Download - Proton NMR Spectroscopy
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Proton NMR Spectroscopy
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The NMR Phenomenon
• Most nuclei possess an intrinsic angular momentum, P.
• Any spinning charged particle generates a magnetic field.
P = [I(I+1)]1/2 h/2where spin quantum #
I = 0, 1/2, 1, 3/2, 2, …
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Which nuclei have a “spin”?• If mass # and atomic # are both even, I = 0 and the
nucleus has no spin. e.g. Carbon-12, Oxygen-16
• For each nucleus with a spin, the # of allowed spin states can be quantized:
• For a nucleus with I, there are 2I + 1 allowed spin states.
1H, 13C, 19F, 31P all have I = 1/2E = h/2)Bo
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Spin states split in the presence of B0
no field applied field
E
+1/2 parallel
-1/2 antiparallel
Bo
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When a nucleus aligned with a magnetic field, B0, absorbs radiation frequency (Rf), it can change spin orientation to a higher energy spin state. By relaxing back to the parallel (+1/2) spin state, the nucleus is said to be in resonance. Hence,
NMR
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Presence of Magnetic Field
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NMR instruments typically have a constant Rf and a variable B0.
A proton should absorb Rf of 60 MHz in a field of 14,093 Gauss (1.4093 T).
Each unique probe nucleus (1H perhaps) will come into resonance at a slightly different - and a very small percentage of - the Rf.
All protons come into resonance between 0 and 12/1,000,000 (0 – 12 ppm) of the B0.
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• Nuclei aligned with the magnetic field are lower in energy than those aligned against the field
• The nuclei aligned with the magnetic field can be flipped to align against it if the right amount of energy is added (DE)
• The amount of energy required depends on the strength of the external magnetic field
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NMR Spectrometer• Schematic diagram of a nuclear magnetic
resonance spectrometer.
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Energy Difference (E) Between Two Different
Spin States of a Nucleus With I=1/2
+1/2
-1/2
E 400 MHz300 MHz200 MHz100 MHz
23,500 47,000 70,500 104,000
parallel
antiparallel
inc. magnetic field strength, Gauss
B0
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What Does an NMR Spectrum Tell You?
• # of chemically unique H’s in the molecule # of signals
• The types of H’s that are present e.g. aromatic, vinyl, aldehyde …
chemical shift• The number of each chemically unique H
integration• The H’s proximity to eachother
spin-spin splitting
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Chemical EquivalenceHow many signals in 1H NMR spectrum?
O OO
O
CH3
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Number of Equivalent Protons
O
OOO
6 4
CH3
1 3 5 2 4
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Homotopic H’s– Homotopic Hydrogens
• Hydrogens are chemically equivalent or homotopic if replacing each one in turn by the same group would lead to an identical compound
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Enantiotopic H’s• If replacement of each of two
hydrogens by some group leads to enantiomers, those hydrogens are enantiotopic
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Diastereotopic H’s• If replacement of each of two hydrogens
by some group leads to diastereomers, the hydrogens are diastereotopic
– Diastereotopic hydrogens have different chemical shifts and will give different signals
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Vinyl Protons
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Typical 1H NMR Scale is 0-10 ppm
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The Scale
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Tetramethylsilane (TMS)
CH3SiCH3
CH3
CH3TMS
Arbitrarily assigned a chemical shift of 0.00
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Chemical Shift Ranges, ppm
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Diamagnetic AnisotropyShielding and Deshielding
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Deshielding in Alkenes
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Shielding in Alkynes
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Integration
CH3
CH3
CH3
Br
H H
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Methyl t-butyl ether (MTBE)
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Toluene at Higher Field
Splitting patterns in aromatic groups can be confusing
A monosubstituted aromatic ring can appear as an apparent singlet or a complex pattern of
peaks
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Integral Trace
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Signal Splitting; the (n + 1) Rule
• Peak:Peak: The units into which an NMR signal is split; doublet, triplet, quartet, multiplet, etc.
• Signal splitting:Signal splitting: Splitting of an NMR signal into a set of peaks by the influence of neighboring nonequivalent hydrogens.
• ((nn + 1) rule: + 1) rule: If a hydrogen has n hydrogens nonequivalent to it but equivalent among themselves on the same or adjacent atom(s), its 1H-NMR signal is split into (n + 1) peaks.
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Spin-Spin Splitting
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The Doublet in 1H NMR
C C
HH
B0
a b
Ha splits into a 1:1 doublet peak
Hb is parallel or anti-parallel to B0
Ha is coupled to Hb
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Hb in 1,1,2-Tribromoethane
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The Triplet in 1H NMR
C C
H
H
H
B0
a b
Ha splits into a 1:2:1 triplet peak
Hb can both be parallel, anti-parallel Ha is coupled to Hb and Hb
b
or one parallel and one anti-parallel
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Ha in 1,1,2-Tribromoethane
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1,1,2-Tribromoethane
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The Quartet in 1HMR
B0
C
H
CH
HH
proton splits into n+1
n = # adjacent H'squartet 1:3:3:1
shieldeddeshielded
Chemical Shift
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Summary of Signal Splitting• The origins of signal splitting patterns. Each arrow
represents an Hb nuclear spin orientation.
