1. Rotating
Molecules
Stretching
Bonds
Exciting
Electrons
Breaking
Bonds

2. Infrared Spectroscopy
Bond Bending and Bond Stretching
Test for C=O, C=C, CC, CN, O-H, N-H
N – H
O – H
C – H
(sp > sp2 > sp3)
C≡C
C≡N
C=C
C=N
C=O
Fingerprint region
Transmittance (%)
100
Alkynes
Nitriles
Alkenes
Imines
Carbonyls
C-C
C-O
C-X
C-N
Wavenumber (cm-1)

3. Infrared Spectroscopy
Bond Bending and Bond Stretching
C=O Huge peak ~ 1700 cm-1
CC, CN Moderate peak ~ 2200 cm-1
O-H Large broad peak > 3000 cm-1 (3300 cm-1)
N-H Like an OH but sharper
1,5-diaminopentane
Benzonitrile
Vanillin
Cinnamaldehyde
Linalool
3-methylindole
Cadaverine
Skatole

4. Benzonitrile
Cinnamaldehyde
Linalool
Vanillin
Skatole
Cadaverine

5. Benzonitrile
Cinnamaldehyde
Linalool
Vanillin
Skatole
Cadaverine

6. Benzonitrile
Cinnamaldehyde
Linalool
Vanillin
Skatole
Cadaverine

7. Benzonitrile
Cinnamaldehyde
Linalool
Vanillin
Skatole
Cadaverine

8. Benzonitrile
Cinnamaldehyde
Linalool
Vanillin
Skatole
Cadaverine

9. Benzonitrile
Cinnamaldehyde
Linalool
Vanillin
Skatole
Cadaverine

10. This is probably a triplet,
Representing the two methyl groups.
The other eight hydrogens are here.
This is much harder to interpret.
You are completely reliant on integration
And chemical shift.

11. It is very difficult to distinguish pentane from hexane.

12. 13C-NMR The three different types of C are visible.
13C-NMR has a large chemical shift range.
Most signals fall between 0 and 220 ppm.
31.9
14.2
22.9

13. 13C Nuclear Magnetic Resonance
13C-NMR has a large chemical shift range.
Most signals fall between 0 and 200 ppm.
13C is NMR active, 12C is invisible in NMR.
The natural abundance of 13C is 1.108%.
So, a molecule that contains 100 carbons will contain one 13C nucleus
One molecule of hexane out of 16 will contain one 13C nucleus
Chances of one molecule containing two 13C nuclei within coupling range- insignificant.
Good News: Second order effects typically do not appear.
Bad News: No coupling information about adjacent carbons.

14. 13C Nuclear Magnetic Resonance
The sensitivity 13C-NMR spectroscopy is quite low.
1H-NMR is about 160 times more sensitive than 13C-NMR.
A typical 1H-NMR spectrum takes 4 minutes.
A 13C spectrum without relying on any “tricks” would take ~ hours.
With “tricks” a 13C spectrum can take only 10 minutes.
Tricks include higher concentration and proton decoupling.
Protons (aka H’s) can couple with carbons 1,2, and 3 bonds away.
This divides the signal strength over several peaks instead of one.
By using a small amount of background frequency, the protons are
continually excited and do not couple. Some of this excess energy is
transferred to the carbons increasing the signal strength.
As a result, you get taller peaks,
but these taller peaks are not necessarily due to the number of carbons.
Therefore, 13C-NMR spectra cannot be reliably integrated.

15. Coupling due to 13C 1H attached to 12C
12C is ~99% abundant and does not couple
So 1H-NMR shows very little coupling to carbon.
Ha = methyl Hb = aromatic
Each unique H-atom environment gives a signal unique = non-equivalent
10 H-atoms, but only 2 non-equiv H-atom
# non-equivalent H-atoms = # signals in NMR spectrum
Draw p-xylene and label each H-atom
1H attached to 13C
1H attached to 13C

16. Effects of decoupling: MeOH
512 scans
30 min
10 M concentration
1H is > 99% abundant, so it couples with the 13C it is attached to (1 bond Hz)
JC-H=144 Hz
CDCl3
Ha = methyl Hb = aromatic
Each unique H-atom environment gives a signal unique = non-equivalent
10 H-atoms, but only 2 non-equiv H-atom
# non-equivalent H-atoms = # signals in NMR spectrum
Draw p-xylene and label each H-atom

17. Effects of decoupling: EtOH
512 scans
30 min
0.5 M concentration
CDCl3
CH3CH2OH
CH3CH2OH
Ha = methyl Hb = aromatic
Each unique H-atom environment gives a signal unique = non-equivalent
10 H-atoms, but only 2 non-equiv H-atom
# non-equivalent H-atoms = # signals in NMR spectrum
Draw p-xylene and label each H-atom

18. Effects of decoupling: Ethyl cyanoacetate 512 scans 30 min
0.35 M concentration d c
CDCl3 a e b a b c d e
Notice the peak height:
Due to relaxation differences,
The area of the peaks is not proportional
to number of carbons.
Ha = methyl Hb = aromatic
Each unique H-atom environment gives a signal unique = non-equivalent
10 H-atoms, but only 2 non-equiv H-atom
# non-equivalent H-atoms = # signals in NMR spectrum
Draw p-xylene and label each H-atom

19. Peak area is proportional to amount of nuclei flipped.
Total Spectra
800
000
First Scan
sp carbon
sp3 carbon
500
300
000
800
Second Scan
sp carbon
Slow
relaxation
sp3 carbon
Fast
800
000
800
000
Relaxation
Due to low abundance of 13C, multiple scans are taken and added together
13C nuclei can take several seconds to relax back down to the lower energy.
Peak intensity/area is proportional to amount of nuclei flipped. 19

20. Effects of decoupling: Ethyl cyanoacetate 512 scans 30 min
0.35 M concentration d c
CDCl3 a e b a b c d e
Notice the peak height:
Due to relaxation differences,
The area of the peaks is not proportional
to number of carbons.
Ha = methyl Hb = aromatic
Each unique H-atom environment gives a signal unique = non-equivalent
10 H-atoms, but only 2 non-equiv H-atom
# non-equivalent H-atoms = # signals in NMR spectrum
Draw p-xylene and label each H-atom

21. Nuclear Magnetic Resonance
Three Characteristics of NMR Spectra
Chemical Shift
Local Magnetic Environment
How Many Types of C’s
Integration
How Many of Each Type of C
Coupling
How Many H’s on the C’s

22. sp2 carbons
13C Chemical Shift Table
sp3 carbons

23. Actual Example: Acetamidobenzenesulfonamide
Integration
2.95
2.05
4.03
0.97

24. Actual Example: Acetamidobenzenesulfonamide