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54 Lecture Supplement: Proton NMR Spectroscopy
Proton Nuclear Magnetic Resonance (1H-NMR) Spectroscopy
Fundamental principle: The energy required to cause nuclear spin flip is a function of
the magnetic environment experienced by the nucleus.
Charged particle (nucleus or electron) in motion creates a magnetic field. In an external magnetic field, the magnetic moment (spin) of 1H nucleus (a
proton) becomes aligned with the external field (lower energy) or against it
(higher energy).
Externalmagnetic
field
No external magnetic field With external magnetic field
Addition of energy results in nuclear spin flip:
E ~ 0.02 cal mol-1
= radio wave photons
Add energy
Relax (release energy)
Ground state
Nuclear spin parallel toexternal magnetic field
Lower energy
Excited state
Nuclear spin antiparallelto external magnetic field
Higher energy
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What does an NMR spectrum look like?
NMR signal observed when radio waves supplied match E for nucleus underobservation in the external magnetic field; proton is said to be in resonance.
Downfield (deshielded)low magnetic field strength
Upfield (shielded)high magnetic field strength
Intensity
NMR signal
Information From NMR Spectrum
What information from the NMR spectrum is useful to deduce molecular structure?
Number of signals Position of signals (chemical shift) Relative intensity of signals (integration) Splitting of signals (spin-spin coupling)
Number of Signals
Protons in the same magnetic environment are equivalent: give same signal Nonequivalent protons give different signals Therefore number of signals = number of equivalent sets of protons in molecule
Examples:
HF versus HCl H C
H
H
Cl
H C
H
H
C
H
H
Cl H C
H
H
C
H
H
OH
H C
F
F
C
Cl
Cl
H F C
H
H
C
Cl
Cl
F
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56 Lecture Supplement: Proton NMR Spectroscopy
HH
H
HH
H
HH
H
CH3H
H
Sample Spectra Verify what we have learned about equivalent protons.
CH3 C
CH3
CH3
CH2OH
H
HH
H
OH
OH
Position of Signals: The Chemical Shift
Magnetic field at nucleus influences E
Magnetic field strength
Spin
state
energy
E
spin parallel to applied magnetic field
spin antiparallel to applied magnetic field
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What contributes to magnetic field felt by nucleus?
Magnetic fields of earth and NMR spectrometer: same for all nuclei in molecule Magnetic fields provided by electrons and other nuclei in molecule: varies
between nuclei in molecule
Chemical shift
E scale that is independent of spectrometers magnetic field Chemical shift identical for a given proton regardless of spectrometer used Chemical shift unit = parts per million (ppm) Most proton chemical shifts 0-15 ppm range Represents E relative to reference proton: (CH3)4Si (tetramethylsilane; TMS) =
0.00 ppm by definition
How does chemical shift depend on molecular structure?
Molecule 1H-NMR chemical shift
(CH3)4Si 0.00 ppm
CH4
CH3I
CH3Br
CH3Cl
CH3F
CH2Cl2
CHCl3
CCl4
How does electronegativity influence chemical shift? Electron cloud shields nucleus from applied magnetic field Electronegative atoms remove electron density from proton Less electron density = less shielding = higher chemical shift
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58 Lecture Supplement: Proton NMR Spectroscopy
Characteristic Proton NMR Chemical Shifts (ppm)
RCH3 0.9 RCCH 2.5 ArH 6.5 - 8
RCH2R acyclic 1.3 RNHCH3 2 3 RCHO, ArCHO 9.5 11
RCH2R cyclic 1.5 RCH2X(X = Cl, Br, I)
3.5 RNH2 1 3
R3CH 1.5 2.0 RCH2NR2 2.3 2.7 ArNH2 3 5
CH3R
R R
1.8 OCH3 3.8 R N
O
R
H
5 9
R CH3
O
2.0 2.6 OCH2 3.3 4.1 ROH 1 5
CH2R
O
2.2 3.0 R2C=CH2 5.0 ArOH 4 7
ArCH3 2.3 RCH=CR2 5.3 RCO2H 10 13
ArCH2R 2.6
The exact chemical shift for each proton type does not need to be memorized. This
table will be provided on an exam if needed.
Similar functional group = similar chemical shift.
Characteristic chemical shifts are averages for typical protons. Vary somewhat between
molecules. Example:
CH3O CH3
O
2.01 ppm
CH3
O
2.59 ppm
Other useful chemical shift observations
Chemical shift RCH3 < RCH2R < R3CH, because EN of C > EN of H Electron-withdrawing or donating effects on chemical shift decrease with
distance. Example:
CH3OH CH3CH2OH CH3CH2CH2OH
3.39 ppm 1.18 ppm3.59 ppm
0.93 ppm1.53 ppm
3.49 ppm
CH4
0.23 ppm
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Unlike IR stretching frequencies, we cannotuse characteristic chemical shiftstable to assign proton types to NMR signals. Example: 3.8 ppm not alwaysROCH3 and 2.2 not always ArCH3.Exception: 6.5 8.0 ppm usually (not always)
benzene ring protons.
Probably benzenering protons
3.7 ppm notnecessarily OCH3
2.2 ppm notnecessarily ArCH3
TMS0.00 ppm
Relative Intensity of Signals: Integration
Beers Law: amount of energy absorbed or transmitted moles of stuff present For NMR, amount of radio energy proportional to area under peak in spectrum
(area measurement = integration)
Relative intensities of signals proportional to relative number of equivalentprotons
Integrals do not always correspond to exact number of protons
Sample NMR Spectra Verify what we have learned so far about equivalent protons,
chemical shifts and integration.
