1H NMR note-1.ppt

Chemistry

1. Interpreting Proton NMR
Spectra

2. Nuclearspinandthesplittingofenergylevelsinamagneticfield
Subatomicparticles(electrons,protonsandneutrons)canbeimaginedas
spinningontheiraxes.Inmanyatoms(suchas12C)thesespinsarepaired
againsteachother,suchthatthenucleusoftheatomhasnooverallspin.
However,insomeatoms(suchas1Hand13C)thenucleusdoespossessan
overallspin.Therulesfordeterminingthenetspinofanucleusareas
follows;
Ifthenumberofneutronsandthenumberofprotonsarebotheven,thenthe
nucleushasNOspin.
Ifthenumberofneutronsplusthenumberofprotonsisodd,thenthe
nucleushasahalf-integerspin(i.e.1/2,3/2,5/2)
Ifthenumberofneutronsandthenumberofprotonsarebothodd,thenthe
nucleushasanintegerspin(i.e.1,2,3)

3. 1. number of signals
2. their intensity (as measured by area
under peak)
3. splitting pattern (multiplicity)
Information contained in an NMR
spectrum includes:

4. Number of Signals
protons that have different chemical shifts
are chemically nonequivalent
exist in different molecular environment

5. 01.02.03.04.05.06.07.08.09.010.0
Chemical shift (d, ppm)
CCH
2OCH
3
N
OCH
3
NCCH
2O

6. are in identical environments
have same chemical shift
replacement test: replacement by some
arbitrary "test group" generates same compound
H
3CCH
2CH
3
chemically equivalent
Chemically equivalent protons

7. H
3CCH
2CH
3
chemically equivalent
CH
3CH
2CH
2ClClCH
2CH
2CH
3
Chemically equivalent protons
Replacing protons at C-1 and C-3 gives same
compound (1-chloropropane)
C-1 and C-3 protons are chemically
equivalent and have the same chemical shift

8. replacement by some arbitrary test group
generates diastereomers
diastereotopic protons can have different
chemical shifts
Diastereotopic protons
CC
Br
H
3C
H
H
d5.3 ppm
d5.5 ppm

9. are in mirror-image environments
replacement by some arbitrary test group
generates enantiomers
enantiotopic protons have the same
chemical shift
Enantiotopic protons

10. C CH
2OH
H
3C
H
H
Enantiotopic
protons
C CH
2OH
H
3C
Cl
H
C CH
2OH
H
3C
H
Cl
R S

11. not all peaks are singlets
signals can be split by coupling of
nuclear spins
Spin-Spin Splitting
in
NMR Spectroscopy

12. 01.02.03.04.05.06.07.08.09.010.0
Chemical shift (d, ppm)
Cl
2CHCH
3
4 lines;
quartet
2 lines;
doublet
CH
3CH

13. Two-bond and three-bond coupling
CC
H
H
CCHH
protons separated by
two bonds
(geminal relationship)
protons separated by
three bonds
(vicinal relationship)

14. in order to observe splitting, protons cannot
have same chemical shift
coupling constant (
2
J or
3
J) is independent
of field strength
Two-bond and three-bond coupling
CC
H
H
CCHH

15. 01.02.03.04.05.06.07.08.09.010.0
Chemical shift (d, ppm)
Cl
2CHCH
3
4 lines;
quartet
2 lines;
doublet
CH
3CH
coupled protons are vicinal (three-bond coupling)
CHsplits CH
3into a doublet
CH
3splits CHinto a quartet

16. Why do the methyl protons of
1,1-dichloroethane appear as a doublet?
CCHH
Cl
Cl
H
H
signal for methyl
protons is split into
a doublet
To explain the splitting of the protonsat C-2,
we first focus on the two possible spin
orientations of the protonat C-1

17. Why do the methyl protons of
1,1-dichloroethane appear as a doublet?
CCHH
Cl
Cl
H
H
signal for methyl
protons is split into
a doublet
There are two orientations of the nuclear spin
for the proton at C-1. One orientation shields
the protons at C-2; the other deshields the C-
2 protons.

18. Why do the methyl protons of
1,1-dichloroethane appear as a doublet?
CCHH
Cl
Cl
H
H
signal for methyl
protons is split into
a doublet
The protons at C-2 "feel" the effect of both the
applied magnetic field and the local field
resulting from the spin of the C-1 proton.

19. Why do the methyl protons of
1,1-dichloroethane appear as a doublet?
CCHH
Cl
Cl
H
H
"true" chemical
shift of methyl
protons (no coupling)
this line corresponds
to molecules in which
the nuclear spin of
the proton at C-1
reinforces
the applied field
this line corresponds
to molecules in which
the nuclear spin of
the proton at C-1
opposes
the applied field

20. Why does the methine proton of
1,1-dichloroethane appear as a quartet?
CCHH
Cl
Cl
H
H
signal for methine
proton is split into
a quartet
The proton at C-1 "feels" the effect of the
applied magnetic field and the local fields
resulting from the spin states of the three
methyl protons. The possible combinations
are shown on the next slide.

21. CCHH
Cl
Cl
H
H
There are eight combinations of
nuclear spins for the three methyl
protons.
These 8 combinations split the
signal into a 1:3:3:1 quartet.
Why does the methine proton of
1,1-dichloroethane appear as a quartet?

22. High-resolution spectra are run using a higher radio
frequency and the peaks have more detail.
Compare the spectra below for methyl propanoate.
High resolution SpectraC
C
C
O
O
C
H
H
H
H
H
H H
H
Low-resolution NMR
for methyl propanoate
High-resolution NMR for
methyl propanoate.

