Chapter 13
Proton Nuclear Magnetic Resonance – H1 NMR
Presented by: Amey Deshpande
Chapter 13
Overview1. Introduction2. Relaxation process3. NMR signals4. Number of signals5. Positions of signals (Chemical Shift)6. Factors affecting chemical shift7. Solvents used8. Peak area and proton counting9. Splitting of signals/ spin-spin coupling10. Coupling constant – “J”11. Important tips for Interpreting an NMR Spectra12. Applications of NMR13. Limitations of NMR14. Conclusion
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1. Introduction
Nucleus is involved Magnetic fields
are involved
Resonance – two oscillating things
(EM wave + precessing
protons)
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1. Introduction
• A nucleus with an odd atomic number or an odd mass number has a nuclear spin
• The spinning charged nucleus generates a magnetic field
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1. Introduction• Precessional Motion spinning axis revolves around vertical axis
• Precessional Frequency (ν) number of revolutions
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1. Introduction
• Little Physics!
ω = γH0
Where, ω= angular Precessional velocity
γ gyro-magnetic ratio/ Nuclear const.
H0 applied magnetic field (Gauss)And γ= 2 π μ
h I μ magnetic moment of spinning bar magnet
h Plank’s constant
I spin quantum number
But γH0= 2πv
v EM radiation frequency
Hence, from 1,2 & 3
1
3
2
angular Precessional velocity, ω = 2πv
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N SN S
Add Energy
N SS N
Aligned = Low Energy
Excited state = High energy
N SS N
Energy Released
Back to low energy ground state
• When the spin falls back into line with the magnetic field it releases energy. We detect this energy and it provides information on:
• The environment of the hydrogen in the molecule
• How many hydrogen atoms are in that environment.
1. Introduction- A proton in magnetic field
N SN S
Add Energy
N SS N
Aligned = Low Energy
Excited state = High energy
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1. Introduction• Precessional Frequency – some more Physics!!
γH0
2 πv = For a proton, γ= 26750
When H0=14092 Gauss, energy required to cause flipping is calculated as…..
26750 X 14092
2 X 3.14v = =60,025,636 ≈60 million cycles per sec
= 60 MHz
= RADIO FREQUENCY!!
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1. Introduction- NMR spectrum
• Different sets of proton different precessional frequency absorb at different radio frequencies
• Practically, It is convenient to keep radiofrequency constant & vary magnetic field strength
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2. Relaxation process(non-radiational transitions)
Spin-spin relaxation• Mutual exchange of
spin• Transfer of energy
from one nucleus to other
• No loss of energy• COUSES LINE
BROADENING (solids)
Spin-Lattice Relaxation (Longitudinal relaxation)
• transfer of energy from the nucleus in its higher energy state to the molecular lattice• energy is transferred as vibrational, transitional & rotational•Less effective in solids• causes broadening•KEEPS EXCESS OF NUCLEI IN THE LOWER ENERGY STATE
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3. NMR signals• The number of signals shows how
many different kinds of protons are present.
• Magnetically equivalent protons are also chemically equivalent protons
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4.No of signalsEquivalent H’s• Two H’s that are in identical environments
(homotopic) have the same NMR signal• Test by replacing each with X
if they give the identical result, they are equivalentProtons are considered homotopic
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Nonequivalent H’s• Replacement of each H with “X” gives a
different constitutional isomer
• Then the H’s are in constitutionally heterotopic environments and will have different chemical shifts – they are nonequivalent under all circumstances
4.No of signals
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4.No of signalsEnantiotopic Distinctions• If H’s are in environments that are mirror images
of each other, they are enantiotopic• Replacement of each H with X produces a set of
enantiomers• The H’s have the same NMR signal (in the
absence of chiral materials)
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5. Position of Signals (Chemical Shift)Magnetic Shielding
• If all protons absorbed the same amount of energy in a given magnetic field, not much information could be obtained
• But protons are surrounded by electrons that shield them from the external field
• Circulating electrons create an induced magnetic field that opposes the external magnetic fieldEffective magnetic field
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5. Position of Signals (Chemical Shift)Shielded Protons
• Magnetic field strength must be increased for a shielded proton to flip at the same frequency
• Differences detected by machine, cause differences in signals (chemical shift, )
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5. Position of Signals (Chemical Shift)
Scale of NMR Spectra : “Tetramethyl silane” as Reference
δ τ
τ=10 - δ
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5. Position of Signals (Chemical Shift)“Tetramethyl silane” as Reference
• miscible with almost all organic substances
• highly volatile – readily removed from the system
• does not interact with sample• Since silicon is less electronegative than carbon, TMS protons are highly shielded. Signal defined as zero.
