chpter 1 uv vis spectroscopy
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UV-Vis Spectroscopy
CHAPTER 1
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UV-VIS SPECTROSCOPY
General info:
Spectrometry is based on the absorption of photons by the analyte.
Spectrometric method: the sample solution absorbs electromagnetic radiation from an appropriate source and the amount absorbed is related to the concentration of the analyte in the solution.
E.g: copper solution is blue because it absorbs the yellow color from white light and transmits the remaining blue light. The higher[cu], the more yellow light is absorbed and the deeper the resulting blue color 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:
λ = wavelength in cm
ѵ = frequency in s-1 or Hz
c = velocity of light (3x1010 cm/s)
The wavelength varies from a few Å to several meters,
λ for UV & Vis – nanometer
λ for IR – micrometer, commonly used as wave number, ṽ in cm-1
c
v
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Continue…
Electromagnetic radiation possesses a certain amount of energy called photon, related to the ν or λ by:
E = energy of the photon in ergs
h = Plank’s constant, 6.62 x 10-34 Jsec
Note: Shorter wavelength, greater energy
UV from the sun burns you!!!!
λ
hchvE
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Working ranges of the UV-Vis and IR spectra
UV 200-380 nm
Vis 380-780 nm
Near IR 0.78-2.5 μm
Mid IR 2.5-15 μm
The most analytically region
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How does matter absorb radiation Three 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 the absorbed radiation, hν. Three type of internal energy are quantized;
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 higher rotational energy level.
Vibrational transition: the atoms or groups of atoms within a molecule vibrate relative to each other, and the energy of this vibration occurs at definite quantized levels. The molecules may then absorb a discrete amount of energy and be raised to a higher vibrational energy level.
Electron transition: the electron of a molecule may be raised to higher electron energy. UV-VIS spectroscopy principal/theory
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Energy level diagram illustrating energy changes associated with absorption of electromagnetic radiation
A = pure rotational changer (far IR) B = rotational + vibrational changes (near IR) C = rotational + vibrational + electronic transition (Vis + UV) Eo = electronic ground state E1 = first electronic exited state
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Electronic Spectra and Molecule Structure
The electronic transition in the UV-VIS regions are due to the 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 electron – not involved in bonding ii. Covalent single bond electron, σ iii. Paired non bonding outer – shell electron (n electrons)
such as those on N, O, S, and halogens. These are less tightly held then electrons and can be excited by visible or UV radiation
iv. Electron in π orbitals, in = or ≡ bonds. These are the most readily excited and are responsible for a majority of electronic spectra in the UV-VIS regions.
* π 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; these correspond to excited- state energy levels (either σ* or π*). Therefore, absorption of radiation results in an electronic transition to antibonding orbitals.
The most common transition:
π π* or,
n π*
n σ*
Above 200 nm
Occur at very short λ
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HOMO: Highest Occupied Molecular Orbital LUMO: Lowest Unoccupied Molecular Orbital
Example, molecule H2
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Example of transition π → π* and n → π* in ketone (acetone)
The probability of π → π* is higher than n → π*
Molar absorptivities, ε
π → π* (1000-100,000)
n → π* (less than 1000)
π → π* n → π*
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ABSORPTION BY ISOLATED CHROMOPHORES
The absorbing groups (the electrons responsible for the absorption) in a molecules are called chromophores
A molecule containing a chromophore – chromogen
The present of auxochrome enhance the absorption by a chromophore or shift the λ of absorption.
Example: hydroxyl (OH), amine (-NH2), halogens (X)
posses n electrons – interact with π electrons in the chromophores (n → π) conjugation.
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Spectra changes can be classed as:
Bathochromic shift (red shift) – λmax shifted to shorter wavelength. It may caused by removal of conjugation.
Hypsochromic shift (Blue shift) - λmax 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 π bond system of the benzene ring is removed on protonation
Neutral aniline absorbs at λmax = 230 nm
ε = 8600
But in acid solution (protonated),
λmax = 203 nm
ε = 7500
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Spectra changes can be classed as: continue….
Hyperchromic effect
– increased absorption intensity (ε↑)
Hypochromic effect
– decreased absorption intensity (ε↓)
Note: λmax = the wavelength of the absorption maximum
= Absorption [ ] of a 1% solution in a cell with 1 cm pathlength- used when MW of a compound is not known
)(log 010 I
I1%
1cmε
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BEER’S LAW-ABSORPTION LAW
Fraction of radiant energy transmitted decays exponentially with pathlength.
Transmittance in exponential form:
Putting in logarithmic form,
0I Ic
b
Transmitted radiation
Incident radiation
Pathlength concentration
kb
I
IT 10
0
kbI
IT
0
loglog (Eq 2)
k is a constant (Eq 1)
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BEER’S LAW-ABSORPTION LAW…continue
Similar law holds for the dependence of T on the concentration,
Combining (Eq 1) and (Eq 3), describes the dependence of T on the pathlength and concentration.
Omit the –ve sign, and rearrange…
Hence,
cck
I
IT '
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 k and k’ (Eq 5)
abcI
IT
0
loglog
abcI
I
TTA 0log
1loglog
abcA
Absorbance Absorptivity Pathlength in cm
Conc in g/L
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BEER’S LAW-ABSORPTION LAW…continue
Note: in absorption spectrum, the absorbance varies with wavelength in direct proportion to a (b & c are held constant). The product of the absorptivity and the molecular weight of the absorbing species is called the molar absorptivity,
Thus bcA
)( 11 Lmolcm
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Example
Amines, RNH2 react with picric acid to form amine picrates, which absorb strongly at 359 nm (ε=1.25 x 104 ). An unknown amine (0.1155 g) is dissolved in water and dilute to 100 mL. A 1 ml aliquot of 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
4127
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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Solvent Choice of solvent Solvent consideration: cheap, good solvent transparent down
to about 210 nm Most commonly used – 95% ethanol
Solvent effect Electronic transition, π → π*: atom do not move, however
electrons 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.
E.g.: Styrene in ethanol (λmax = 270 nm) Styrene in hexane (λmax = 250 nm) about 20 nm λ shift (red shift, longer wavelength)
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Electronic transition, n → π* (the weak transition of the O lone pair in ketones.