Chapter 14 Applications of UVVIS Molecular absorption spectroscopy

1
Chapter 14 Applications of UV-VIS Molecular
absorption spectroscopy

• Magnitude of molar absorptivity e 8.7
  x1019 PA
• P transition probability
• A cross section target area (absorbing species)
  in cm2. Typical organic molecule has A ? 10-15
  cm2.
• P can range from 0 to 1.
• If transition is quantum mechanically allowed P ?
  0.1-1 and e ? 104 -105.
• If transition is forbidden then P lt 0.01 and e lt
  100.
• Example of a forbidden transition is d-d
  transition in octahedral Co(H2O)62,
• Co2(aq), and Cr(H2O)6 3, Cr3(aq), species you
  studied in lab.
• these d-d transitions are between non bonding
  (or slightly bonding) and antibonding d orbitals
  in metal coordination compounds

2
Types of orbitals involved when molecular species
absorb light are s, p, n, p, and s molecular
orbitals. What are their relative energies and
what are allowed transitions?
s _____ p _____
n _____ p _____
s _____
E
What are relative energies of transitions?
relative frequencies?
3
Examples to determine orbitals involved in
molecular absorption (and emission) and allowed
transitions

• C2H6
• C2H4
• CH2O

4
charge transfer band

• Charge transfer complex electron donor group
  bonded to an electron acceptor group
• When complex absorbs light the promoted electron
  is transferred from a filled orbital on donor to
  an empty orbital on the acceptor (intramolecular
  redox)
• These transitions are allowed and have very large
  molar absorptivities (often e gt 10,000).
• Although lmax is most often in UV, broadness of
  absorption band usually causes compound to have a
  color because part of band trails into visible.

5
Charge Transfer Example
Fe(NCS)2
Fe(NCS)2
Fe(NCS)2
Fe(III) NCS- Fe(II)
NCS
Figure 14-10
6
Effects of conjugation

• As number of double bonds in a conjugated system
  increases, p orbital drops in E.
• This causes DE between p and p orbitals involved
  in transition to decrease
• Results
• absorption shifts to longer wavelength
  (bathochromic shift)
• molar absorptivity increases (hyperchromic
  effect)

7
Example of Conjugation Effects
Table 2-2 Approximate Absorption Maxima and Molar
Absorptivity of p p
Transitions in Various Carbon-Carbon bonds
Banwell, C. N. "Fundamentals of Molecular
Spectroscopy", p. 230, McGraw Hill, New York,
1966.
From Olsen, Modern Optical Methods of Analysis,
p. 89, McGraw-Hill, New York, 1975.
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Procedural details for quantitative analysis
(14D-2)

• 1) lmax from spectrum
• 2) variables that influence A
• solvent effects, pH, T, electrolyte
  concentrations (ionic strength effects), and
  presence of interferences
• 3) cleaning and handling of cells
• 4) calibration curves external standard,
  internal
  standard, and standard addition

9
Applications

• simultaneous analysis of two or more compounds
• photometric titrations

10
Simultaneous Analysis of two compounds

• Three conditions for analysis
• 1) Need one wavelength per analyte
• 2) simultaneous (independent) equations (one per
  analyte)
• A1 (eb)11C1 (eb)12C2 (eb)1nCn
• A2 (eb)21C1 (eb)22C2 (eb)2nCn
• simultaneous means independent so cannot have
  combinations like
• (eb)11 (eb)21 and (eb)12 (eb)22 or (eb)11
  (eb)21 (eb)12 (eb)22

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Example wavelengths
0.60
0.40
A
0.20
0.10
0
Figure 14-14 (modified)
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Simultaneous Analysis of two compounds

• A1 (eb)11C1 (eb)12C2 (eb)1nCn
• A2 (eb)21C1 (eb)22C2 (eb)2nCn
• then when solved for C1 and C2

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Simultaneous Analysis of two compounds

• 3) analytes cannot interact with each other in
  solution and change each other's spectra. How to
  tell?
• a) Take spectrum of each compound independently
• b) mathematically add spectra together
• c) take spectrum of same concentrations of same
  two analytes together in solution
• d) compare results of steps b and c
• e) if spectra results are identical then there
  is no interaction
• f) simultaneous analysis can proceed
• g) if results are different, there is
  interaction - simultaneous analysis cannot be
  done

14
Example wavelengths
0.60
0.40
A
0.20
0.10
0
Figure 14-14 (modified)
15
Example wavelengths
0.40
A
0.20
Modified
From Sawyer, Heineman, and Beebe, Chemistry
Experiments for Instrumental Methods,
p. 179, Wiley, New York, 1984.
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Photometric Titrations

• Determine endpoint by following change in A of
• 1) reactant (decrease)
• 2) product (increase)
• 3) titrant (increase after endpoint)

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Example Titration curves for S T
PS analyte being titrated, T titrant, P
product
Figure 14-18
18
Example on board

• hexaureabismuth(III) (Bi(tu)63) complex has lmax
  470 nm
• ethylenediaminetetraacetatebismuth(III) (BiY-)
  has lmax 265 nm
• EDTA4- has much larger formation constant with
  Bi3 than thiourea
• Predict shape of curve for titration of Bi(tu)63
  with EDTA monitored at 470 nm. Reaction
• Bi(tu)63 H2Y2- BiY-
  6 tu 2 H