UV-Vis spectroscopy

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Whats the difference between absorbance and transmission spectra?
Absorption increases when radiation is absorbed. Transmission decreases when radiation is absorbed.
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What determines the intensity of the peaks in a spectrum?
The amount of sample used (concentration and path length) beer Lamberts Law, The population of energy level which is undergoing a transition- Boltzmann Distribution, the individual spectroscopic selection rules.
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What are the units for electronic spectroscopy and Vibration spectroscopy?
Electronic: wavelength (nm) Vibration: wavenumber (cm-1)
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What can UV-Vis spectroscopy be used for?
Provides information on the electronic levels within the molecules, determine the analyte concentration using the Beer-Lambert law, used to study conjugated systems.
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What is transmittance and its equation?
Transmittance, T, of the solution is defined by the fraction of incident light that passes through the sample: T= I(t)/ I(0)
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Why is the relationship between concentration and transmittance not linear?
As the concentration gets higher the amount of light passing through gets lower, making the transmittance value lower. The amount of light passing through the sample decays exponentially with increasing concentration.
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What is the equation associated with Beer-Lamberts law and their respective units?
Absorbance (no units)= molar absorbance coefficient (dm3 mol-1 cm-1) x concentration (mol dm-3) x path length (cm)
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What conditions must be met for Beer-Lamberts law to be valid?
1) Absorbing species must not exist in an equilibrium in different forms. 2) The solvent and the species in solution do not form a complex. 3) The light source should not be too intense.
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What conditions must be met for Beer-Lamberts law to be valid? continued
4) Thermal equilibrium must not exist between ground state and excited state. 5) There is no scattering of light due to particles in solution.
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What conditions must be met for Beer-Lamberts law to be valid? Continued
6) Fluorescent compounds must not be present. 7) The concentration of the solution must not be higher than 0.01 M
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How does the 6th and 7th condition invalidate Beer- Lamberts law?
6) Fluorescence compounds emit light when excited which interferes with the linearity of the Beer-Lambert equation. 7) At higher concentrations electrostatic interactions may occur and affect the absorptivity.
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Describe the energy profile of a diatomic molecule.
Bond is constantly vibrating, will have an equilibrium distance of Re this will be the lowest energy. Bond stretches, the energy will increase. If the bond stretches too much the bond will break. Bond is compressed, 2 nuclei repel (energy increases)
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What is the Franck- Condon Principle?
Electronic transitions occur much more readily than nuclear motions.
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Explain the Franck- Condon Principle?
As the nuclei are much heavier than the electrons an electronic transition takes place much faster than the nuclei can respond.
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What are the transitions called and what occurs?
Vertical transitions occur between states of the same internuclear separation in order to maximise the overlap between the vibrational wave functions in the two states.
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How many molecular orbitals form when atomic orbitals combine?
The same amount of molecular orbitals form as there are atomic orbitals. Forming an antibonding orbital and bonding orbital.
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Define HOMO and LUMO.
HOMO ( highest occupied molecular orbital) LUMO (lowest unoccupied molecular orbital).
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What are non-bonding orbitals?
Orbitals are those in which the atomic orbital has not undergone any overlap with any atomic orbital. They accommodate electrons that are not involved in bonding.
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What are the difference between allowed and forbidden transitions?
Allowed gives more intense signals whereas forbidden gives weak signals.
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What are the two categories of selection rules?
Gross selection rules and specific selection rules.
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Define gross selection rules.
Characteristic requirements for atoms or molecules to display a spectrum of a given kind. There must be no change in the spin quantum number.
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What is the specific selection rule?
For a transition to occur there must be a change in orbital angular momentum.
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Define a chromophore.
The functional group or part of the molecule that is responsible for the absorption of UV-Vis radiation.
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Why are transition metal complexes colourful and absorb in the visible region?
This occurs due to d-d transitions between the metal and ligand orbitals and transitions between the ligands themselves.
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Other cards in this set

Card 2

Front

What determines the intensity of the peaks in a spectrum?

Back

The amount of sample used (concentration and path length) beer Lamberts Law, The population of energy level which is undergoing a transition- Boltzmann Distribution, the individual spectroscopic selection rules.

Card 3

Front

What are the units for electronic spectroscopy and Vibration spectroscopy?

Back

Preview of the front of card 3

Card 4

Front

What can UV-Vis spectroscopy be used for?

Back

Preview of the front of card 4

Card 5

Front

What is transmittance and its equation?

Back

Preview of the front of card 5
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