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1,1-Dichloroethane
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Ethyl benzene
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CH3CH2OCH3
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Equivalent Protons do not Couple
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Pascal’s Triangle
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Signal Splitting (n + 1)
ProblemProblem: Predict the number of 1H-NMR signals and the splitting pattern of each.
CH3CCH2CH3O
CH3CH2CCH2CH3O
CH3CCH(CH3)2O
(a)
(b)
(c)
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Differentiate using 1H NMR
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Methyl Isopropyl Ketone
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1-Nitropropane
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Para Nitrotoluene
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Coupling Constants (J values)
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Bromoethane
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Coupling Constants– An important factor in vicinal coupling is the angle
between the C-H sigma bonds and whether or not it is fixed.
– Coupling is a maximum when is 0° and 180°; it is a minimum when is 90°.
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Physical Basis for (n + 1) Rule• Coupling of nuclear spins is mediated through
intervening bonds.– H atoms with more than three bonds between them
generally do not exhibit coupling.– For H atoms three bonds apart, the coupling is called
vicinal coupling.
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Physical Basis for (n + 1) Rule
• Coupling that arises when Hb is split by two different nonequivalent H atoms, Ha and Hc.
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More Complex Splitting Patterns
– Complex coupling that arises when Hb is split by Ha and two equivalent atoms Hc.
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More Complex Splitting Patterns– Since the angle between C-H bond determines the extent of
coupling, bond rotation is a key parameter.– In molecules with free rotation about C-C sigma bonds, H
atoms bonded to the same carbon in CH3 and CH2 groups are equivalent.
– If there is restricted rotation, as in alkenes and cyclic structures, H atoms bonded to the same carbon may not be equivalent.
– Nonequivalent H on the same carbon will couple and cause signal splitting.
– This type of coupling is called geminal couplinggeminal coupling.
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More Complex Splitting Patterns
– In ethyl propenoate, an unsymmetrical terminal alkene, the three vinylic hydrogens are nonequivalent.
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More Complex Splitting Patterns– Tree diagram for the complex coupling seen for the
three alkenyl H atoms in ethyl propenoate.
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More Complex Splitting Patterns– Cyclic structures often have restricted rotation about
their C-C bonds and have constrained conformations.– As a result, two H atoms on a CH2 group can be
nonequivalent, leading to complex splitting.
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More Complex Splitting Patterns– A tree diagram for the complex coupling seen for the
vinyl group and the oxirane ring H atoms of 2-methyl-2-vinyloxirane.
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More Complex Splitting Patterns
• Complex coupling in flexible molecules.– Coupling in molecules with unrestricted bond rotation often
gives only m + n + I peaks.– That is, the number of peaks for a signal is the number of
adjacent hydrogens + 1, no matter how many different sets of equivalent H atoms that represents.
– The explanation is that bond rotation averages the coupling constants throughout molecules with freely rotation bonds and tends to make them similar; for example in the 6- to 8-Hz range for H atoms on freely rotating sp3 hybridized C atoms.
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More Complex Splitting Patterns– Simplification of signal splitting occurs when
coupling constants are the same.
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More Complex Splitting Patterns
– Peak overlap occurs in the spectrum of 1-chloro-3-iodopropane.
– Hc should show 9 peaks, but because Jab and Jbc are so similar, only 4 + 1 = 5 peaks are distinguishable.
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Styrene
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Ha splitting in Styrene“Tree” Diagram
C CH
HHa
b
c
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In the system below, Hb is split by two different sets of hydrogens : Ha and Hc
– Theortically Hb could be split into a triplet of quartets (12 peaks) but this complexity is rarely seen in aliphatic systems
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0.750.50
60 MHz
100 MHz
300 MHz
300 240 180 120 60 0 Hz
300 240 180 120 60 0 Hz
300 240 180 120 60 0 Hz
Why go to a higher field strength?
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0.75 (t, 2H, J=10)0.50 (t, 2H, J=10)
60 MHz
100 MHz
300 MHz
300 240 180 120 60 0 Hz
300 240 180 120 60 0 Hz
300 240 180 120 60 0 Hz
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Homonuclear Decoupling
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Allyl Bromide
CH2=CHCH2Br
CH2=CHCH2Br
irradiate
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1-Phenyl-1,2-dihydroxyethane
C
H
OH
C
H
H
OH
a b
c
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1-Phenyl-1,2-dihydroxyethane
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Irradiate at 4.8