Transcript Proton NMR Spectroscopy The NMR Phenomenon • Most nuclei possess an intrinsic angular momentum, P. • Any spinning charged particle generates a magnetic field. P.

Proton NMR Spectroscopy
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/2p
where I = spin quantum #
I = 0, 1/2, 1, 3/2, 2, …
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 = 1/2
DE = g(h/2p)Bo
Spin states split in the presence of B0
-1/2 ant iparallel
E
+1/2 parallel
no field
applied field
Bo
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
Presence of Magnetic Field
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 Rf.
Energy Difference (DE) Between Two Different
Spin States of a Nucleus With I=1/2
-1/2
ant iparallel
E
100 MHz
200 MHz
300 MHz
400 MHz
parallel
+1/2
23,500
47,000
70,500
inc. magnet ic field st rengt h, Gauss
B0
104,000
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
How many signals in the
Proton NMR Spectrum?
OCH3
CH3CH2CH2CH3
CH3
O
CH3CH2OCCH2CH3
CH3
OCH3
CH3CH2CH2CH3
2
1
4
CH3
O
CH3CH2OCCH2CH3
4
4
2
CH3
Shielded Protons
• A naked proton will absorb at 70,459 gauss.
• A shielded proton will not absorb at 70,459 gauss so the
magnetic field must be increased slightly to achieve
resonance.
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Chemical Shift
• The variations in the positions of NMR
absorptions, arising from electronic shielding and
deshielding, are called chemical shifts.
• The chemical shift (in ppm) is independent of the
spectrometer used.
• Most common scale is the d (delta) scale.
Typical 1H NMR Scale is
0-10 ppm
The d Scale
Tetramethylsilane (TMS)
Arbitraril y assigne d a ch e m i cal shi
of d 0.00
CH3
CH3SiCH 3
CH3
TMS
Electronegative Atoms
• More electronegative
atoms deshield more and
give larger shift values.
• Additional electronegative
atoms cause an increase in
chemical shift.
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Chapter 13
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Location of Electronegative
Atoms
• The deshielding effect of an electronegative
substituent drops off rapidly with distance.
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Chapter 13
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Chemical Shift Ranges, ppm
Diamagnetic Anisotropy
Shielding and Deshielding
Deshielding in Alkenes
Shielding in Alkynes
Chemical Shift is Affected by
Electron Density Around Nucleus
Increased elect ron density
Shields nucleus
u pfie l d sh ift
Decreased electron density
Deshields nucleus
down fie ld sh ift
from chemistry.msu.edu
Number of Signals
• Methyl tert-butyl ether has two types of protons, giving
two NMR signals.
• Chemically equivalent hydrogens have the same chemical
shift. All the methyl groups of the tert-butyl group are
equivalent, and they produce only one signal.
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Chapter 13
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Intensity of Signals: Integration
• The amount the integral trace rises is proportional to the
area of that peak.
• The integration will have a trace for the tert-butyl
hydrogens that is three times as large as the trace for the
methyl hydrogens. The relative area for methyl and tertbutyl hydrogens is 1:3.
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tert-Butyl Acetoacetate
• The spectrum of tert-butyl acetoacetate has only three
signals. The most shielded protons are the methyl groups
of the tert-butyl. The most deshielded signal is the
methylene (CH2) because it is in between two carbonyl
groups.
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Chapter 13
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Toluene at Higher Field
Integral Trace
How many protons give rise to each signal?
Spin-Spin Splitting
The Doublet in
1H
NMR
H a Hb
C
B0
C
Ha i s cou ple d to bH
Hb i s parall e l or an ti -parall e l 0to B
Ha spli ts i n to a 1:1
doublet pe ak
Hb in 1,1,2-Tribromoethane
The Triplet in
1H
NMR
H a Hb
C
C
Hb
Ha i s cou ple d to bHan d Hb
B0
Hb can both be parall e l, anti-parall e
or on e paral le l an d on e an ti -paral l
Ha spli ts i n to a 1:2:1
triplet pe ak
Ha in 1,1,2-Tribromoethane
1,1,2-Tribromoethane
The Quartet in 1HMR
C
H
H
C H
H
proton spl its i n to n +1
qu arte t 1:3:3:1
n = # adjacent H's
B0
deshielded
shielded
Chemical Shift
The N + 1 Rule
If a proton is coupling to N equivalent protons,
(on adjacent atoms) it is split into N + 1 peaks.
Spin-Spin Splitting Distance
• Equivalent protons do not split each other.
• Protons bonded to the same carbon will split
each other if they are nonequivalent.
• Protons on adjacent carbons normally will split
each other.
• Protons separated by four or more bonds will
rarely split each other.
Predict Splitting
ClCH2CH2Br
CH3CHCl2
ClCH2CH2Cl
CH3
CH3CH2NO2
CHCl
CH3
CH3
CH3
C CH2Br
CH3
1,1-Dichloroethane
Ethyl benzene
Methyl Isopropyl Ketone
1-Nitropropane
O2N C H2C H2C H3
Ethyl, propyl and isopropyl groups
are common. Learn to recognize
them from their splitting patterns.
2-Methyl-1-propanol
CH3
HO C H2C H
CH3
Solved Problem 3
Propose a structure for the compound of molecular formula C 4H10O whose proton NMR spectrum
follows.
Solution
Solved Problem 3 (Continued)
Solution (Continued)
Para Nitrotoluene
C-13 NMR Spectroscopy
C-13 chemical shifts
One signal for each chemically unique carbon
Coupling in C-13 NMR
the doublet in C-13 NMR
H
C
met hine group
B0
C is cou pl e d to H
H is paral le l or an ti-paral le l0to B
C spl i ts i n to a 1:1
doublet pe ak
the triplet in C-13 NMR
B0
Ha
C
Hb
met hylene group
C i s cou pl e d to aHan d Hb
Ha & Hb can both be paral l e l , an ti -par
or on e paral l e l an d on e an ti -paral l e l
C spl i ts i n to a 1:2:1
triplet pe ak
the quartet in C-13 NMR
H
C H
H
methyl group
carbon splits into n+1
quarte t 1:3:3:1
n = # attached H's
B0
deshielded
shielded
Butanone - Coupled and Decoupled
1,2,2-Trichloropropane
1H and 13C NMR Spectra
Coupled C-13 NMR Spectrum
HC
CCH2CH2CH2CH3
CH2CO 2H
SP A
O
O
CH3CH2OCCH2CCH3
coupled spectrum