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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/2 p where I = spin quantum # I = 0, 1/2, 1, 3/2, 2, …. Which nuclei have a “spin”?. - PowerPoint PPT Presentation
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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/2where 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 = 1/2E = h/2)Bo
Spin states split in the presence of B0
no field applied field
E
+1/2 parallel
-1/2 antiparallel
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 (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
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?
CH3CH2CH2CH3
OCH3
CH3CH2OCCH2CH3
O
CH3
CH3
CH3CH2CH2CH3
OCH3
CH3CH2OCCH2CH3
O
CH3
CH3
2 1 4
4 4 2
© 2013 Pearson Education, Inc. Chapter 13 12
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.
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 (delta) scale.
Typical 1H NMR Scale is 0-10 ppm
The Scale
Tetramethylsilane (TMS)
CH3SiCH3
CH3
CH3
TMS
Arbitrarily assigned a chemical shift
of 0.00
© 2013 Pearson Education, Inc. Chapter 13 22
Electronegative Atoms
• More electronegative atoms deshield more and give larger shift values.
• Additional electronegative atoms cause an increase in chemical shift.
© 2013 Pearson Education, Inc. Chapter 13 23
Location of Electronegative Atoms
• The deshielding effect of an electronegative substituent drops off rapidly with distance.
Chemical Shift Ranges, ppm
Diamagnetic AnisotropyShielding and Deshielding
Deshielding in Alkenes
Shielding in Alkynes
Chemical Shift is Affected by Electron Density Around Nucleus
Increased electron density
Decreased electron density Deshields nucleus
upfield shiftShields nucleus
downfield shift
from chemistry.msu.edu
© 2013 Pearson Education, Inc. Chapter 13 31
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.
© 2013 Pearson Education, Inc. Chapter 13 32
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 tert-butyl hydrogens is 1:3.
© 2013 Pearson Education, Inc. Chapter 13 33
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.
Toluene at Higher Field
Integral TraceHow many protons give rise to each signal?
Spin-Spin Splitting
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
Hb in 1,1,2-Tribromoethane
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
Ha in 1,1,2-Tribromoethane
1,1,2-Tribromoethane
The Quartet in 1HMR
B0
C
H
C
H
H
H
proton splits into n+1
n = # adjacent H'squartet 1:3:3:1
shieldeddeshielded
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.
• 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.
Spin-Spin Splitting Distance
Predict Splitting
ClCH2CH2Br
ClCH2CH2Cl
CH3CHCl2
CH3CH2NO2
CHCl
CH3
CH3
CH3 C
CH3
CH3
CH2Br
1,1-Dichloroethane
Ethyl benzene
Methyl Isopropyl Ketone
1-Nitropropane
CH2CH2CH3O2N
Ethyl, propyl and isopropyl groups are common. Learn to recognize them from their splitting patterns.
2-Methyl-1-propanol
HOCH2CH
CH3
CH3
Propose a structure for the compound of molecular formula C4H10O whose proton NMR spectrum follows.
Solved Problem 3
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
methine group
B0
the doublet in C-13 NMR
C splits into a 1:1 doublet peak
H is parallel or anti-parallel to B0
C is coupled to H
C
H
methylene group
C
H
HB0 a
the triplet in C-13 NMR
C splits into a 1:2:1 triplet peak
Ha & Hb can both be parallel, anti-parallel
C is coupled to Ha and Hb
b
or one parallel and one anti-parallel
B0
C
H
H
H
the quartet in C-13 NMR
carbon splits into n+1
n = # attached H'squartet 1:3:3:1
shieldeddeshielded
methyl group
Butanone - Coupled and Decoupled
1,2,2-Trichloropropane1H and 13C NMR Spectra
Coupled C-13 NMR Spectrum
HC CCH2CH2CH2CH3
CH2CO2H
SPA
CH3CH2OCCH2CCH3
O O
coupled spectrum