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Indicator Electrodes, Potentiometry, Electrochemistry, Electrode Kinetics, Electrogravimetry, Polarography, Square Wave Voltammetry, Wavelength Selection, Types of Spectroscopy, Detectors, Theory of Molecular Absorption, Absorption by Organic Molecules, Applying UV-vis Spectroscopy are major topics of this course. Main points from these slides are: Molecular Absorption in Uv-Vis Region, Crystalline Quartz, Amorphous Quartz, Vacuum Uv, Spectroscopic Information, Electronic Energy, Vibrational Ene
Typology: Slides
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Practical considerations – discussion limited to
species absorbing at
(^) λ ’s > 200 nm. Work
below 200 nm is extremely difficult because:
2 absorbs starting at ~195 nm & below
2 absorbs starting at ~145 nm & below
(^2) O absorbs starting at ~178 nm & below
Crystalline quartz absorbs below ~185 nm
Amorphous quartz absorbs below ~170 nm
CaF
2 & LiF only suitable optical materials
Few good sources – rare gas discharge
lamps
Generally there is very little
200 nm & its not worth the troublespectroscopic information available below
Region below 200 nm is known as the vacuum UV
(^) because one approach to
absorbing gas like He or Armonochromator or purge it with a nonwork in this region is to evacuate the
Region of interest is from 195 – 200 nm up to 650 – 800 nm
Electronic energy involves changes in moleculeenergy levels of the outer electrons of a
energy of the ultraviolet-visible radiation
quanta of light)discrete levels exist corresponding to
elec.^
vib.
∆ E rot.
transition for absorptionEnergy change or
energyLargest
energySmallest
Simplified Energy Level Diagram
Levels (2)Electronic
Levels (4)Vibrational
Levels (5)Rotational
Tetrazine absorption various conditionsspectrum under
Vapor phase spectrum structuregives greatest fine
In hexane solution solventinteractions with thelost due to tetrazinemany features are
In aqueous solution all merge into onelost & all peaksspectral features are
The “smearing out” effect is greater in the molecules)(polar solvents always interact more withdegree of solvation of the molecule- In liquid phase it increases with the- In gas phase it increases with pressureliquid phase than in gas phase:
Some molecules inherently interact less with solvents so spectra have more features
Rigid molecules like PAHs interact less with solvents & therefore have more features
ε Molar Absorptivity or Absorption Probabilities wavelength and therefore a transitionabsorption probability – specific for a^ is a measure of, or can be thought of as
ε In general (^) = 10
(^4) to 10
(^5)
(very strong absorption)highly probable transition
ε (^) = 10
(^3) to 10
(^4)
strong absorption
ε (^) < 10
(^3)
weak absorption
Atomic orbital combine to give molecularOrganic Molecules – Structure & Absorption orbitals
Two types of orbitals:
bonding orbitals
(^) – more stable than
(^) – less stable than
by *atomic orbitals (higher energy) – denoted
Must also consider electrons that are not involved in bonds, e.g. C=O
has two pairs
of electrons on the oxygen =
(^) non bonded
electrons (n electrons)
(^) – present when
heteroatom is present (N, S, O, S, etc.)
Bonding orbitals are filled, antibonding orbitals are empty
orbitals* or antibondingare promoted tooccurs, electronsWhen absorption
E
Orbital Atomic
OrbitalAtomic
OrbitalsMolecular
σ σ * π π * n
●● ●●
Another view of molecular orbitals and the possible electronic transitions
Note energy gaps for different transitions
σ
(^) occur at short
(^) λ ’s i.e. below 200 nm and
importance (vacuum UV)are neither easily accessible or of great
n- π
electrons),compounds containing heteroatoms (n
(^) ε (^) usually low (
(^3)
) because^2
orbitalsthere is very little overlap between the two
Sulfide oxygenthanpromotedeasilymoreelectrons
Br and I than Clpromotedeasilyare moreelectrons
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