1. NUCLEAR MAGNETIC RESONANCE II
Instrumental Analysis
NUCLEAR MAGNETIC RESONANCE II
Dr. Nermin Salah
12th Lecture

2. Objectives Magnetic Shielding Chemical Shift
Factors Affecting Chemical Shift
Inductive Effect by Electronegative Groups
s-character (Hybridization Effect)
Magnetic Anisotropic Effect
Hydrogen Bonding
Equivalent and Nonequivalent Protons
Integration of NMR Signal
Spin-Spin Splitting (Spin-Spin Coupling)

3. MAGNETIC SHIELDING
NMR would not be very valuable if all protons absorbed at the same frequency.
Different protons usually absorb different radiofrequencies (ν ).
The electrons in a bond shield the nuclei from the magnetic field. A moving charge (electron) creates a magnetic field, and the field created by the moving electrons opposes the applied magnetic field Bo.
The electrons in a bond shield the nuclei from the magnetic field.

4. The more electron density around a proton, the more the shield, the lower magnetic field affecting the proton ,
The less electron density , the less the shield , the higher magnetic field.

5. Shielded Protons
The effective magnetic field, therefore, what the hydrogen nuclei actually “sense” through the surrounding electronic environment is always less than the actual applied magnetic field B0.
Beffective = Bapplied  Blocal
1 Gauss= Tesla
In the classical NMR experiment, magnetic field strength must be increased for a shielded proton to flip at the same frequency.

6. Low external applied field
Higher external applied field

7. Protons in a molecule upfield Lower frequency downfield
higher frequency

8. The CHEMICAL SHIFT shift in Hz chemical = d = = ppm shift
The “chemical shift” is a field independent value.
Chemical shift is : the difference in frequency between the sample and
the standard over the operation frequency.
chemical
shift
= d =
shift in Hz
spectrometer frequency in MHz
= ppm
This division gives a number independent
of the instrument used.
parts per
million
A particular proton in a given molecule will always come at the same chemical shift (constant value).
Of course, we don’t do any of this, it’s all done automatically by the NMR machine.

9. Tetramethylsilane (TMS)
TMS is added to the sample as internal standard.
TMS protons are all identical, highly shielded providing a single sharp peak always isolated from peaks of interest. The TMS was assigned d = 0.00.
Organic protons absorb downfield (to the left) of the TMS signal.
TMS is inert , highly soluble in organic liquids and easily removed from samples by distillation.

10. FACTORS AFFECTING CHEMICAL SHIFT
Four major factors account for the resonance
positions (on the ppm scale) of most protons
Deshielding by electronegative elements Inductive effect by electronegative groups
s-character (hybridization effect)
Magnetic Anisotropic effect (magnetic fields usually due to -bonded electrons in the molecule(
Hydrogen bonding.

11. C H Cl d- d+ 1. DESHIELDING BY ELECTRONEGATIVE ELEMENTS
Chlorine “deshields” the proton,
that is, it takes valence electron
density away from carbon, which
in turn takes more density from
hydrogen deshielding the proton.
“highly shielded”
protons appear
at upfield
(lower )
“deshielded“
at downfield
(higher )
deshielding moves proton
resonance to lower field and higher 
NMR CHART

12. ELECTRONEGATIVITY DEPENDENCE OF
CHEMICAL SHIFT
Dependence of the Chemical Shift of CH3X on the Element X
Compound CH3X
CH3F CH3OH CH3Cl CH3Br CH3I CH4 (CH3)4Si
Element X
F O Cl Br I H Si
Electronegativity of X
Chemical shift d
most
deshielded
TMS
deshielding increases with the
electronegativity of atom X

13. SUBSTITUTION EFFECTS ON CHEMICAL SHIFT
Cumulative Effect
most
deshielded
The effect
increases with
greater numbers
of electronegative
atoms.
CHCl3 CH2Cl2 CH3Cl
ppm
CH2Br CH2CH2Br  CH2CH2CH2Br
ppm
most
deshielded
The effect decreases with increasing distance from the electronegative atom. The effect completely vanished at the fourth bond from the electronegative atom.  X

