Download - Kuliah UV-Molecular Spectrometry
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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
ionconcentrat
pathlength
tyabsorptivi
loglog0
0
c
b
a
kcabcA
P
PTA
P
P
P
PT
solvent
solution
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 = A1+A2++An A total = e1bc1+e2bc2++enbcn
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
2
12
2
12
0 )(
)(
I
Ir
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Chemical EquilibriaConsider the equilibrium:
A + C AC
If eis different for A and AC then the absorbancedepends on the equilibrium.
[A] and [AC] depend on [A]total.
A plot of absorbance vs. [A]totalwill not be linear.
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Instrumental deviation with
polychromatic radiation
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Effects of Stray Light
kcabcA
PP
PPA
TA
PP
PPT
P
S
S
S
S
S
0
0
log
log
lightstray
1000
PPS
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Instrument Noise
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1 2 3 4 5 6 7 8 9 10 11
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
% RELATIVE CONCENTRATION UNCERTAINTIES
TRANSMISSION
Effects of Signal-to-Noise
Bad at Low T
Bad at High 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 (160375 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 quantizedenergy of D2*, EDand EDare kinetic energy of the two deuterium atoms.
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SourcesTungsten lamps (350-2500 nm)
Blackbody type , temperature dependent
Why add I2in the lamps?
W + I2 WI2
Low limit: 350 nm
1)Low density
2)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
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 withwavelength
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Multi-channel Design
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Molar absorptivities
e= 8.7 x 10 19P 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
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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 orbitalis the nonlocalized fieldsbetween atoms that are occupied by bonding
electrons. (when two atom orbitals combine, eithera low-energy bonding molecular orbital or a highenergy antibonding molecular orbital results.)
Sigma () orbitalThe molecular orbital associated with single bondsin organic compounds
Pi () orbitalThe molecular orbital associated with paralleloverlap 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 Diagramfor
Formaldehyde
(CH2O)
HC
H
O
= =
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
*High energy required, vacuum UV range
CH4: = 125 nm n *
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- *) (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- * and *) (Bathochromic shift) Increasing polarity of solvent, then increase the
attractive polarization forces between solvent andabsorber, thus decreases the energy of the unexcitedand excited stateswith the later greater
peaks shift to the red
5 nm
i i A i C
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UV-Visible Absorption Chromophores
T i l UV Ab i S
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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 increasin their intensit .
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Now solvents are your container
They need to be transparent and do not erase thefine structure arising from the vibrational effects
Polar solvents generallytend to cause this
problem
Same solvent must be
Used when comparing
absorption spectra for
identification purpose.
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Summary of transitions for organic
molecules
* transition in vacuum UV (single bonds)
n* saturated compounds with non-bondingelectrons
l~ 150-250 nm e~ 100-3000 ( not strong)
n*, * requires unsaturated functionalgroups (eq. double bonds) most commonly used,
energy good range for UV/Vis l~ 200 - 700 nm
n* : e~ 10-100
*: 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 orqualitative analysis of types of bonds is possible.
Effects of solvent & molecular details complicate comparison
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If greater then one single bond apart
- eare relatively additive (hyperchromic shift)
- lconstant
CH3CH2CH2CH=CH2 lmax= 184 emax= ~10,000
CH2=CHCH2CH2CH=CH2 lmax=185 emax= ~20,000
If conjugated- shifts to higher ls (red shift)
H2C=CHCH=CH2 lmax=217 emax= ~21,000
Rule of thumb for conjugation
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Spectral nomenclature of shifts
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What about inorganics? Common anions n * nitrate (313 nm), carbonate (217 nm)
Most transition-metal ions absorb in the UV/Vis region.
In the lanthanide and actinide series the absorption processresults 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 anelectron-donor group bonded to anelectron acceptor. When this productabsorbs radiation, an electron from thedonor is transferred to an orbital that islargely associated with the acceptor.
1) Large molar absorptivity (max>10,000)
2) Many organic and inorganic complexes
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