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UV-VIS SPECTROSCOPY
General info:
Spectrometryis based on the absorption ofphotons by the analyte.Spectrometric method: the sample solution absorbselectromagnetic radiation from an appropriate source
and the amount absorbed is related to theconcentration of the analyte in the solution.
Eg: copper solution is blue because it absorbs the
yellow color from white light and transmits theremaining blue light. The higher[cu], the more yellowlight is absorbed and the deeper the resulting bluecolor of the solution.
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The electromagnetic spectrum- Review
A form of radiant energy that is propagated as a
transverse wave Vibrate perpendicular to the direction of
propagation - a wave motion to the radiation
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Continue The relationship between wavelength and
frequency:;
= wavelength in cm
= frequency in s-1or Hzc = velocity of light (3x1010cm/s) The wavelength varies from a few to several
meters, for UV & Visnanometer for IRmicrometer, commonly used as wave
number, in cm-1
c
v
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Continue
Electromagnetic radiation possesses a
certain amount of energy called photon,related to the or by:
energy of the photon in ergs= Planks constant,
Note: Shorter wavelength, greater energyUV from the sun burns you!!!!
v
hchvE
E
h sec1062.6 34
Joule
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Working ranges of the UV-Vis and IR spectra
UV 200-380 nmVis 380-780 nmNear IR 0.78-2.5 mMid IR 2.5-15 m
The most
analytically region
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How does matter absorb radiationThree basic processes by which a molecule can absorb radiation;all involve raising the molecule to a higher internal energy level,the increase in energy being equal to the energy of theabsorbed radiation, . Three type of internal energy arequantized;
Rotational transition: the molecule rotates about various axes,the energy of rotation being a definite energy levels, so the
molecule may absorb radiation and be raised to a higherrotational energy level.
Vibrational transition: the atoms or groups of atoms within amolecule vibrate relative to each other, and the energy of this
vibration occurs at definite quantized levels. The molecules maythen absorb a discrete amount of energy and be raised to ahigher vibrational energy level.
Electron transition: the electron of a molecule may be raised tohigher electron energy. UV-VIS spectroscopy principal/theory
hv
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Energy level diagram illustrating energy changes associated with absorption ofelectromagnetic radiation
A = pure rotational changer (far IR)B = rotational + vibrational changes (near IR)C = rotational + vibrational + electronic transition(Vis + UV)Eo= electronic ground stateE1= first electronic exited state
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Electronic Spectra and Molecule Structure
The electronic transition in the UV-VIS regions are due tothe absorption of radiation by specific types of: Groups,
bonds and functional groups within the molecule Kinds of transitions: Electrons in a molecule can be
classified into 4 different types:i. Closed shell electronnot involved in bondingii. Covalent single bond electron,iii. Paired non bonding outershell electron (n electrons)
such as those on N, O, S, and halogens. These are lesstightly held then electrons and can be excited by visibleor UV radiation
iv. Electron in orbitals, in = or bonds. These are themost readily excited and are responsible for a majorityof electronic spectra in the UV-VIS regions.
* and n electrons responsible for UV-VIS spectrum
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Other electron remain/occupate in orbitals
A molecule also possesses unoccupied
orbitals called antibonding orbitals; thesecorrespond to excited- state energy levels(either * or * ).Therefore, absorption ofradiation results in an electronic transition
to antibonding orbitals. The most common transition:
* orn *n *
above 200 nm
occur at very short
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HOMO: Highest Occupied Molecular OrbitalLUMO: Lowest Unoccupied Molecular Orbital
Example, molecule H2
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Example of transition * and n *in ketone (acetone)
* n *The probability of * is higher thann *
Molar absorptivities,* (1000-100,000)
n * (less than 1000)
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ABSORPTION BY ISOLATED CHROMOPHORES
The absorbing groups (the electrons responsible forthe absorption) in a molecules are calledchromophores
A molecule containing a chromophorechromogen
The present of auxochrome enhance theabsorption by a chromophore or shift the ofabsorption.
