Infrared Spectroscopy

1
Infrared Spectroscopy

• Gives information about the functional groups in
  a molecule

2
Infrared Spectroscopy

• region of infrared that is most useful lies
  between2.5-16 ?m (4000-625 cm-1)
• depends on transitions between vibrational
  energy states
• stretching
• bending

3
Fig. 13.30
4
Fig. 13.31
IR Spectrum of Hexane
5
Fig. 13.32
6
Fig. 13.33
7
Fig. 13.34
8
Fig. 13.35
9
Fig. 13.36
10
Ultraviolet-Visible (UV-VIS) Spectroscopy

• Gives information about conjugated ? electron
  systems

11
Transitions between electron energy states

• gaps between electron energy levels are greater
  than thosebetween vibrational levels
• gap corresponds to wavelengthsbetween 200 and
  800 nm

?E h?
12
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

13
UV Spectrum of cis,trans-1,3-cyclooctadiene
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

15

• Methyl groups on double bond cause ?max to shift
  to longer wavelengths

H
H
H
CH3
H
CH3
H
H
?max 170 nm
?max 188 nm
16

• Extending conjugation has a larger effect on
  ?max shift is again to longer wavelengths

H
H
H
H
H
H
H
H
H
H
?max 170 nm
?max 217 nm
17
?max 217 nm(conjugated diene)
?max 263 nmconjugated triene plus two methyl
groups
18
Lycopene

• orange-red pigment in tomatoes

?max 505 nm
19
Mass Spectrometry
mass spec is different because it is not related
to electromagnetic radiation
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

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

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.

23
Principles of Electron-Impact Mass Spectrometry
The molecular ion dissociates to a cationand a
radical.

24
Principles of Electron-Impact Mass Spectrometry
The molecular ion dissociates to a cationand a
radical.

• Usually several fragmentation pathways are
  available and a mixture of ions is produced.

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

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

27
Fig. 13.39
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.
29
Isotopesin Chlorobenzene
30
Isotopic Clustersin Chlorobenzene

• ion responsible for m/z 77 peak does not contain
  Cl

31
Alkanes undergo extensive fragmentation
Decane, C10H22
The largest peak may not be the parent ion, or
may not be visible at all.
32
Propylbenzene fragments mostlyat the benzylic
position
91
120
33
Molecular Formulaas aClue to Structure
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.

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.

93
36
Exact Molecular Weights
CH3(CH2)5CH3
Heptane
Cyclopropyl acetate
C7H16
C5H8O2
Molecular formula
100
100
Molecular weight
Exact mass
100.1253
100.0524

• Mass spectrometry can measure exact masses.
• Therefore, mass spectrometry can be used to
  distinguish between molecular formulas.

37
Index of Hydrogen DeficiencyDegree 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 2 - h - x n)
38
Rings versus Multiple Bonds
Index of hydrogen deficiency tells us the sum
ofrings plus multiple bonds. Using catalytic
hydrogenation, the number ofmultiple bonds can
be determined.