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UV-VIS Molecular
Spectroscopy
Chapter 13-14
From 190 to 900 nm!
Reflection and Scattering Losses
LAMBERT-BEER LAW
Psolution
P
T 

Psolvent
P0
Power of radiation
after passing
through the solvent
Power of radiation after
passing through the
sample solution
 P
A   log T   log  
 P0 
A  abc  kc
a  absorptivi ty
b  pathlength
c  concentrat ion
Absorption Variables
Beer’s law and mixtures
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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
Assumptions
Ingle and Crouch, Spectrochemical Analysis
Deviations from Beer’s Law
I r (2 1 ) 2

I 0 (2  1 ) 2
Successful at low analyte concentrations (0.01M)!
High concentrations of other species may also affect
Chemical Equilibria
Consider the equilibrium:
A+C
AC
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.
Instrumental deviation with
polychromatic radiation
Effects of Stray Light
PS  stray light
T 
P  PS
P0  PS
A   log T
 P  PS 

A   log 
 P0  PS 
A  abc  kc
PS
 100
P0
Instrument Noise
Effects of Signal-to-Noise
1
0.9
Bad at High T
TRANSMISSION
0.8
0.7
0.6
0.5
0.4
0.3
0.2
Bad at Low T
0.1
0
1
2
3
4
5
6
7
8
9
10
% RELATIVE CONCENTRATION UNCERTAINTIES
11
Components of instrumentation:
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Sources
Sample Containers
Monochromators
Detectors
Components of instrumentation:
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Sources: Agron, Xenon, Deuteriun, or Tungsten lamps
Sample Containers: Quartz, Borosilicate, Plastic
Monochromators: Quarts prisms and all gratings
Detectors: Pohotomultipliers
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.
Sources
Tungsten lamps (350-2500 nm)
Blackbody type , temperature dependent
Why add I2 in the lamps?
W + I2 → WI2
Low limit: 350 nm
1)Low density
2)Glass envelope
General Instrument Designs
Single beam
Requires a stabilized voltage supply
General Instrument Designs
Double Beam: Space resolved
Need two detectors
General Instrument Designs
Double Beam: Time resolved
Double Beam Instruments
1.Compensate for all but the most short term fluctuation in
radiant output of the source
2.Compensate drift in transducer and amplifier
3.Compensate for wide variations in source intensity with
wavelength
Multi-channel Design
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)
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P>0.1-1  allowable transitions
P<0.01  forbidden transitions
Molecular Absorption
M  h  M* (absorption 10-8 sec)
 M*  M  heat (relaxation process)
 M*  A+B+C (photochemical decomposition)
 M*  M  h (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
Molecular Transitions
for UV-Visible Absorptions
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What electrons can we use for these
transitions?
MO Diagram
for
Formaldehyde
(CH2O)
H
C
O
H
=
=
n=
Singlet vs. triplet
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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.
Type of Transitions
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σ → σ*
High energy required, vacuum UV range
CH4: l = 125 nm
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n → σ*
Saturated compounds, CH3OH etc (l = 150 - 250 nm)
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n → * and  → *
Mostly used! l = 200 - 700 nm
Examples of
UV-Visible Absorptions
LOW!
UV-Visible Absorption Chromophores
Effects of solvents
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Blue shift (n- *) (Hypsocromic shift)
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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- * and  –*) (Bathochromic shift)
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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
UV-Visible Absorption Chromophores
Typical UV Absorption Spectra
Chromophores?
Effects of Multiple Chromophores
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.
Now solvents are your “container”
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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.
Summary of transitions for organic
molecules
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  * transition in vacuum UV (single bonds)
n  * saturated compounds with non-bonding
electrons
l ~ 150-250 nm
e ~ 100-3000 ( not strong)
n  *,   * requires unsaturated functional
groups (eq. double bonds) most commonly used,
energy good range for UV/Vis
l ~ 200 - 700 nm
n  * : e ~ 10-100
  *: e ~ 1000 – 10,000
List of common chromophores and their
transitions
Organic Compounds
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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
Rule of thumb for conjugation
If greater then one single bond apart
- e are relatively additive (hyperchromic shift)
- l constant
CH3CH2CH2CH=CH2
lmax= 184
emax = ~10,000
CH2=CHCH2CH2CH=CH2
lmax=185
emax = ~20,000
If conjugated
- shifts to higher l’s (red shift)
H2C=CHCH=CH2
lmax=217 emax = ~21,000
Spectral nomenclature of shifts
What about inorganics?
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Common anions n* nitrate (313 nm), carbonate (217 nm)
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Most transition-metal ions absorb in the UV/Vis region.
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In the lanthanide and actinide series the absorption process
results from electronic transitions of 4f and 5f electrons.
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For the first and second transition metal series the absorption
process results from transitions of 3d and 4d electrons.
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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.
Transition metal ions
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.
1)
2)
Large molar absorptivity (εmax >10,000)
Many organic and inorganic complexes