Molecular Absorptions

1
Molecular Absorptions

• Interactions of light with matter

2
Emission and Absorption of Light

• Emission generates a pattern of narrow bright
  lines
• Absorption generates a pattern of narrow dark
  lines in a bright background

3
What does the absorbed light (electromagnetic
radiation)do to the molecule?
UV
IR
visible
700 nm
400 nm
Energy increasing

• high energy UV ionizes electrons
• low energy UV and visible promotes electrons to
  higher energy orbitals
• (absorption of visible light leads to a colored
  solution)
• IR causes molecules to vibrate (more later)

4
The process of light being absorbed by a solution
concentration 2
with sample I lt Io
concentration 1
blank where Io I
light source
detector
Io
I
As concentration increased, less light was
transmitted (more light absorbed).
b
Cell with Pathlength, b, containing solution
5
Some Terminology

• I-Intensity where I0 is the initial intensity
• T-Transmittance or T 100 X T
• T I/ I0
• Absorption
• Abs 1 T Abs 100 - T
• A Absorbance
• A - log T -log I/ I0

6
Beers Law

• The thicker the sample, the more light is
  absorbed
• The higher the concentration the more light is
  absorbed
• Absorbance A abc
• a molar absorptivity
• b pathlength
• c concentration in moles

7
Beers Law

• Beers Law lets you quantify the amount of
  material in a sample provided
• Its the only thing absorbing at that wavelength
• Or
• It is the only component that is changing
  concentration during the tests

8
Analyze at what wavelength?
Scan visible wavelengths from 400 650 nm
(detector range) to produce an absorption
spectrum (A vs. l)
phototube detector range
lmax - wavelength where maximum absorbance occurs
9
The Blank

• Contains all substances except the analyte (the
  compound you are trying to measure)
• Is used to set the Absorbance to zero
• Absblank 0
• Removes any effect of any light absorption or
  other losses due to these materials and the cell
• All Absorbance is due to the analyte

10
The components of a Spec-20D
Light source
- white light of constant intensity
slits
filter
occluder
Grating
slits
Separates white light into various colors
Phototube
detects light measures intensity
Rotating the grating changes the wavelength going
through the sample
Sample
When blank is the sample Io is
determined otherwise I is measured
11
Double Beam Spectrophotometer

• Allows direct comparison of sample and reference
• Grating is used to disperse the light
• Slit selects wavelength
• Turning grating tunes the selected wavelength

12
Double Beam Spectrophotometer with a Chopper
Uses the Same Detector
13
Multi-channel Detector Looks at All Wavelengths
Simultaneously
14
Quantifying Concentration

• Standard with measured absorbance
• Astd abcstd
• Unknown with measured absorbance
• Aunk abcunk
• Ratio the two equations
• Aunk / Astd cunk/cstd

15
When calibration curves go bad!

• The linear Beers Law relationship starts to show
  curvature at high concentrations
• Single-point calibration assumes a linear
  calibration curve

Non-linear
16
Covalent Bond Formation in H2

• As atoms begin to approach electron interactions
  are attractive
• Very close together, electron screening of the
  nuclei becomes less and nuclear repulsion
  dominates.
• Movie clip

17
Covalent Bonds

• When electrons are shared in covalent bonds they
  are delocalized over two or more atoms.
• The electrons reorganize to minimize repulsions
  between electron clouds
• e.g. formation of tetrahedral shape in methane
• Movie Clip

18
sp3 Hybridization Single bonds
19
Hybridized Orbitals Between Atoms Overlap to form
Bonds

• Bond directions minimize repulsions

20
sp2 Hybridization Double bonds

• 3rd p orbital is perpendicular to the plane and
  not hybridized

21
sp2 Hybridization Ethylene
22
sp Hybridization Triple Bonds

• Remaining 2 p orbitals are perpendicular and
  unhybridized

23
sp Hybridization Acetylene
24
Bonding in Organic Molecules

• Electrons can reside in 3 types of molecular
  electronic orbitals
• s orbitals from overlap of sp3, sp2 and sp
  orbitals
• p orbitals from overlap of p orbitals in double
  and triple bonds
• n orbitals which arise from lone pair electrons
  like those on O or N

25
Excited State Orbitals

• Just like there are higher lying empty orbitals
  in atoms, there are higher lying empty molecular
  orbitals.
• Anti-bonding characterlittle elctron density
  between the atoms
• Symbolized by a

26
Bonding and Antibonding Orbitals
27
Antibonding orbitals are at higher energy
28
Types of Transitions
29
Types of Transitions

• ??? Very high energy, not usually observed (125
  nm)
• n?? 150-250 nm (UV)
• ??? and n?? (200-700 nm)
• Cause visible colors
• Chains of alternating double bonds (conjugation)
  lower the energies of the transitions

30
Spectra are complicated by molecular vibrations
and rotations
31
Absorption Wavelength Depends on Conjugation
The more double bonds the more the spectrum
shifts towards the visible.
32
Another example of conjugation effects
Colorless colorless orange
Violet green
33
Visible Absorption of Dyes
The color of the dye depends on the colors it
transmits
34
Eosin Y
Absorption Spectrum
Fluorescence Spectrum
35
What problems do you see?

• Spectra have relatively few features
• Spectra all tend to overlap in the ultraviolet
• How can you separate features in mixtures?

36
Answer

• Use a separation technique like column
  chromatography or HPLC FIRST!
• Hopefully you then have single component samples
  to identify