Electronic Spectroscopy

Question 1

What is electronic spectroscopy?

Answer 1

This is the study of how EM radiation interacts with a molecule to change its electronic energy

Question 2

Which part of the EM spectrum does it correspond to?

Answer 2

The energies associated with transition correspond to UV/VIS region

Absorptions in visible region are associated with colour in compounds

Question 3

The electrons in a molecule occupy molecular orbitals. Describe these electronic energy levels

Answer 3

They are quantised

They are widely spaced (large difference in energy and most occupy lowest state)

Question 4

What is a chromophore?

Answer 4

This is a molecule or group that absorbs in the UV visible region

Question 5

What colours are observed?

Answer 5

If one colour is absorbed, the complementary colour is observed
Red- green
Violet- yellow
Blue- orange

Question 6

What groups act as chromophores?

Answer 6

These are normally conjugated systems in which the electrons are delocalised and the electronic transitions involve pi oribitals

Question 7

What is the beer lambert law?

Answer 7

A= E c l

E has units dm3mol-1cm-1
It is for a specific molecule at a specific wavelength

Question 8

What is the relationship between wavelength and frequency

Answer 8

Wavelength is inversely proportional to frequency and energy so a higher wavelength corresponds to a lower frequency and lower energy

Question 9

When is measuring absorbance useful?

Answer 9

When you know the conc- you can now determine E

If you know E- you can now determine the conc

Question 10

What is involved in an electronic transition?

Answer 10

An electron moves from one molecular orbital to another

Question 11

What are the gross selection rules?

Answer 11

There must be a change in dipole as the electron moves

Large dipole change give a strong band

Question 12

What determines the MOs involved?

Answer 12

The specific molecule determines the specific MO involved

• specific energy gaps (absorption wavelengths
• specific band strengths (E values )

Question 13

Other than MO transitions, what other transitions are there?

Answer 13

D-d transitions

Change transfer transitions

Question 14

What does each band in the spectrum correspond to?

Answer 14

Each band corresponds to electrons transitioning to a different molecular orbital

Question 15

How many bands will be seen in the spectrum

Answer 15

As there are many different occupied and occupied MOs in a molecule, there will be several bands in the spectrum

Question 16

What happens as conjugation increases??

Answer 16

The pi- pi* absorption bands shifts to longer wavelengths (lower energy)

• this is because there is more resonance so the molecule is more stable

Question 17

What are the applications?

Answer 17

-Identification of chromophores
- identify types and energies of excited electronic states (photochemistry)
Measure absorbance to determine concentrations

Question 18

How do you calculate total absorbance?

Answer 18

Total absorbance is the some of the absorbances of the components

Question 19

How can you calculate the concentrations of A and B in a mixture?

Answer 19

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

Question 20

What happens to spectra when H+ is added

Answer 20

Large change in UV vis absorption spectrum due to change in donor and acceptor properties

Question 21

What is an isobestic point?

Answer 21

Constant absorbance at a specific wavelength as two species interact
It occurs when two interconverting species have the same E at specific A

Question 22

How can you calculate the total energy?

Answer 22

Electronic, vibrational and electronic energies are very different, so independent and the total is the sum of the three energies

Question 23

Can electronic transition occur with v and r?

Answer 23

An electronic transition can occur with simultaneous change in vibrational level v and rotational level j