Optical

Question 1

Whare are chromophores?

Answer 1

Functional groups which give rise to observed transitions

Question 2

UV-VIS

Why does spectrum broadening happen?

Answer 2

• large number of overlapping vibronic transitions
• transitions broadened by collisions with solvent

Question 3

Beer Lambert Law

What are the fundamental limitations?

3 points

Answer 3

1. Invalid at high concentrations
2. Refractive index depends on concentration
3. Extinction coefficient depends on refractive index

Question 4

Why is Beer-Lamberts law invalid at high concentrations?

Answer 4

Aggregation of molecules affects extinction coefficicient

Question 5

Beer-Lambert Law

What type of deviation does high sample concentration lead to?

Answer 5

Positive

Question 6

Beer-Lambert Law

What are instrumental limitations?

2 points

Answer 6

1. Stray radiation reaching detector without passing through sample
2. Polychromatic radiation, as BL assumes monochromatic radiation

Question 7

Beer-Lambert Law

What type of deviation does stray radiation lead to?

Answer 7

Negative

Question 8

Define bandwidth

Answer 8

The full-width of the radiation at half of its maximum intensity

Question 9

Beer-Lambert Law

Why do you measure absorbance at the top of the absorbance peak

Answer 9

BL assumes monochromatic radiation, spectrometer has finite bandwidth. Effect is minimised if the value of epsilon is constant over bandwidth (top of the abs peak)

Question 10

Describe the procedure for quantitative analysis using Beer-Lamber Law

4 points

Answer 10

1. Measure absorbance and apply BL law
2. Construct a calibration curve
3. Linear regression of absorbance versus concentration to give calibration equation
4. Substitute sample’s absorbance into calibration equation to give analyte concentration

Question 11

How is qualitative analysis carried out using UV-VIS?

Answer 11

1. Use characteristic absorption spectrum of chromophore to identify compound
2. Compare to the library of digital compounds
3. Use of derivative spectra can improve discrimination between similar spectra
4. UV-vis spectra tend to be broad and unstructured – not “fingerprints”
5. Use PCA for multi-component mixtures

Question 12

Define Luminescence

Answer 12

Emission of light by electronically excited molecules

Question 13

Define Fluorescence

Answer 13

Molecule excited by absorption of light

Question 14

Compare and contrast Fluorescence and Phosphorescence

Answer 14

F: spin allowed and very short lived
P: Spin forbiddena and long lived

Question 15

What is a singlet electronic state?

Answer 15

When all the electron spins are paired in the molecular electronic state and the electronic energy levels do not split when the molecule is exposed into a magnetic field

Question 16

What is a triplet electronic state?

Answer 16

Two electrons in different molecular orbitals have parallel spins

Question 17

Fluorescence

What is vibrational relaxation?

Answer 17

• Collision with solvent molecules -rapid loss of vibrational energy to solvent as heat.
• Moving down one energy level within the same state

Question 18

Fluorescence

What is Excitation (absorption)?

Answer 18

Promotion of electrons into S1/2 etc into the same energy level

Question 19

Fluorescence

What is fluorescence?

Answer 19

Transition from lowest vibrational level(s) of S1 state to S0

Question 20

Fluoresence

What is internal conversion?

Answer 20

Conversion of electronicenergy to vibrational energy S1 -> S1

Question 21

What is Kasha’s Rule?

Answer 21

Fluorescence always from lowest vibrational levels of S1 state to any level of S0

Question 22

Fluorescnce

What is intersystem crossing?

Answer 22

Transition between states of different multiplicity (S1 to T1/ T1 to S0)

Question 23

Fluorescence

Fate of radiactive decay

3 points

Answer 23

1. Radiative
2. Non radiative
3. Intramoleculay decay

Question 24

Fluorescence

What is radiative decay?

3 points

Answer 24

1. Energy lost by emission of a photon
2. Fluorescence S1 to S0 spin allowed
3. Phosphorescence T1 to S0 spin forbidden

Question 25

What is on the axes of Stern-Volmer plot?

Answer 25

F0/F vs [Q]

Question 26

What is the **gradient** and **intercept** of Stern-Volmer plot?

Answer 26

grad: K_SV (Stern-Volmer constant) intercept: 1

Question 27

# Fluorescence What is the innter filter effect? ## Footnote 2 points

Answer 27

1. If concentration is too high maximum fluorescnce happens in the middle of the sample 2. This results in a distorted spectrum, as intensity in the middle of cuvette is reduced.

Question 28

# Fluorescence How can the inner effect be prevented?

Answer 28

Absorbance should be < 0.1 at excitation wavelength

Question 29

# Fluorescence What is spectrum reabsorption? ## Footnote 3 points

Answer 29

- If absorption and emission spectra overlap, and concentration is too high fluorescence is reabsorbed as it passes through solution - Fluorescence intensity is reduced - Emission spectrum distorted

Question 30

What is a fluorophore?

Answer 30

Fluorescent molecule or group

Question 31

* What groups do flurophores contain? * Which transition does it allow?

Answer 31

1. Aromatic groups 2. pi to pi*

Question 32

List advantages of fluorescence over UV-VIS ## Footnote 5 points

Answer 32

- High detection sensitivity - High sensitivity - Use as probe - Sensitive to molecular environment - Sensitive to molecular interactions

Question 33

Which 3 characteristic parameters are responsible for high sensitivity of fluorescence over UV-VIS

Answer 33

1. Excitation wavelength 2. