Electronic Spectroscopy Flashcards
What is electronic spectroscopy?
This is the study of how EM radiation interacts with a molecule to change its electronic energy
Which part of the EM spectrum does it correspond to?
The energies associated with transition correspond to UV/VIS region
Absorptions in visible region are associated with colour in compounds
The electrons in a molecule occupy molecular orbitals. Describe these electronic energy levels
They are quantised
They are widely spaced (large difference in energy and most occupy lowest state)
What is a chromophore?
This is a molecule or group that absorbs in the UV visible region
What colours are observed?
If one colour is absorbed, the complementary colour is observed
Red- green
Violet- yellow
Blue- orange
What groups act as chromophores?
These are normally conjugated systems in which the electrons are delocalised and the electronic transitions involve pi oribitals
What is the beer lambert law?
A= E c l
E has units dm3mol-1cm-1
It is for a specific molecule at a specific wavelength
What is the relationship between wavelength and frequency
Wavelength is inversely proportional to frequency and energy so a higher wavelength corresponds to a lower frequency and lower energy
When is measuring absorbance useful?
When you know the conc- you can now determine E
If you know E- you can now determine the conc
What is involved in an electronic transition?
An electron moves from one molecular orbital to another
What are the gross selection rules?
There must be a change in dipole as the electron moves
Large dipole change give a strong band
What determines the MOs involved?
The specific molecule determines the specific MO involved
- specific energy gaps (absorption wavelengths
- specific band strengths (E values )
Other than MO transitions, what other transitions are there?
D-d transitions
Change transfer transitions
What does each band in the spectrum correspond to?
Each band corresponds to electrons transitioning to a different molecular orbital
How many bands will be seen in the spectrum
As there are many different occupied and occupied MOs in a molecule, there will be several bands in the spectrum
What happens as conjugation increases??
The pi- pi* absorption bands shifts to longer wavelengths (lower energy)
- this is because there is more resonance so the molecule is more stable
What are the applications?
-Identification of chromophores
- identify types and energies of excited electronic states (photochemistry)
Measure absorbance to determine concentrations
How do you calculate total absorbance?
Total absorbance is the some of the absorbances of the components
How can you calculate the concentrations of A and B in a mixture?
If you know E of A and B at two wavelengths:
1) measure absorption at the two wavelengths at known l
A1= (EA1 x [A] xl) + (EB1 x [B] xl)
A2= (EA2 x [A] xl) + (EB2 x [B] xl)
- then solve as simultaneous equations
- remember A= E c l
Total A is the sum of all absorbances
What happens to spectra when H+ is added
Large change in UV vis absorption spectrum due to change in donor and acceptor properties
What is an isobestic point?
Constant absorbance at a specific wavelength as two species interact
It occurs when two interconverting species have the same E at specific A
How can you calculate the total energy?
Electronic, vibrational and electronic energies are very different, so independent and the total is the sum of the three energies
Can electronic transition occur with v and r?
An electronic transition can occur with simultaneous change in vibrational level v and rotational level j
