proton nuclear magnetic resonance – h1 nmr

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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Alkynes/ Acetylenic Protons

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Aromatic Protons

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3. Van der Waal’s Deshielding


• 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


• 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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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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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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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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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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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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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

Chapter 13

THANK YOU

proton nuclear magnetic resonance – h1 nmr

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