Basic NMR

NMR Prsentation at IICT, hyderabad

2. Overview
•NMR is a sensitive, non-destructive method
for elucidating the structure of organic
molecules
•Information can be gained from the hydrogens
(proton NMR, the most common), carbons
(
13
C NMR) or other elements like
31
P,
15
N,
19
F.

3. Types of NMR
1D NMR
2D NMR: Data plotted in a space defined by
two frequency axes rather than one
representing the chemical shift
Types of 2DNMR:
Correlation spectroscopy(COSY)
J-spectroscopyexchange spectroscopy(EXSY)
Nuclear Overhausereffectspectroscopy
(NOESY)

4. PRINCIPLE
Absorption of energy occurs and a NMR signal is recorded.
Applied Frequency = Precessional frequency,
Radiofrequency is applied
Spin of nuclei is aligned with the externally applied magnetic field.
The nucleus spins on its axis and a magnetic moment is created in a
precessional orbit, with a frequency called precessional frequency (Larmor
frequency).
Application of an external magnetic field (H
o)
Proton or nucleus with odd mass number spins on its own axis.

6. TYPES OF PROTONS
EQUIVALENT PROTON
A set of protons with same environment are
called equivalent protons
NON –EQUIVALENT PROTON
Sets of protons with different environments are
called non equivalent protons

7. NMR Signals
•The number of signals shows how many different
kinds of protons are present.
•The location of the signals shows how shielded or
deshielded the proton is.
•The intensity of the signal shows the number of
protons of that type.
•Signal splitting shows the number of protons on
adjacent atoms.

8. EQUIVALENT PROTONS
NON EQUIVALENT PROTONS

10. NMR signal splits into doublet, triplet, quartet
etc.

14. COUPLING CONSTANT
•Separation between the lines of each doublet is
equal.Thisconstant is called the coupling constant
J value.
•It is measured in HZ
•Identical for each signal
•Independent of field strength

16. Fourier Transform-What it transforms?
a) Single frequency Sine Waves
b) Single frequency FID
c) Three sine wave combination

17. Instrumentation

18. Working-A Closer Look

19. Instrumentation-Sample Holder
Material:Borosilicate Glass
Dimensions:
1.3mm –10mm in diameter
2.7-8 inches in height
Specifications:
1.Concentricity
Difference between two radial centers using the outer and
inner tube's circumference as reference points.
Larger differenceNon-homogeneity of magnetic field
2.Camber
Measurement of the degree of curvature of the tube
Larger valueCauses wobbling of the tube while
spinning
Spinner
Sample tube

20. Minimum Concentration: 5mg/mL
Solvents : CCl
4, CDCl
3, D
2O, C
6D
6
Why deuteratedsolvents?
To avoid swamping of the solvent signal
To stabilize the magnetic field
To accurately define 0ppm
Cleaning of tubes:
PiranhanaSolution/ Aqua Regia
Instrumentation-Sample Preparation

21. NMR Spectrum-Chemical Shift
Chemical Shift: Difference in parts per million between the resonance frequency of
the observed proton and TMS (Trimethylsilane) hydrogens.
TMS ppm is set to 0.
Shielding/ Deshieldingof protons
Factors affecting the Chemical Shift:
1.Electronegativity
2.Magnetic Anisotrophy
3.Hydrogen Bonding
High
Electron density
Low
Electron density

22. NMR Spectrum-Chemical Shift

23. NMR Spectrum-Intensity
Intensity No. of Absorbing Protons

24. NMR Spectrum-Spin-Spin Coupling
Signals for different protons are split into
more than one
peaktermed as Spin-Spin Coupling or
Splitting.
Spin-spin splitting occurs only between
nonequivalent protons on the same carbon
or adjacent carbons.
The frequency difference, measured in Hz
between two peaks of the doublet is called
the coupling constant, J
Multiplicity Rule
n+1 rule, where n is the number of
neighboring spin-coupled nuclei with the
same (or very similar) Js

25. NMR Spectrum-Interpretation
Number of signals: Indicates how many different kinds of
protons are present
Position of signals: Indicates Magnetic environment of the
signal
Relative Intensity: Proportional to number of protons present
Splitting: Indicates number of splitting Nuclei [Usually
Protons]

26. Difference between Proton NMR and C13
NMR
Proton NMR
13
C NMR
1. It is study of spin changes of
proton nuclei.
1. It is study of spin changes of
carbon nuclei.
2. Chemical shift range is
0-14 ppm.
2. Chemical shift range is
0-240 ppm.
3. Continuous wave method 3. Fourier transform Method
4.slow process 4.Very fast process.

27. Signature Ranges

28. In a magnetic field the states have different energies

Alignment with the magnetic field (called ) is
lower energy than against the magnetic field
(called ). How much lower it is, depends on the
strength of the magnetic field

29. Chemical shift
•Protons in different environments absorb at
different field strengths (for the same frequency)
•Different environment = different electron density
around the H

30. Aromatic Protons, 7-8

31. Vinyl Protons, 5-6

32. Acetylenic Protons, 2.5

33. O-H and N-H Signals
•Chemical shift depends on concentration.
•Hydrogen bonding in concentrated solutions deshield
the protons, so signal is around 3.5 for N-H and 4.5
for O-H.
•Proton exchange between the molecules broaden the
peak.