14. 2. s-CHARACTER
(HYBRIDIZTION OF CARBON ATOM)

15. But in fact, we have the following order:
As s-character of carbon atom increases, the electronic cloud is held more closer to the carbon and provides less electron density for shielding of protons, and thus the chemical shift, δ, increases (shifted downfield).
According to the above reasoning, the following trend for the chemical shift is expected:
expected to be observed at  > 7
(down field - higher )
But in fact, we have the following order:
actually observed at  = 2-3
This suggested that other factors than the sp character of carbon might affect the chemical shift in this case. The above discrepancy can be explained by what is called Magnetic Anisotropic Effect.

16. 3. MAGNETIC ANISOTROPIC FIELDS
DUE TO THE PRESENCE OF -BONDS
The presence of a nearby pi bond or pi system
greatly affects the chemical shift.
Induced magnetic fields due to the  - electrons have
greatest effect.

17. Aromatic protons  = 7-8 ppm
Prediction of direction of magnetic field using right hand rule.

18. Vinyl (Olefinic) protons,
 = 5-6 ppm

19. Acetylene protons
̃ ≈ ppm

20. Aldehyde proton = 9-10
ppm
Electronegative
oxygen atom

21. HYDROGEN BONDING DESHIELDS PROTONS
O-H and N-H Signals
HYDROGEN BONDING DESHIELDS PROTONS
The chemical shift depends
on how much hydrogen bonding
is taking place (observed in high
concentrated solutions). O H R
Hydrogen bonding lengthens the
O-H bond and reduces the valence
electron density around the proton
it is deshielded and shifted
downfield in the NMR spectrum.
Alcohols vary in chemical shift
from 0.5 ppm (free OH) to about
5.0 ppm (lots of H bonding).
D2O-exchangeable (peak for OH proton in alcohol and NH in amines disappears upon shaking with D2O)

22. SOME MORE EXTREME EXAMPLESδ- δ+ -I
Carboxylic acids have strong
hydrogen bonding - they
form dimers.
Resonance, electronegativity of oxygen and the formation of hydrogen bonding withdraw electron cloud from the acid protons.
Thus, protons attached to carboxylic acids are the least shielded protons and have a chemical shift of ppm.
In methyl salicylate, which has strong
internal hydrogen bonding, the NMR
absorption for O-H is at about 14 ppm,
(highly downfield(
Notice that a stable 6-membered ring is formed

23. disappears upon shaking
NMR CORRELATION CHART
Chemical shift gives the electronic environment of protons
(Shielding and Deshielding)
PROTON IN
ELECTRON-POOR
ENVIRONMENTS
DESHIELDED
DOWNFIELD
HIGHFREQUENCY
LARGE 
PROTON IN
ELECTRON-RICH
ENVIRONMENTS
SHIELDED
UPFIELD
LOWFREQUENCY
SMALL 
disappears upon shaking
with D2O
7-8
CHCl3 ,
aromatic
-OH
-NH
0 - 5
3 - 5
10-12
RCOOH
acid
TMS
9-10
RCHO
aldehyde
0 – 2
d (ppm) 12 11 10 9 8 7 6 5 4 3 2 1 H
CH2F
CH2Cl
CH2Br
CH2I
CH2O
CH2NO2
sp3
CH2Ar
CH2NR2
CH2S
C C-H
C=C-CH2
CH2-C- O
sp3
C-CH-C C
C-CH2-C
C-CH3
sp3
C=C
olefinic
sp2 sp

24. EQUIVALENT AND NONEQUIVALENT PROTONS
All of the protons in a molecule which are in chemically identical environments will often exhibits the same chemical shift i.e., shows one signal in NMR spectrum at the same value of .
The protons in this case are said to be chemically equivalent
On the other hand, molecules which have sets of protons which are chemically distinct (have different chemical environments) from one another give rise to different absorption signals from each other.
Chemically nonequivalent protons

