UV-VIS Molecular Spectroscopy

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UV-VIS Molecular Spectroscopy

• Chapter 13-14
• From 190 to 900 nm!

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Reflection and Scattering Losses
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LAMBERT-BEER LAW
Power of radiation after passing through the
solvent
Power of radiation after passing through the
sample solution
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Absorption Variables
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Beers law and mixtures

• Each analyte present in the solution absorbs
  light!
• The magnitude of the absorption depends on its e
• A total A1A2An
• A total e1bc1e2bc2enbcn
• If e1 e2 en then simultaneous determination
  is impossible
• Need nls where es are different to solve the
  mixture

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Assumptions
Ingle and Crouch, Spectrochemical Analysis
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Deviations from Beers Law
Successful at low analyte concentrations
(0.01M)! High concentrations of other species may
also affect
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Chemical Equilibria
Consider the equilibrium
If e is different for A and AC then the
absorbance depends on the equilibrium. A and
AC depend on Atotal. ? A plot of absorbance
vs. Atotal will not be linear.
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Instrumental deviation with polychromatic
radiation
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Effects of Stray Light
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Instrument Noise
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Effects of Signal-to-Noise
Bad at High T
Bad at Low T
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Components of instrumentation

• Sources
• Sample Containers
• Monochromators
• Detectors

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Components of instrumentation

• Sources Agron, Xenon, Deuteriun, or Tungsten
  lamps
• Sample Containers Quartz, Borosilicate, Plastic
• Monochromators Quarts prisms and all gratings
• Detectors Pohotomultipliers

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SourcesDeuterium 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.
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SourcesTungsten 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

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General Instrument Designs Single beam
Requires a stabilized voltage supply
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General Instrument Designs Double Beam Space
resolved
Need two detectors
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General Instrument Designs Double Beam Time
resolved
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• 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

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Multi-channel Design
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Molar absorptivities

• e 8.7 x 10 19 P A
• A cross section of molecule in cm2 (10-15)
• P Probability of the electronic transition (0-1)
• Pgt0.1-1 ? allowable transitions
• Plt0.01 ? forbidden transitions

Molecular Absorption

• M hn ? M (absorption 10-8 sec)
• M ? M heat (relaxation process)
• M ? ABC (photochemical decomposition)
• M ? M hn (emission)

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

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• 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

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Molecular Transitions for UV-Visible Absorptions

• What electrons can we use for these transitions?

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MO Diagram for Formaldehyde (CH2O)
H
C
O
H
s
p
n
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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.

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Type of Transitions

• s ? s
• High energy required, vacuum UV range
• CH4 ? 125 nm
• n ? s
• Saturated compounds, CH3OH etc (? 150 - 250 nm)
• n ? ? and ? ? ?
• Mostly used! ? 200 - 700 nm

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Examples of UV-Visible Absorptions
LOW!
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UV-Visible Absorption Chromophores
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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

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UV-Visible Absorption Chromophores
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Typical UV Absorption Spectra
Chromophores?
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Effects of Multiple Chromophores
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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.
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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 absorptio
n spectra for identification purpose.
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Summary of transitions for organic molecules

• s ? s transition in vacuum UV (single bonds)
• n ? s saturated compounds with non-bonding
  electrons
• l 150-250 nm
• e 100-3000 ( not strong)
• n ? p, p ? p requires unsaturated functional
  groups (eq. double bonds) most commonly used,
  energy good range for UV/Vis
• l 200 - 700 nm
• n ? p e 10-100
• p ? p e 1000 10,000

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List of common chromophores and their transitions
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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

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Rule of thumb for conjugation
If greater then one single bond apart - e are
relatively additive (hyperchromic shift) - l
constant CH3CH2CH2CHCH2 lmax 184 emax
10,000 CH2CHCH2CH2CHCH2 lmax185 emax
20,000 If conjugated - shifts to higher ls
(red shift) H2CCHCHCH2 lmax217 emax
21,000
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Spectral nomenclature of shifts
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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.

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Transition metal ions
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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 (emax gt10,000)
• Many organic and inorganic complexes

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