1. NMR Spectroscopy

2. Nuclear Magnetic Resonance Spectroscopy
Electron has a spin and that spinning creates and associated magnetic field.
Electron can behave like a tiny bar magnet.
Atomic nucleus can spin and behave as a tiny bar magnet. These spinning nuclei generate tiny magnetic fields
Any nucleus will spin (odd mass or odd atomic number)
Will not spin (if even mass or even atomic number)
Tiny magnets interact with an external magnetic field, denoted B0
Proton (1H) and carbon (13C) are the most important nuclear spins to organic chemists

3. Recall
Mass number (atomic mass) is number of protons plus neutrons and can vary from isotopes of the same element
Atomic number (number of protons or electrons) is the same for all isotopes of that element

4. the nuclei of some atoms spin: 1H, 13C, 19F, …
Nuclear Magnetic Resonance (NMR)
the nuclei of some atoms spin: 1H, 13C, 19F, …
Any nucleus will spin (odd mass or odd atomic number)
the nuclei of many atoms do not spin: 2H, 12C, 16O, …
Will not spin (if even mass or even atomic number)
moving charged particles generate a magnetic field ()
when placed between the poles of a powerful magnet, spinning nuclei will align with or against the applied magnetic field creating an energy difference.
Mass of the element

5. the nuclei of some atoms spin: 1H, 13C, 19F, …
Nuclear Magnetic Resonance (NMR)
the nuclei of some atoms spin: 1H, 13C, 19F, …
Any nucleus will spin (odd mass or odd atomic number)
the nuclei of many atoms do not spin: 2H, 12C, 16O, …
Will not spin (if even mass or even atomic number)
moving charged particles generate a magnetic field ()
when placed between the poles of a powerful magnet, spinning nuclei will align with or against the applied magnetic field creating an energy difference.
Using a fixed radio frequency, the magnetic field is changed until the ΔE = EEM. When the energies match, the nuclei can change spin states (resonate) and give off a magnetic signal. ΔE
Mass of the element

6. Nuclear Magnetic Resonance Spectroscopy
Nuclear spins are oriented randomly in the absence (a) of an external magnetic field but have a specific orientation in the presence (b) of an external field, B0
Some nuclear spins are aligned parallel to the external field
Lower energy orientation
More likely
Some nuclear spins are aligned antiparallel to the external field
Higher energy orientation
Less likely

7. Nuclear Magnetic Resonance Spectroscopy
When nuclei that are aligned parallel with an external magnetic field are irradiated with the proper frequency of electromagnetic radiation the energy is absorbed and the nuclei “spin-flips” to the higher-energy antiparallel alignment
Nuclei that undergo “spin-flips” in response to applied radiation are said to be in resonance with the applied radiation - nuclear magnetic resonance
Frequency necessary for resonance depends on strength of external field and the identity of the nuclei

8. Nuclear Magnetic Resonance Spectroscopy
The energy difference DE between nuclear spin states depends on the strength of the applied magnetic field
Absorption of energy with frequency n converts a nucleus from a lower to a higher spin state
DE = 8.0 x 10-5 kJ/mol for magnetic field strength of 4.7 T a
For field strength of 4.7 T a radiofrequency (rf) of n = 200 MHz is required to bring 1H nuclei into resonance
For a field strength of 4.7 T a radiofrequency (rf) of n = 50 MHz is required to bring 13C nuclei into resonance

9. Nuclear Magnetic Resonance Spectroscopy
Many nuclei exhibit NMR phenomenon
All nuclei with odd number
of protons
of neutrons
Nuclei with even numbers
of both protons and neutrons
do not exhibit NMR
phenomenon

10. The Nature of NMR Absorptions
The absorption frequency is not the same for all 1H and 13C nuclei
Nuclei in molecules are surrounded by electrons
Electrons set up tiny local magnetic fields that act in opposition to the applied field, shielding the nucleus from the full effect of the external magnetic field
The effective field actually felt by the nucleus is the applied field reduced by the local shielding effects
Beffective = Bapplied – Blocal

11. The Nature of NMR Absorptions
The absorption frequency is not the same for all 1H and 13C nuclei
Each chemically distinct nucleus in a molecule has a slightly different electronic environment and consequently a different effective field
Each chemically distinct 13C or 1H nucleus in a molecule experiences a different effective field and will exhibit a distinct 13C or 1H NMR signal
Hydrogens in most organic molecules are surrounded by electrons and by other atoms.

