frequency equals
the energy difference between parallel and antiparallel
nuclei, low-energy parallel nuclei absorb the radio
energy and undergo spin-flip to a higher energy
antiparallel state..
∆Ε = hν
Energy-absorbing nuclei are said to be in resonance.
Nuclear Spin Energy and
Magnetic Field Strength
Antiparallel nuclei exist at higher energy levels than
parallel nuclei.
The greater the strength of the magnetic field, the
greater the energy difference between the nuclei with
parallel spins and nuclei with antiparallel spins.
Shielding
Molecular structural factors affect electron densities near
different kinds of protons.
Different kinds of protons lie in different electrical
environments.
High electron densities tend to shield protons from the
full effects of an applied magnetic field.
Local shielding can decrease the full effects of an
applied magnetic field.
A local magnetic field can reduce the external field
applied by the magnet, producing a smaller
effective,local field.
Different kinds of protons, in different, local magnetic
environments, resonate at different radio frequencies.
Beffective = Bapplied - Blocal
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δ (Delta), The Chemical Shift
If all protons were shielded to the very same extent, all
protons would experience the same magnetic field
strength and all protons would spin-flip with the same
energy and therefore at the very same radio frequency.
Protons experiencing different shielding absorb energy
at different radio frequencies.
The absorption frequency is measured as the chemical
shift, δ, (delta).
Delta is measured with reference to TMS,
tetramethylsilane, (CH3)4Si, a convenient standard
reference.
The 1H NMR Region
TMS defines δ = 0 ppm
Upfield and high-field lie to the right, toward
lower values of δ.
Downfield and low-field lie to the left, toward
higher values of δ.
Number of Absorption Regions
Only one kind of H (12 allylic) in
2,3-dimethyl-2-butene.
NMR absorption in only one region, about 1.7 ppm.
Definition of δ
Observed chemical shift (separation
from TMS in Hz)
δ = ---------------------------------------------Spectrometer frequency in MHz
1H
NMR or PMR (Proton
Magnetic Resonance)
1) Number of absorption regions - How
many chemically different kinds of
hydrogen are present.
2) Chemical shifts - Chemical
environments of the hydrogens present.
3) Relative peak areas - Relative
numbers of each kind of hydrogen.
4) Spin-spin splitting patterns Numbers of β hydrogens present.
Number of Absorption Regions
Two different hydrogens of methyl acetate
produce absorptions in two different NMR
regions.
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Chemical Shifts
Chemical Shifts
Chemical Shifts
Chemical Shifts
Allylic H’s absorb at about 1.7 ppm.
Chemical Shifts
The CH3 adjacent to the C=O (“methyl ketone”)
absorbs at 2.1 ppm.
The CH3 bonded to the O absorbs at 3.7 ppm
(not shown in Table.)
Chemical Shifts
The CH3 hydrogens of the tert-butyl group absorb at
1.2 ppm.
As in methyl acetate, the CH3 hydrogens of the methyl
group bonded to the O absorb at 3.6 ppm.
The stepped trace gives relative peak areas.
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Relative Peak Areas
Every individual H absorbs with the same intensity as
every other H.
Relative peak areas are given by the heights of the
steps
Peak area ratios equal ratios of numbers of H’s
producing the peaks.
In this case, 2:6 = 1:3 = 3:9.
Spin-Spin Splitting - Origin
Spin-Spin Splitting - Origin
Spin-Spin Splitting
Steps ratio is about 3:4.5.
This equals the 2:3 ratio of the H’s associated
with each peak.
The inserted multiplets reveal spin-spin splitting.
Spin-Spin Splitting - Origin
Spin-Spin Splitting
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Spin-Spin Splitting
Spin-Spin Splitting - The (n+1) Rule
n refers to the number of mutually equivalent
protons on the neighboring carbon(s).
Spin-Spin Splitting - The Ethyl
Group
The CH3 protons appear as a triplet because they face 2
neighboring protons.
The CH2 protons appear as a quartet because they face
3 neighboring protons.
The triplet-quartet is the NMR signature of an ethyl
group.
Isopropyl Bromide
Area ratio - 1.1:6.5 = 1:6
Multiplicity - 7:2
Absorptions in 2 NMR regions
Spin-Spin Splitting Multiplicity
An unsplit peak is the signature of
hydrogens on a carbon that face no
neighboring protons.
A triplet-quartet (3:4) is the signature of
an ethyl group.
A doublet-heptet (2:7) is the signature of
an isopropyl group
Summary of Multiplicities
For a pentet, the ratio of intensities is
1:5:10:10:5:1
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13C
NMR Spectrometry
Spin-spin splitting patterns are of little use in 13C NMR
spectrometry.
Area ratios are also of little use.
Chemical shifts are of some value.
The number of different C atoms is reflected by the
number of absorption regions.
Methyl Acetate: 1H And 131C NMR
H at top
Protons on
chemically
different CH3’s
have different
values of δ.
13C at bottom
Chemically
different 13C’s
have different
values of δ.
End
Structure Determination:
Nuclear Magnetic Resonance
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