Example: hydroxyl (OH), amine (-NH2), halogens(X)
posses n electronsinteract with electronsin the chromophores (n ) conjugation.
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Spectra changes can be classed as:
Bathochromic shift (red shift) shifted to shorter
wavelength. It may caused by removal of conjugation. Hypsochromic shift (Blue shift) - shifted to shorter
wavelength. It may caused by removal of conjugation.Eg. The conjugation of the lone pair electrons on the
N atom on aniline with the bond system of thebenzene ring is removed on protonationNeutral aniline absorbs at = 230 nm
= 8600
But in acid solution (protonated) = 203 nm= 7500
max
max
max
max
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Spectra changes can be classed as: continue.
Hyperchromic effectincreased absorption
intensity ( ) Hypochromic effectdecreased absorption
intensity ( )
Note: = the wavelength of the absorptionmaximum
= Absorption [ ] of a 1%solution in a cell with 1 cmpathlength- used when MW of a
compound is not known
max
%11cm )(log 010 I
I
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BEERS LAW-ABSORPTION LAW
Fraction of radiant energy transmitted decays exponentially withpathlength.
Transmittance in exponential form:
is a constant (eq 1)
Putting in logarithmic form,
0I Ic
b
Transmittedradiation
Incidentradiation
Pathlength concentration
kb
I
IT
100
k
kb
I
IT
0
loglog (Eq 2
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BEERS LAW-ABSORPTION LAWcontinue
Similar law holds for the dependence of T on the concentration,
Combining (Eq 1) and (Eq 3), describes the dependence of T on thepathlength and concentration.
Omit theve sign, and rearrange
Hence,
c
ck
IIT '0
10
'k is a new constant (Eq 3)
ckI
IT 'loglog
0
(Eq 4)
abc
I
IT 10
0
a is combined constant of kand k (Eq 5)
abcIIT 0
loglog
abcI
I
TTA
0log1
loglog
abcA
Absorbance Absorptivity Pathlength in cm
Conc in g/L
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BEERS LAW-ABSORPTION LAWcontinue
Note: in absorption spectrum, the absorbance varies withwavelength in direct proportion to a (b & c are held
constant). The product of the absorptivity and themolecular weight of the absorbing species is called themolar absorptivity,
Thus bcA )( 11 Lmolcm
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Example
Amines, RNH2react with picric acid to form amine picrates, which
absorb strongly at 359 nm (=1.25 x 104). An unknown amine
(0.1155 g) is dissolved in water and dilute to 100 mL. A 1 ml aliquotof this is diluted to 250 ml for measurement. If this final solution
exhibit an absorbance of 0.454 at 359 nm using a 1.00 cm cell,
what is the formula weight of the amine? What is a probable
formula?
Solution:
in original flask
bcA ccmLmolcm 00.11025.1454.0 114
Lmolc /1063.3 5
mL
mLLLmol 00.1
100)250.0)(/1065.3(
5
mol41008.9
1
4 127
1008.9
1155.0
gmol
mol
g
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Measurement of the UV Spectrum:
Using a dilute solution: 1 mg of compound with MW
(100-400 g/mol) is dissolved in 100 mL volumetric
flask. Hexane is used as a solvent. Portion of the
solution is transferred to a cuvett quartz of 1 cm
(Styrene, MW = 104 g/mol
max=250 nm
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Choice of solvent
Solvent consideration: cheap, good solvent transparent downto about 210 nm.
Most commonly used95% ethanolSolvent Effect
Electronic Transition, *: atom do not move, howeverelectrons of the atom and solvent molecules may
reorganise Transition state is more polar than ground state due to
dipole-dipole interaction with solvents, thus lower the
energy of the excited state.
Eg: Styrene in ethanol (max
= 270 nm)
Styrene in hexane (max= 250 nm)
about 20 nm shift (red shift, longer wavelength)
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Electronic transition, n * (the weaktransition of the O lone pair in ketones