12. Chemical Shifts
The NMR Chart
The downfield, deshielded side is on the left, and requires a lower field strength for resonance
The upfield, shielded side is on the right, and requires a higher
field strength for resonance
The tetramethylsilane (TMS) absorption is used as a reference point

13. Shielding and Deshielded
Shielded (by the local environment)
Greater electron density around each hydrogen
Due to poor electron withdrawing group
Due to poor inductive effect
Deshielded (by the local environment)
Less electron density around each hydrogen
Due to high electronegative element
Due to larger inductive effect

14. 1H NMR Spectroscopy and Proton Equivalence
1H NMR spectroscopy determines how many kinds of electronically nonequivalent hydrogens are present in a molecule
Equivalence or nonequivalence of two protons determined by replacing each H by an X (halogen group)
Equivalent H give the same H-NMR signal
Non equivalent H give different H-NMR signal
Symmetrical compounds tend to contain a higher amount of equivalent hydrogens and thus fewer resonance signals in their NMRspectra

15. Protons are chemically identical and thus electronically equivalent
Four possibilities:
Protons are chemically unrelated and thus nonequivalent
Protons are chemically identical and thus electronically equivalent
Protons are electronically equivalent but not identical
Replacement of a hydrogen gives a unique diastereomer (not mirror images of each other) with a second chirality center

16. 1H NMR Spectroscopy and Proton Equivalence
1H NMR spectroscopy determines how many kinds of electronically nonequivalent hydrogens are present in a molecule
Four possibilities:
Protons that are unrelated and thus nonequivalent

17. 1H NMR Spectroscopy and Proton Equivalence
Protons are chemically identical and thus electronically equivalent
Chemically identical protons are said to be homotopic

18. 1H NMR Spectroscopy and Proton Equivalence
Protons are electronically equivalent but not identical
The two –CH2 – hydrogens on C2 of butane (as well as the two C3 hydrogens) are not identical because replacing one or the other would lead to a new chirality center
Different enantiomers would result if pro-R or pro-S hydrogen were replaced
Prochiral hydrogens are electronically equivalent and thus have the same NMR absorption

19. 1H NMR Spectroscopy and Proton Equivalence
Protons are electronically equivalent but not identical
The two –CH2 – hydrogens on C2 of butane (as well as the two C3 hydrogens) are not identical because replacing one or the other would lead to a new chirality center
Non-identical but electronically equivalent protons are said to be enantiotopic
Different enantiomers would result if pro-R or pro-S hydrogen were replaced
Prochiral hydrogens are electronically equivalent and thus have the same NMR absorption

20. 1H NMR Spectroscopy and Proton Equivalence
Replacement of a hydrogen gives a unique diastereomer (not mirror images of each other) with a second chirality center
These hydrogens are neither electronically or chemically equivalent and will most likely show different 1H NMR absorptions

21. 1H NMR Spectroscopy and Proton Equivalence
Replacement of a hydrogen gives a unique diastereomer (not mirror images of each other) with a second chirality center
Such hydrogens are diastereotopic
Diastereotopic hydrogens are neither electronically or chemically equivalent and will most likely show different 1H NMR absorptions

22. Information from 1H-nmr spectra:
Number of signals: How many different types of hydrogens in the molecule.
Position of signals (chemical shift): What types of hydrogens.
Relative areas under signals (integration): How many hydrogens of each type.
Splitting pattern: How many neighboring hydrogens.

23. Number of signals: How many different types of hydrogens in the molecule

24. Practice, how many signals

28. Position of signals (chemical shift): what types of hydrogens.
primary 0.9 ppm
secondary 1.3
tertiary 1.5
aromatic 6-8.5
allyl 1.7
benzyl 2.2-3
chlorides 3-4 H-C-Cl
bromides H-C-Br
iodides 2-4 H-C-I
alcohols H-C-O
alcohols H-O- (variable)
Note: combinations may greatly influence chemical shifts. For example, the benzyl hydrogens in benzyl chloride are shifted to lower field by the chlorine and resonate at 4.5 ppm.

29. reference compound = tetramethylsilane (CH3)4Si @ 0.0 ppm
remember: magnetic field 
 chemical shift
convention: let most upfield signal = a, next most upfield = b, etc.
… c b a tms

30. reference compound = tetramethylsilane (CH3)4Si @ 0.0 ppm
remember: magnetic field 
 chemical shift
convention: let most upfield signal = a, next most upfield = b, etc.
… c b a tms

31. toluene b a

32. chemical shifts

33. Integration (relative areas under each signal): how many hydrogens of each type.
a b c
CH3CH2CH2Br a 3H a : b : c = 3 : 2 : 2
b 2H
c 2H
a b a
CH3CHCH3 a 6H a : b = 6 : 1
Cl b 1H

34. integration

35. Integration of 1H NMR Absorptions: Proton Counting
The area under each 1H NMR peak is proportional to the number of protons causing that peak
Integrating (electronically measuring) the area under each peak makes it possible to determine the relative number of each kind of proton in a molecule
Integrating the peaks of 2,2-dimethylpropanoate in a “stair-step” manner shows that they have 1:3 ratio, corresponding to the ratio of the numbers of protons (3:9)