H C
H
H
OH4.19 ppm: integral = 1.0 (1 H)
3.41 ppm: integral = 3.0 (3 H)
Smallest integral often set = 1
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60 Lecture Supplement: Proton NMR Spectroscopy
3.19 ppm: integral = 1.0 (6 H)
1.33 ppm: integral = 1.0 (6 H)
Proton count = 6 : 6Integration = 1 : 1
Integration gives ratio of
protons, which is often notthe exact number of protons.
Splitting of NMR Signals: Spin-Spin Coupling
1
H-NMR spectrum of CH3CH2Br: more complex than we might have predicted!
CH3CH2Br
Four lines
a quartet
Three lines
a triplet
3.43 ppm: integral = 1.0 (2 H)
1.68 ppm: integral = 1.5 (3H)
What is the origin of this extra complexity?
Ha C C Hb
Ha, Hb parallel
Ha, Hb antiparallel
Different spin flip energies (E) for Ha
two chemical shifts for Ha Ha signal is
split into two lines (a doublet)
Spin-spin coupling: spin of one nucleus influences the spin of another nucleus
Splitting: effect on NMR spectrum caused by spin-spin coupling
Coupling constant(J): spacing between lines in a splitting pattern J
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What happens to splitting when there is more than one neighbor?
Example: Ha has two neighbors (Hb and Hc)Ha C C Hc
Hb
equalenergy
Two neighborsparallel to Ha
One parallel, one
antiparallel to Ha
Two neighbors
antiparallel to Ha
Three possible neighbor spin
states; Ha signal split into three
lines (a triplet) in 1:2:1 ratio
a triplet
In general: the signal for a proton with n neighbors is split into n+1 lines.
No neighbors singlet; one neighbor doublet; two neighbors triplet;
three neighbors quartet; four neighbors pentet, etc.
Rules and Restrictions for Proton-Proton Spin-Spin Coupling
What are the restrictions on spin-spin coupling? What constitutes a coupling neighbor?
1. Only nonequivalent protons couple.
Ha C
Hb
H
C
Hc
H
C
Hd
H
C
H
H
H
X
X
Hb couples with Hc.
Hb and Ha do not couple because they are equivalent.
Hc and Hd do not couple because they are equivalent.
2. Protons separated by more than three single bonds usually do not couple.
Ha C
Hb
C C
Hc
Hd
X Ha can couple with HbHa can couple with HcHa cannot couple with Hd
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Pi bonds do not count toward the bond limit, but the coupling constants may be too
small to be resolved.
C
Hb
Ha
C
C
Hc
Hd
Ha can couple with Hb, Hc and Hd, butJad may be small.
CH2CH3
CH3
H
X
X
The benzene ring coupling club: a benzene ring
blocks some coupling that otherwise should beobserved (i.e,Jis too small).
All nonequivalent benzene ring protons may couple with each other, but the coupling
constants may be too small to be resolved.
CH3
OCH3Hb
Hc
Ha
Hd
Ha, Hb, Hc and Hd all couple with each other, butJad may be small.
3. Signals for O-H and N-H protons are usually singlets. (Splittingof O-H or N-H protons is observed only in highly pure samples,
and is very rare.)H2N C
H
H
C
H
H
OH
Singlets
Sample Spectra Verify what we have learned about equivalency, chemical shifts,
integration and splitting.
1H-NMR (ppm):
3.39 (triplet, integral = 1.0)
1.87 (sextet, integral = 1.0)
1.03 (triplet, integral = 1.5)
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1H-NMR (ppm):3.79 (septet, integral = 1.0)
1.31 (doublet, integral = 6.0)
1
H-NMR (ppm):5.66 (multiplet, integral = 1.0)
1.98 (multiplet, integral = 2.0)
1.16 (multiplet, integral = 2.0)
Multiplet: a splitting patternthat is too complex to
decipher.
Non-First Order Splitting (such as a doublet of doublets)
Normal splitting patterns occur when coupling constants of neighboring protons are
equal. Example: The CH of CHCH2 is a triplet whenJab = Jac. When Jab Jac, more
complex patterns arise, such as a doublet of doublets.
C C
Ha Hb
Hc
J
Jab =Jacnormal triplet
JabJacdoublet of doublets
For this course, we will assume that coupling constants are usually equal, and that
normal splitting patterns will usually be observed. Exceptions are plentiful.
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1H-NMR (ppm):5.83 (doublet of doublets,
integral = 1.0)
Doublet of doublets = | | | |_
4.93 - 4.82 (multiple, integral
= 2.0)
1.00 (singlet, integral = 9.0)
H
O 1H-NMR(ppm):
10.00 (singlet, integral = 1.0)
7.87 - 7.56 (multiplet, integral
= 5.0)
1H-NMR (ppm):7.38 - 7.00 (multiplet, integral
= 1.7)
2.34 (singlet, integral = 1.0)
Splitting Jab seems to
disappear as Ha Hbchemical shift difference
becomes smaller.
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How do we decude molecular structure from NMR spectra?
Formula: C9H12
1H-NMR: 7.40 7.02 ppm (multiplet, integral = 5); 2.57 ppm (triplet, integral = 2); 1.64ppm (sextet, integral = 2) and 0.94 ppm (triplet, integral = 3).