Title: Nuclear Magnetic Resonance Spectroscopy, cont'
1Nuclear Magnetic Resonance Spectroscopy, cont.
Dr. Clower Chemistry 2412L
2Intensity 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)
5So 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)
6Next
- Determine specific information about adjacent
groups - In particular, how many H atoms on the adjacent
atoms - Signal splitting
7Spin-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
81,1,2-Tribromoethane
Nonequivalent protons on adjacent carbons
9Doublet 1 Adjacent Proton
10Triplet 2 Adjacent Protons
11The (n 1) Rule
- If a signal is split by n equivalent protons, it
is split into (n 1) peaks - n neighboring
12Range 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
13Ethyl Iodide
14(No Transcript)
15Splitting for Ethyl Groups
163-Methyl-2-butanone
17(No Transcript)
18Splitting for Isopropyl Groups
193-Methyl-2-butanone
- How many signals in the 13C-NMR?
20(No Transcript)
21Which isomer best fits this spectrum? or
22Which isomer best fits this spectrum? or
23Splitting of Hydroxyl Proton
- Ultrapure samples of ethanol show splitting
- Ethanol with a small amount of acidic or basic
impurities will not show splitting
24N-H Proton
- Moderate rate of proton transfer
- Peak may be broad
25Coupling 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
26Values for Coupling Constants
27Stereochemical 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
28Some Nonequivalent Protons
29How 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
30Interpreting 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
31Solving NMR Problems
- Given
- Molecular formula
- 1H-NMR
- IR (sometimes)
- 13C-NMR (sometimes)
- Goal
- Determine structure
- First step
- Determine the Index of Hydrogen Deficiency
32IHD 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
33In-class 1H-NMR Problem What is the structure
of the compound with the following 1H-NMR
spectrum?
a b c d e