36. Splitting pattern: how many neighboring hydrogens.
In general, n-equivalent neighboring hydrogens will split a 1H signal into an ( n + 1 ) Pascal pattern.
“neighboring” – no more than three bonds away
n n + 1 Pascal pattern:
singlet
doublet
triplet
quartet
quintet

37. Spin-Spin Splitting in 1H NMR Spectra
Multiple absorptions, called spin-spin splitting, are caused by the interaction (coupling) of the spins of nearby nuclei
Tiny magnetic fields produced by one nucleus affects the magnetic field felt by neighboring nuclei
If protons align with the applied field the effective field felt by neighboring protons is slightly larger
If protons align against the applied field the effective field felt by neighboring protons is slightly smaller

38. note: the alcohol hydrogen –OH usually does not split neighboring hydrogen signals nor is it split. Normally a singlet of integration 1 between 1 – 5.5 ppm (variable).

39. splitting pattern?

40. Spin-Spin Splitting in 1H NMR Spectra
The absorption of a proton can split into multiple peaks called a multiplet
Remember ( n + 1 ) for splitting signals
1H NMR spectrum of bromoethane shows four peaks (a quartet) at 3.42 d for –CH2Br protons and three peaks (a triplet) at 1.68 d for –CH3 protons

41. Spin-Spin Splitting in 1H NMR Spectra
Each –CH2Br proton of CH3CH2Br has its own nuclear spin which can align either with or against the applied field, producing a small change in the effective field experienced by the –CH3 protons
Three possible spin states (combinations)
Both protons spin in alignment with applied field
Effective field felt by neighboring –CH3 protons is larger
Applied field necessary to cause resonance is reduced
One proton spin is aligned with and one proton spin is aligned against the applied field (two possible combinations)
No effect on neighboring protons
Both proton spins align against applied field
Effective field felt by neighboring –CH3 is smaller
Applied field necessary to cause resonance is increased

42. Spin-Spin Splitting in 1H NMR Spectra
The origin of spin-spin splitting in bromoethane. The nuclear spins of – CH2Br protons, indicated by horizontal arrows, align either with or against the applied field, causing the splitting of –CH3 absorptions into a triplet

43. Spin-Spin Splitting in 1H NMR Spectra
n + 1 rule
Protons that have n equivalent neighboring protons show n + 1 peaks in their 1H NMR spectrum
The septet is caused by splitting of the –CHBr- proton signal at 4.28 d by six equivalent neighboring protons on the two methyl groups (n = 6 leads to 6+1 = 7 peaks)
The doublet at 1.71 d is due to signal splitting of the six equivalent methyl protons by the single –CHBr- proton (n = 1 leads to 2 peaks)

44. cyclohexane
a singlet 12H

45. isopropyl chloride
a b a
CH3CHCH3 Cl
a doublet 6H
b septet H

47. The Isopropyl Group: Equivalent Neighbors

48. Peak Intensity Variation

49. alcohols

50. C13 NMR: Differences from 1H NMR
Signal for a carbon atom is 10-4 times weaker than hydrogen
Carbon signals are spread over a much wider range
All signals appear as singlets.
Signal intensity varies and can not be used to determine how many carbon atoms are responsible for each signal

51. 13C – nmr C ~ 1.1% of carbons
number of signals: how many different types of carbons
splitting: number of hydrogens on the carbon
chemical shift: hybridization of carbon sp, sp2, sp3
chemical shift: evironment

52. Characteristics of 13C NMR Spectroscopy
Factors that affect chemical shifts:
Chemical shift affected by nearby electronegative atoms
Carbons bonded to electronegative atoms absorb downfield from typical alkane carbons
Hybridization of carbon atoms
sp3-hybridized carbons generally absorb from 0 to 90 d
sp2-hybridized carbons generally absorb from 110 to 220 d
C=O carbons absorb from 160 to 220 d

53. 13C-nmr
2-bromobutane
a c d b
CH3CH2CHCH3 Br

54. Characteristics of 13C NMR Spectroscopy
13C spectrum for butan-2-one
Butan-2-one contains 4 chemically nonequivalent carbon atoms
Carbonyl carbons (C=O) are always found at the low-field end of the spectrum from 160 to 220 d

55. Practice Predicting Chemical Shifts in 13C NMR Spectra
At what approximate positions would you expect ethyl acrylate, H2C=CHCO2CH2CH3, to show 13C NMR absorptions?

57. Solution Predicting Chemical Shifts in 13C NMR Spectra
Ethyl acrylate has five distinct carbons: two different C=C, one C=O, one C(O)-C, and one alkyl C. From Figure 11.7, the likely absorptions are
The actual absorptions are at 14.1, 60.5, 128.5, 130.3, and d