UV/Vis Molecular Absorption Spectroscopy Flashcards
what is present in uv/vis molecular absorption
vibrational, rotational and electronic transitions
what are the two absorbing species
- organic
2. inorganic
allowed molecular transitions
σ→σ*
π→π*
n→σ*
n→π*
λ range for σ→σ*
< 180 nm (vacuum UV)
λ range for n→σ*
15-250 nm
λ range for n→π*
200-700 nm
λ range for π→π*
170-700 nm
ε range for n→σ*
100-3000
ε range for n→π*
10-100
ε range for π→π*
1000-15000
molecules that contain unsaturated function group to provide π orbitals
chromophores
what absorption are considered for inorganic species?
- Elements of the first and second transition metal series
- Lanthanide and actinide ions
- Charge transfer absortion
what does absorption involve for first and second transition metal series
transtiosn between filled and unfilled d-orbitals
what does the energy difference between d-orbitals depend on
- the position of the element in the periodic table
- the oxidation state of the element
- The nature of the ligand bonded to it
what does a different position on the periodic table lead to for first and second transition metal series
a different observed λ max
what says the energy difference between d orbitals depends on the ligands bonded to the metal ions
ligand crystal field theory
what does octaheadral ligand field say for the d orbitals
Dx2-y2 = Dz2
Greater than
Dxy = Dxz = Dyz
Tetraheadral ligand field
Dxy=Dxz+Dyz
greater than
Dx2-y2 = Dz2
square planar ligan field
Dx2-y2
greater than
Dxz
greater than
Dz2
greater than
Dxy = Dyz
what is the magnitude of ∆ relate to
the spectrochemical series
what is the order of the spectrochemical series
CN->NO2>o-phen>EN>NH3>SCN->H2O~C2O4 2->OH->F->Cl->Br->I-
CN has highest ligand field strength while I- has the lowest ligand field strength
In spectrochemical series, as λ decreases, ∆ ___
increases
what electrons are responsible for absorption of lanthanides
4f electrons
what electrons are responsible for the absorption of actinides
5f electrons
what leads to the narrow absorption bands of lathanides and actinides
the full d orbital shields the f orbital responsible for absorption which leads to narrower bands.
what is charge transfer absorption
when the product of an electron donor is bonded to an electron acceptor absorbs radiation, an electron from the donor is transferred to an orbital that is largely associated with the acceptor
(Internal redox reaction)
what usually serves as the electron acceptor in charge transfer absorption
the metal
five components of uv/vis molecular absorption spectroscopy
- radiation source
- wavelength selector
- sample container
- transducer
- signal processor and readout
radiation sources in UV/Vis
- tungsten filament lamp
- deuterium and hydrogen lamps
- xenon arc lamps
tungsten filament lamp range
VIS and Near IR
deuterium and hydrogen lamp range
UV
xenon arc lamp range
VIS and UV
Are radiation sources continuum sources or linear?
continuum
what are tungsten filament lamps based on
black body radiation
original tungsten filament lamp
Mostly visible range
glass container
Ne gas
Tungsten filament(2870K)
A tungstein-halogen lamp
quarts container
Newith 2
Tungsten filament (3000K)
what was the advantage of tungsten halogen over the original tungsten lamp
the addition of I2 basically forms WI2 in the vapor of the lamp, and when the lamp is turned off the volatile WI2 hits the hot filament lamp and the W redeposits onto the filament and the I2 is dispersed back into the “air”
what does the higher temperature of tungsten halogen lead to
- Higher intensities
- Extended range into UV