• Organic protons absorb downfield (to the left) of the TMS signal
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6. Factors influencing Chemical Shift
1. Inductive effect
How does electronegativity influence chemical shift?•Chemical shift related to magnetic field strength at nucleus•Electron cloud shields nucleus from effects of Bo
Increasing EN of X
H X H X H X H X
Decreasing electron density around HDeshielding
Downfield chemical shift
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2. Anisotropic effect6. Factors influencing Chemical Shift
Alkenes/ vinyl protons
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2. Anisotropic effect6. Factors influencing Chemical Shift
Alkynes/ Acetylenic Protons
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2. Anisotropic effect6. Factors influencing Chemical Shift
Aromatic Protons
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3. Van der Waal’s Deshielding
6. Factors influencing Chemical Shift
• Observed in overcrowded molecule, with sterically hindered protons
• Adjacent bulky group repels the cloud of e- around H+
• Causes Deshielding – Downfield Shift
H
CF3
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4. Hydrogen bonding
6. Factors influencing Chemical Shift
• hydrogen bonded proton is bound to highly EN group/ atom
•EN group pulls the e- cloud of proton deshielding downfield chemical shift
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Typical Values
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7. Solvents Used• Important characteristics for solvents
1. Chemically inert and magnetically isotropic
2. Devoid of hydrogen atom
3. Should dissolve the sample to a reasonable extent
•Examples
Carbon tetra chloride (CCl4), Carbon disulphide (CS2), Duteriochloroform (CDCl3), Hexachloroacetone((CCl3)2C=O)Some other solvents require corrections of the order 0.5 ppm or more in tau value
e.g. Pyridine, Dioxane, acetonitrile, Trichloro acetonitrile
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8. Splitting of the signals (Spin-spin coupling)
Some Useful Terms
Spin-spin coupling: one nuclear spin influences spin of another nucleus
Splitting: effect on NMR signal caused by spin-spin coupling
Coupling constant (J): spacing between lines in a splitting pattern
J
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9. Splitting of the signals (Spin-spin coupling)
Rules and Restrictions
General rule: the signal for a proton with n neighbors is split into n+1 lines
Rules and Restrictions for Proton-Proton Spin-Spin Coupling1. Only nonequivalent protons couple
•Hb couples with Hc
Ha C
Hb
H
C
Hc
H
C
Hd
H
C
H
H
H
X
X
•Hb and Ha do not couple because they are equivalent
•Hc and Hd do not couple because they are equivalent
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9. Splitting of the signals (Spin-spin coupling)
Rules and Restrictions
2. Protons separated by more than three single bonds usually do not couple
•Ha couples with Hb
•Ha couples with Hc
•Ha does not couple with Hd
Pi bonds do not count toward this bond limit, but J may be too small to observe
•Ha couples with Hb
•Ha couples with Hc
•Ha couples with Hd but J may be very small
Ha C
Hb
C C
Hd
Hc
X
free spacer
CC
Ha
Hd
C
HcHb
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9. Splitting of the signals (Spin-spin coupling)
Rules and Restrictions
2. Protons separated by more than three single bonds usually do not couple
•Benzene ring = one big free spacer
•All benzene ring protons may couple with each other but J may be small
•Ha, Hb, Hc, and Hd all couple with each other
•Jad may be too small to observe
Benzene ring blocks some coupling that we expect to observe
CH3
H
CH2CH3
F
Hd
Ha
Cl
Hc
Hb
X
X
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9. Splitting of the signals (Spin-spin coupling)
3. Signals for O-H and N-H are usually singlets
•Splitting of O-H or N-H protons may be observed in rare circumstances
singletsinglet
triplettriplet
Signal SplittingRules and Restrictions
H2N C
H
H
C
H
H
OH
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10. Coupling constant – ‘J’
• Distance between the peaks of a split signal
• Measured in Hz (usually 0-18)
• Not dependent on strength of the external field
• Gives info on type of HMultiplets with the same coupling constants may
come from adjacent groups of protons that split each other
Structural features
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11. Important tips Interpreting an NMR Spectra
1. regarding chemical shift valuesi. tau values of :- methyle> methylene >methine
ii. tau value depends upon – nature of substituent on carbon aton bearing proton i.e. greater EN of substituent lower is tau value
iii. tau value depends on type of hybrid orbital holding proton :- sp3 > sp > sp2
iv. tau value of aromatic proton is always < 4 ppm the value depends on the degree & nature of substitution
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11. Important tips Interpreting an NMR Spectra
1. regarding chemical shift valuesv. tau value of aldehydic proton are usually lower, i.e. 0.8
ppm or lower
vi. Tau value in cyclic compound is always higher than that of any other proton. Protons in cyclopropane has maximum tau value
vii. tau for –OH an –NH proton depends on temp, solvent,
conc. and neighboring group…e.g- for alcoholic –OH its 4.5 - 9 τ and for phenolic –OH its -2 to 6 τ
viii. for –COOH tau is -0.5 to -2.0
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11. Important tips Interpreting an NMR Spectra
2. The number of signals in an NMR spectrum tells the number of sets of different the protons in different chemical environments
3. It also tells the number of equivalent protons causing the splitting of signals
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12.
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13. Limitations of NMR
1. Lack of sensitivity – abt 1 ml sample of at least 1% conc.
2. Overlapping of spectra in some compounds –difficult to interpret
3. NO information about molecular weight is given but RELATIVE number of different protons present are only known
4. Mostly ONLY LIQUIDS can be studied (solid polymers are preheated with solvents)
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14. Conclusion
• NMR uses magnetic property of nucleus for obtaining Spectra
• Phenomenon like chemical shift, spin-spin coupling and signal splitting help to analyze the sample by interpreting the spectra
• Although it has wide range of applications, it has got few limitations also
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THANK YOU