1. UV-VIS Molecular Spectroscopy
Chapter 13-14
From 190 to 900 nm!

2. Reflection and Scattering Losses

3. LAMBERT-BEER LAW Power of radiation after passing through the solvent
Power of radiation after passing through the sample solution

4. Absorption Variables

5. Beer’s law and mixtures
Each analyte present in the solution absorbs light!
The magnitude of the absorption depends on its e
A total = A1+A2+…+An
A total = e1bc1+e2bc2+…+enbcn
If e1 = e2 = en then simultaneous determination is impossible
Need nl’s where e’s are different to solve the mixture

6. Assumptions
Ingle and Crouch, Spectrochemical Analysis

7. Deviations from Beer’s Law
Successful at low analyte concentrations (0.01M)!
High concentrations of other species may also affect

8. Chemical Equilibria A + C AC Consider the equilibrium:
If e is different for A and AC then the absorbance depends on the equilibrium.
[A] and [AC] depend on [A]total.
 A plot of absorbance vs. [A]total will not be linear.

9. Instrumental deviation with polychromatic radiation

10. Effects of Stray Light

11. Instrument Noise

12. Effects of Signal-to-Noise
Bad at High T
Bad at Low T

13. Components of instrumentation:
Sources
Sample Containers
Monochromators
Detectors

14. Components of instrumentation:
Sources: Agron, Xenon, Deuteriun, or Tungsten lamps
Sample Containers: Quartz, Borosilicate, Plastic
Monochromators: Quarts prisms and all gratings
Detectors: Pohotomultipliers

15. Sources Deuterium and hydrogen lamps (160 – 375 nm)
D2 + Ee → D2* → D’ + D’’ + h
Excited deuterium molecule with fixed quantized energy
Dissociated into two deuterium atoms with different kinetic energies
Ee = ED2* = ED’ + ED’’ + hv
Ee is the electrical energy absorbed by the molecule. ED2* is the fixed quantized
energy of D2*, ED’ and ED’’ are kinetic energy of the two deuterium atoms.

16. Sources Tungsten lamps (350-2500 nm)
Blackbody type , temperature dependent
Why add I2 in the lamps?
W + I2 → WI2
Low limit: 350 nm
Low density
Glass envelope

17. General Instrument Designs
Single beam
Requires a stabilized voltage supply

18. General Instrument Designs Double Beam: Space resolved
Need two detectors

19. General Instrument Designs Double Beam: Time resolved

20. Double Beam Instruments
Compensate for all but the most short term fluctuation in
radiant output of the source
Compensate drift in transducer and amplifier
Compensate for wide variations in source intensity with
wavelength

21. Multi-channel Design

22. Molar absorptivities Molecular Absorption
e = 8.7 x P A
A: cross section of molecule in cm2 (~10-15)
P: Probability of the electronic transition (0-1)
P>0.1-1  allowable transitions
P<0.01  forbidden transitions
Molecular Absorption
M + hn  M* (absorption 10-8 sec)
M*  M + heat (relaxation process)
M*  A+B+C (photochemical decomposition)
M*  M + hn (emission)

23. Visible Absorption Spectra

24. The absorption of UV-visible radiation generally results from excitation of bonding electrons.
can be used for quantitative and qualitative analysis

25. Molecular orbital is the nonlocalized fields between atoms that are occupied by bonding electrons. (when two atom orbitals combine, either a low-energy bonding molecular orbital or a high energy antibonding molecular orbital results.)
Sigma () orbital
The molecular orbital associated with single bonds in organic compounds
Pi () orbital
The molecular orbital associated with parallel overlap of atomic P orbital.
n electrons
No bonding electrons

26. Molecular Transitions for UV-Visible Absorptions
What electrons can we use for these transitions?

27. MO Diagram for Formaldehyde (CH2O)
s =
p =
n =

28. Singlet vs. triplet
In these diagrams, one electron has been excited (promoted) from the n to * energy levels (non-bonding to anti-bonding).
One is a Singlet excited state, the other is a Triplet.

29. Type of Transitions σ → σ* n → σ* n → * and  → *
High energy required, vacuum UV range
CH4:  = 125 nm
n → σ*
Saturated compounds, CH3OH etc ( = nm)
n → * and  → *
Mostly used!  = nm

30. UV-Visible Absorptions
Examples of
UV-Visible Absorptions
LOW!

31. UV-Visible Absorption Chromophores

32. Effects of solvents Blue shift (n- p*) (Hypsocromic shift)
Increasing polarity of solvent  better solvation of electron pairs (n level has lower E)
 peak shifts to the blue (more energetic)
30 nm (hydrogen bond energy)
Red shift (n- p* and p –p*) (Bathochromic shift)
Increasing polarity of solvent, then increase the attractive polarization forces between solvent and absorber, thus decreases the energy of the unexcited and excited states with the later greater
 peaks shift to the red
5 nm

33. UV-Visible Absorption Chromophores

34. Typical UV Absorption Spectra
Chromophores?

35. Effects of Multiple Chromophores

36. The effects of substitution
Auxochrome function group
Auxochrome is a functional group that does not absorb in UV region but
has the effect of shifting chromophore peaks to longer wavelength as well
As increasing their intensity.

37. Now solvents are your “container”
They need to be transparent and do not erase the fine structure arising from the vibrational effects
Polar solvents generally
tend to cause this
problem
Same solvent must be
Used when comparing
absorption spectra for
identification purpose.

39. Summary of transitions for organic molecules
s  s* transition in vacuum UV (single bonds)
n  s* saturated compounds with non-bonding electrons
l ~ nm
e ~ ( not strong)
n  p*, p  p* requires unsaturated functional groups (eq. double bonds) most commonly used, energy good range for UV/Vis
l ~ nm
n  p* : e ~
p  p*: e ~ 1000 – 10,000

40. List of common chromophores and their transitions

41. Organic Compounds Most organic spectra are complex
Electronic and vibration transitions superimposed
Absorption bands usually broad
Detailed theoretical analysis not possible, but semi-quantitative or qualitative analysis of types of bonds is possible.
Effects of solvent & molecular details complicate comparison

42. Rule of thumb for conjugation
If greater then one single bond apart
- e are relatively additive (hyperchromic shift)
- l constant
CH3CH2CH2CH=CH2 lmax= emax = ~10,000
CH2=CHCH2CH2CH=CH2 lmax= emax = ~20,000
If conjugated
- shifts to higher l’s (red shift)
H2C=CHCH=CH2 lmax=217 emax = ~21,000

43. Spectral nomenclature of shifts

44. What about inorganics?
Common anions np* nitrate (313 nm), carbonate (217 nm)
Most transition-metal ions absorb in the UV/Vis region.
In the lanthanide and actinide series the absorption process results from electronic transitions of 4f and 5f electrons.
For the first and second transition metal series the absorption process results from transitions of 3d and 4d electrons.
The bands are often broad.
The position of the maxima are strongly influenced by the chemical environment.
The metal forms a complex with other stuff, called ligands. The presence of the ligands splits the d-orbital energies.

45. Transition metal ions

46. Charge-Transfer-Absorption
A charge-transfer complex consists of an electron-donor group bonded to an electron acceptor. When this product absorbs radiation, an electron from the donor is transferred to an orbital that is largely associated with the acceptor.
Large molar absorptivity (εmax >10,000)
Many organic and inorganic complexes