23. C
C
C
O
C
C
H
H
H
H
H
H
H
H
H
H CH
3
CH
2
This spectrum is for pentan-3-one. The peaks show that there are
two proton environments. Can you assign these peaks to the
structure?

24. When the spectrum is expanded it can be seen that each peak is made
up of a number of peaks. These are called multiplets.
Quartet: four peaks in
the group.
Other multiplets include
singlets and doublets.
Triplet: three
peaks in the group.

25. n+ 1 rule
•Thenumberofpeaksinamultipletcangiveadditional
informationaboutthestructure.
•Thesplittingofpeaksiscausedbytheneighbouringcarbon’s
hydrogenatoms.
•Protonsinthesameenvironmentaresaidtobeequivalentandas
suchbehaveasoneproton.
•Thisfollowsthen+1rule.
–nisthenumberofhydrogenatomsattachedtothenext-door
carbon
–n+1ishowmanypeakswillbeseeninthecluster.

26. Splitting Patterns of Common Multiplets
Number of equivalentAppearanceIntensities of lines
protons to which H of multipletin multiplet
is coupled
0 singlet 1
1 Doublet 1:1
2 Triplet 1:2:1
3 Quartet 1:3:3:1
4 Pentet 1:4:6:4:1
5 Sextet 1:5:10:10:5:1
6 Septet 1:6:15:20:15:6:1
Table

27. Splitting Patterns:
The Ethyl Group
CH
3CH
2X is characterized by a triplet-quartet
pattern (quartet at lower field than the triplet)

28. 01.02.03.04.05.06.07.08.09.010.0
Chemical shift (d, ppm)
BrCH
2CH
3
4 lines;
quartet
3 lines;
triplet
CH
3
CH
2

29. Splitting Patterns:
The Isopropyl Group
(CH
3)
2CHX is characterized by a doublet-
septet pattern (septet at lower field than the
doublet)

30. 01.02.03.04.05.06.07.08.09.010.0
Chemical shift (d, ppm)
BrCH(CH
3)
2
7 lines;
septet
2 lines;
doublet
CH
3
CH

31. Splitting Patterns:
Pairs of Doublets
Splitting patterns are not always symmetrical,
but lean in one direction or the other.

32. Pairs of Doublets
Consider coupling between two vicinal
protons.
If the protons have different chemical shifts,
each will split the signal of the other into a
doublet.
CCH H

33. Pairs of Doublets
Let Dnbe the difference in chemical shift in Hz
between the two hydrogens.
Let J be the coupling constant between them
in Hz.
CCH H

34. AX
When Dnis much larger than J the signal for
each proton is a doublet, the doublet is
symmetrical, and the spin system is called
AX.
CCH H
J J
Dn

35. AM
As Dn/J decreases the signal for each proton
remains a doublet, but becomes skewed. The
outer lines decrease while the inner lines increase,
causing the doublets to "lean" toward each other.
CCH H
J J
Dn

36. AB
When Dnand J are similar, the spin system is
called AB. Skewing is quite pronounced. It is
easy to mistake an AB system of two doublets
for a quartet.
CCH H
J J
Dn

37. A
2
When Dn= 0, the two protons have the same
chemical shift and don't split each other. A
single line is observed. The two doublets
have collapsed to a singlet.
CCH H

38. 01.02.03.04.05.06.07.08.09.010.0
Chemical shift (d, ppm)
OCH
3
skewed doublets
H H
HH
Cl OCH
3

39. Complex Splitting Patterns
Multiplets of multiplets

40. m-Nitrostyrene
Consider the proton shown in red.
It is unequally coupled to the protons shown
in blueand white.
J
cis= 12 Hz; J
trans= 16 Hz
H
H
O
2N
H

41. m-Nitrostyrene
16 Hz
12 Hz 12 Hz
The signal for
the proton
shown in red
appears as a
doublet of
doublets.
H
H
O
2N
H

42. H
H
O
2N
H
doublet of doublets
doubletdoublet

43. 1
H NMR Spectra of Alcohols
What about H bonded to O?

44. O—H
The chemical shift for O—His variable (d0.5-
5 ppm) and depends on temperature and
concentration.
Splitting of the O—Hproton is sometimes
observed, but often is not. It usually appears
as a broad peak.
Adding D
2O converts O—Hto O—D. Then
O—Hpeak disappears.
COH H

45. 1
HNMR—IntensityofSignals
•TheareaunderanNMRsignalisproportionaltothenumberof
absorbingprotons.
•AnNMRspectrometerautomaticallyintegratestheareaunderthe
peaks,andprintsoutasteppedcurve(integral)onthespectrum.
•Theheightofeachstepisproportionaltotheareaunderthepeak,
whichinturnisproportionaltothenumberofabsorbingprotons.
•ModernNMRspectrometersautomaticallycalculateandplotthe
valueofeachintegralinarbitraryunits.
•Theratioofintegralstooneanothergivestheratioofabsorbing
protonsinaspectrum.Notethatthisgivesaratio,andnotthe
absolutenumber,ofabsorbingprotons.

48. Excercise:Molecular formula: C4H9BrO
1H-NMR: 4.49 ppm (sextet, integral=1); 3.73 ppm (triplet,
integral = 2); 3.20 ppm (singlet, integral =1); 1.95 ppm (quartet,
integral=2) and 1.70 ppm (doublet, integral = 3)