Nuclear Magnetic Resonance Spectroscopy, cont'

1
Nuclear Magnetic Resonance Spectroscopy, cont.
Dr. Clower Chemistry 2412L
2
Intensity of Signals

• The area under each peak is proportional to the
  number of protons
• Shown by integration line
• Height a area under peak a Hs in set
• Measure height with ruler or look at graph paper
• Ratio of height ratio of hydrogens

3
(No Transcript)
4
(No Transcript)
5
So far

• Determine the number of sets of equivalent
  hydrogen atoms
• Number of signals on spectrum
• Determine the number of hydrogen atoms in each
  set
• Integration line
• Determine general information about adjacent
  groups
• Chemical shift (d)

6
Next

• Determine specific information about adjacent
  groups
• In particular, how many H atoms on the adjacent
  atoms
• Signal splitting

7
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
• All possibilities exist, so signal is split into
  multiple peaks

8
1,1,2-Tribromoethane
Nonequivalent protons on adjacent carbons
9
Doublet 1 Adjacent Proton
10
Triplet 2 Adjacent Protons
11
The (n 1) Rule

• If a signal is split by n equivalent protons, it
  is split into (n 1) peaks
• n neighboring

12
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

13
Ethyl Iodide
14
(No Transcript)
15
Splitting for Ethyl Groups
16
3-Methyl-2-butanone
17
(No Transcript)
18
Splitting for Isopropyl Groups
19
3-Methyl-2-butanone

• How many signals in the 13C-NMR?

20
(No Transcript)
21
Which isomer best fits this spectrum? or
22
Which isomer best fits this spectrum? or
23
Splitting of Hydroxyl Proton

• Ultrapure samples of ethanol show splitting
• Ethanol with a small amount of acidic or basic
  impurities will not show splitting

24
N-H Proton

• Moderate rate of proton transfer
• Peak may be broad

25
Coupling Constants (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 H
• Multiplets with the same coupling constants may
  come from adjacent groups of protons that split
  each other
• Structural features

26
Values for Coupling Constants
27
Stereochemical Nonequivalence

• Usually, two protons on the same C are equivalent
  and do not split each other
• If the replacement of each of the protons of a
  -CH2 group with an imaginary Z gives
  stereoisomers, then the protons are
  non-equivalent and will split each other

28
Some Nonequivalent Protons
29
How do 1H- and 13C-NMR differ?

• Signals in 13C-NMR are weak
• Noisy baseline, many scans are necessary
• Peak areas in 13C-NMR are not proportional to
  number of carbons
• No integration
• Signals in 13C-NMR are not split
• C-C splitting is negligable
• C-H splitting too difficult to analyze
  H-decoupled mode

30
Interpreting 13C-NMR

• The number of different signals indicates the
  number of different kinds of carbon
• The chemical shift indicates the functional group
• Use to support 1H-NMR analysis

31
Solving NMR Problems

• Given
• Molecular formula
• 1H-NMR
• IR (sometimes)
• 13C-NMR (sometimes)
• Goal
• Determine structure
• First step
• Determine the Index of Hydrogen Deficiency

32
IHD C ½(H X) ½N 1

• Sum of number of rings p bonds
• Remember alkynes have 2 p bonds
• C6H12 IHD 6 ½(12) 1 1
• C5H3N2O2Cl IHD 5 ½(31) ½(2) 1 5
• Anytime IHD gt 4 and C 6, think benzene
• C6H6 IHD 6 ½(6) 1 4

33
In-class 1H-NMR Problem What is the structure
of the compound with the following 1H-NMR
spectrum?
a b c d e