1. Infrared Spectroscopy
Gives information about the functional groups in a molecule 1

2. Infrared Spectroscopy
region of infrared that is most useful lies between m ( cm-1)
depends on transitions between vibrational energy states
stretching
bending 2

3. Fig

4. Fig
IR Spectrum of Hexane

5. Fig

6. Fig

7. Fig

8. Fig

9. Fig

10. Ultraviolet-Visible (UV-VIS) Spectroscopy
Gives information about conjugated  electron systems 1

11. Transitions between electron energy states
gaps between electron energy levels are greater than those between vibrational levels
gap corresponds to wavelengths between 200 and 800 nm
E = h 2

12. X-axis is wavelength in nm
Conventions in UV-VIS
X-axis is wavelength in nm
Y axis is a measure of absorption of electromagnetic radiation expressed as molar absorptivity ()
max is the wavelength of maximum absorption and is related to electronic makeup of molecule,especially  electrons 3

13. UV Spectrum of cis,trans-1,3-cyclooctadiene
max 230 nm
max 2630 4

14. * Transition in Alkenes
HOMO-LUMO energy gap is affected by substituents on double bond
as HOMO-LUMO energy difference decreases (smaller E), max shifts to longer wavelengths 5

15. Methyl groups on double bond cause max to shift to longer wavelengths
CH3 C C H
CH3 H H
max 170 nm
max 188 nm 5

16. Extending conjugation has a larger effect on max; shift is again to longer wavelengths
max 170 nm
max 217 nm 5

17. max 217 nm (conjugated diene)H C
max 217 nm (conjugated diene) C H
CH3
H3C
max 263 nm conjugated triene plus two methyl groups 5

18. orange-red pigment in tomatoes
Lycopene
orange-red pigment in tomatoes
max 505 nm 5

19. Mass Spectrometry
mass spec is different because it is not related to electromagnetic radiation 1

20. Principles of Electron-Impact Mass Spectrometry
Atom or molecule is hit by high-energy electron from an electron beam at 10ev e– e–
beam
+ •
forming a positively charged, odd-electron species called the molecular ion 2

21. Principles of Electron-Impact Mass Spectrometry
Molecular ion passes between poles of a magnet and is deflected by magnetic field
deflection depends on mass-to-charge ratio
highest m/z deflected least
lowest m/z deflected most
+ • 5

22. Principles of Electron-Impact Mass Spectrometry
If the only ion that is present is the molecular ion, mass spectrometry provides a way to measure the molecular weight of a compound and is often used for this purpose.
However, the molecular ion often fragments to a mixture of species of lower m/z. 6

23. Principles of Electron-Impact Mass Spectrometry
The molecular ion dissociates to a cation and a radical.
+ • 2

24. Principles of Electron-Impact Mass Spectrometry
The molecular ion dissociates to a cation and a radical. + •
Usually several fragmentation pathways are available and a mixture of ions is produced. 2

25. Principles of Electron-Impact Mass Spectrometry
mixture of ions of different mass gives separate peak for each m/z
intensity of peak proportional to percentage of each ion of different mass in mixture
separation of peaks depends on relative mass + + + + + + 6

26. Principles of Electron-Impact Mass Spectrometry
mixture of ions of different mass gives separate peak for each m/z
intensity of peak proportional to percentage of each ion of different mass
separation of peaks depends on relative mass + + + + + + 6

27. Fig

28. Some molecules undergo very little fragmentation
Benzene is an example. The major peak corresponds to the molecular ion.
m/z = 78
The largest peak is called the base peak and is reference to 100 to give relative intensity. 9

29. Isotopes in Chlorobenzene
35Cl
37Cl
Isotopes in Chlorobenzene 11

30. Isotopic Clusters in Chlorobenzene+
ion responsible for m/z 77 peak does not contain Cl 11

31. Alkanes undergo extensive fragmentation
Decane, C10H22
The largest peak may not be the parent ion, or may not be visible at all. 13

32. Propylbenzene fragments mostly at the benzylic position91
CH2—CH2CH3
120 14

33. Molecular Formula as a Clue to Structure1

34. Molecular Weights
One of the first pieces of information we try to obtain when determining a molecular structure is the molecular formula.
We can gain some information about molecular formula from the molecular weight.
Mass spectrometry makes it relatively easy to determine molecular weights. 6

35. The Nitrogen Rule
A molecule with an odd number of nitrogens has an odd molecular weight.
A molecule that contains only C, H, and O or which has an even number of nitrogens has an even molecular weight.
NH2 93
138
NH2
O2N
183
NH2
O2N
NO2 6

36. Exact Molecular Weights
CH3CO O
CH3(CH2)5CH3
Heptane
Cyclopropyl acetate
Molecular formula
C7H16
C5H8O2
Molecular weight
100
100
Exact mass
Mass spectrometry can measure exact masses.
Therefore, mass spectrometry can be used to distinguish between molecular formulas. 6

37. Index of Hydrogen Deficiency Degree of Unsaturation
relates molecular formulas to multiple bonds and rings
For a molecular formula, CcHhNnOoXx, the degree of unsaturation can be calculated by:
Degree = ½ (2c h - x + n) 6

38. Rings versus Multiple Bonds
Index of hydrogen deficiency tells us the sum of rings plus multiple bonds.
Using catalytic hydrogenation, the number of multiple bonds can be determined. 6