25. INTEGRATION OF A PEAK Integration = determination of the area
Not only does each different type of hydrogen give a
distinct peak in the NMR spectrum, but we can also tell
the relative numbers of each type of hydrogen by a
process called integration.
Integration = determination of the area
under a peak

26. The integrated area measured by a ruler are 5 : 2.5 : 22
In the NMR spectrum, the area under each peak is proportional to the number of hydrogens generating that peak.
The NMR spectrometer has the capability to electronically integrate the area under each peak.
The integrated area measured by a ruler are 5 : 2.5 : 22
Divide by the smallest number give us the simplest ratio of 2 : 1 : 9
Note that the integration gives only ratios, not absolute values for the number of the hydrogen present in the sample
2,2-dimethyl-1-propanol (C4H12O)

27. Modern spectrometers automatically print out the integrals as numbers on the spectrum
NMR spectrum can reveal the number of protons assigned for each signal
Integral lines : no. of protons in each signal

28. SPIN-SPIN SPLITTING

29. SPIN-SPIN SPLITTING
Often a group of hydrogens will appear as a multiplet
rather than as a single peak.
Multiplets are named as follows:
Singlet (s) Quintet (quin)
Doublet (d) Sixtet (six)
Triplet (t) Septet (sept)
Quartet (q) Multiplet (m)
This happens because of interaction with neighboring
hydrogens and is called
SPIN-SPIN SPLITTING
Spin-spin coupling

30. Nonequivalent protons on adjacent carbons.
1,1,2-Tribromoethane
Nonequivalent protons on adjacent carbons.
Chapter 13

31. THE ORIGIN OF SPIN-SPIN SPLITTING HOW IT HAPPENS
Spin-spin splitting arises because hydrogen on adjacent carbon atoms can “sense” one another

32. Doublet 1 adjacent proton
Chapter 13

33. Triplet 2 Adjacent Protons
Chapter 13

34. n + 1 RULE singlet doublet triplet quartet quintet sixtet septet
MULTIPLETS
this hydrogen’s peak
is split by its two neighbors
these hydrogens are
split by their single
neighbor
singlet
doublet
triplet
quartet
quintet
sixtet
septet
two neighbors
n+1 = 3
triplet
one neighbor
n+1 = 2
doublet
Where n is the number of EQUIVALENT protons on
Adjacent carbon atoms

35. EXCEPTIONS TO THE n+1 RULE
IMPORTANT ! 1)
Protons that are equivalent by symmetry usually do not split one another
splitting if x  y
two triplets
no splitting if x = y 2)
Protons in the same group usually do not split one another or 3)
Splitting is not observed if the protons are separated by more than three  bonds

36. 1 1 1 1 2 1 1 3 3 1 INTENSITIES OF MULTIPLET PEAKS PASCAL’S TRIANGLE
singlet 1
Intensities of
multiplet peaks
1 1
doublet
triplet
quartet

37. NMR spectrum indicates the carbon skeleton
Spin-spin splitting gives the number of equivalent protons on adjacent carbon atoms

38. INFORMATION WE CAN GET FROM NMR SPECTRUM
Summary
INFORMATION WE CAN GET FROM NMR SPECTRUM
Number of 1H-NMR signals = Number of kinds of protons.
Chemical shift gives the electronic environment of protons
(Shielding and Deshielding).
NMR spectrum can reveal the number of protons assigned for each signal (integral lines : no. of protons in each signal).
NMR spectrum indicates the carbon skeleton Spin-spin splitting gives the number of equivalent protons on adjacent carbon atoms.

39. Typical Values
Note that these are typical values and that there are lots of exceptions!

40. Resources and references
Textbook: Principles of Instrumental Analysis, Skoog, Holler, Nieman
Recommended further reading:
“Principles of instrumental analysis, 5th ed. by Skoog, Holler, Nieman” Chapter 19.
Extra resources are available on the intranet.
Relevant web sites