34. Identifying the O-H or N-H Peak
•Chemical shift will depend on concentration and
solvent.
•To verify that a particular peak is due to O-H or N-H,
shake the sample with D
2O.
•Deuterium will exchange with the O-H or N-H
protons.
•On a second NMR spectrum the peak will be absent,
or much less intense.

35. Carboxylic Acid Proton, 10+

36. Intensity of Signals
•The area under each peak is proportional to
the number of protons.
•Shown by integral trace.

37. Spin-Spin Splitting
•Nonequivalent protons on adjacent carbons have
magnetic fields that may align with or oppose the
external field.
•This magnetic coupling causes the proton to absorb
slightly downfield when the external field is reinforced
and slightly upfield when the external field is opposed.

38. The N + 1 Rule
If a signal is split by N equivalent protons,
it is split into N + 1 peaks.
=>

39. Chapter 13
Doublet: 1 Adjacent Proton
=>

40. Chapter 13
Triplet: 2 Adjacent Protons
=>

41. Range of Magnetic Coupling
•Equivalent protons do not split each other.
•Protons bonded to the same carbon will split
each other only if they are not equivalent.
•Protons on adjacent carbons normally will
couple.
•Protons separated by four or more bonds will not
couple.

42. Coupling Constants
•Distance between the peaks of multiplet
•Measured in Hz
•Not dependent on strength of the external field
•Multiplets with the same coupling constants may
come from adjacent groups of protons that split each
other.

43. Values for Coupling Constants

44. Complex Splitting
•Signals may be split by adjacent protons, different
from each other, with different coupling constants.
•Example:
H
a
of styrene which is split by an adjacent H trans to it
(J = 17 Hz) and an adjacent H cis to it (J = 11 Hz).

C C
H
H
H
a
b
c

45. General Regions of Chemical Shifts
Aldehydic
Aromatic and heteroaromatic
Olefinic
-Disubstitutid aliphatic
-Monosubstituted aliphatic
Acetylenic
-Substituted aliphatic
Aliphatic alicyclic
0  1 3 4 5 6 10 2 7 8 9 = TMS
CH
3-CH
2-CH
2-CH
2-CH
2-CH=CH-CH
2-CH=CH-CH
2-CH
2-CH
2-CH
2-CH
2-CH
2-CH
2-COOCH
2
HOCH
HOCH
2

46. cyclohexane
a singlet 12H

47. 2,3-dimethyl-2-butene
C
CH
3
C
H
3C
H
3C
CH
3
a singlet 12H

48. benzene
a singlet 6H

49. p-xylene
H
3C CH
3
a
a
b
a singlet 6H
b singlet 4H

50. tert-butyl bromide
C CH
3H
3C
Br
CH
3 a singlet 9H

51. 13
C ~ 1.1% of carbons

1)number of signals: how many different types of carbons

2)chemical shift: hybridization of carbon sp, sp
2
, sp
3

13
C – NMR

52. 2-bromobutane

a c d b
CH
3CH
2CHCH
3
Br

53. NUCLEAR MAGNETIC RESONANCE
SPECTROSCOPY
NMRisatechniquewhichisbasedontheabsorptionelectromagnetic
radiationintheradiofrequencyregion4to900MHzbynucleiofthe
atoms.
Studyofspinchangesatthenuclearlevelwhenaradiofrequency
energyisabsorbedbythenucleiinthepresenceofanexternalmagnetic
field.

57. ThefrequencyνatwhichenergyisabsorbedoremittedisgivenbyBohr̕s
relationship:
ν=E
2–E
1/h
ν=-1/2[γh/2π]B
0+1/2[γh/2π]B
0/h
ν=(γ/2π)B
0
ThisistheLarmorequationwhichisthemathematicalbasisforNMR.

58. 1 D NMR SPECTROSCOPY
•1-DNMRspectroscopyissimpletechniqueinwhicha90degree
pulseisprovidedtothesamplewhichisplacedinuniformmagnetic
field.
•AsaresultaFID(FreeInductionDecay)signalortimedomain
signalisproduced.
•TogetmeaningfuldatafouriertransformprogramisappliedonFID
togetfrequencysignal.

59. THE 2 STEPS OF A 1 D NMR
1)Preparation
•90degreepulseexcitethenucleiinsample.
2)Detection
•DetectthesignalormeasuringofFIDsignal.

60. CONVERSION OF TIME DOMAIN SIGNAL INTO
FREQUENCY DOMAIN SIGNAL

61. LIMITATIONS OF 1 D NMR SPECTROSCOPY
Itisnotpossibletocorrelatetwodifferentchemicalshiftby1D
NMRspectroscopy.
Itisdifficulttoanalysea1DNMRspectruminthecaseoflarge
moleculesduetooverlapofpeaks.