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Molecular Absorption in UV-vis Region - Advanced Analytical Chemistry - Lecture Slides, Slides of Analytical Chemistry

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

2012/2013

Uploaded on 08/30/2013

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Molecular Absorption in UV-vis Region
Practical considerations – discussion limited to
species absorbing at λ’s > 200 nm. Work
below 200 nm is extremely difficult because:
1) O2absorbs starting at ~195 nm & below
2) N2absorbs starting at ~145 nm & below
3) H2O absorbs starting at ~178 nm & below
4) Crystalline quartz absorbs below ~185 nm
5) Amorphous quartz absorbs below ~170 nm
6) CaF2& LiF only suitable optical materials
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Molecular Absorption in UV-vis Region

Practical considerations – discussion limited to

species absorbing at

(^) λ ’s > 200 nm. Work

below 200 nm is extremely difficult because:

O

2 absorbs starting at ~195 nm & below

N

2 absorbs starting at ~145 nm & below

H

(^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

  • these changes correspond to the

energy of the ultraviolet-visible radiation

  • these changes are quantized (i.e.

quanta of light)discrete levels exist corresponding to

E =

E

elec.^

E

vib.

∆ E rot.

transition for absorptionEnergy change or

energyLargest

energySmallest

Simplified Energy Level Diagram

E

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

  • atomic orbitals (lower energy) antibonding orbitals

(^) – 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

  • σ
  • transitions are high energy transitions

(^) occur at short

(^) λ ’s i.e. below 200 nm and

importance (vacuum UV)are neither easily accessible or of great

n- π

  • transitions are observed in saturated

electrons),compounds containing heteroatoms (n

(^) ε (^) usually low (

(^3)

  • 10

) because^2

orbitalsthere is very little overlap between the two

Sulfide oxygenthanpromotedeasilymoreelectrons

Br and I than Clpromotedeasilyare moreelectrons

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