Title: UV-VIS Molecular Spectroscopy
1UV-VIS Molecular Spectroscopy
- Chapter 13-14
- From 190 to 900 nm!
2Reflection and Scattering Losses
3LAMBERT-BEER LAW
Power of radiation after passing through the
solvent
Power of radiation after passing through the
sample solution
4Absorption Variables
5Beers 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
6Assumptions
Ingle and Crouch, Spectrochemical Analysis
7Deviations from Beers Law
Successful at low analyte concentrations
(0.01M)! High concentrations of other species may
also affect
8Chemical 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.
9Instrumental deviation with polychromatic
radiation
10Effects of Stray Light
11Instrument Noise
12Effects of Signal-to-Noise
Bad at High T
Bad at Low T
13Components of instrumentation
- Sources
- Sample Containers
- Monochromators
- Detectors
14Components of instrumentation
- Sources Agron, Xenon, Deuteriun, or Tungsten
lamps - Sample Containers Quartz, Borosilicate, Plastic
- Monochromators Quarts prisms and all gratings
- Detectors Pohotomultipliers
15SourcesDeuterium and hydrogen lamps (160 375
nm)
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.
16SourcesTungsten 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
17General Instrument Designs Single beam
Requires a stabilized voltage supply
18General Instrument Designs Double Beam Space
resolved
Need two detectors
19General 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
21Multi-channel Design
22Molar 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)
23Visible 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
26Molecular Transitions for UV-Visible Absorptions
- What electrons can we use for these transitions?
27MO Diagram for Formaldehyde (CH2O)
H
C
O
H
s
p
n
28Singlet 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.
29Type 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
30Examples of UV-Visible Absorptions
LOW!
31UV-Visible Absorption Chromophores
32Effects 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
33UV-Visible Absorption Chromophores
34Typical UV Absorption Spectra
Chromophores?
35Effects of Multiple Chromophores
36The 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.
37Now 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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39Summary 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
40List of common chromophores and their transitions
41Organic 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
42Rule 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
43Spectral nomenclature of shifts
44What 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.
45Transition metal ions
46Charge-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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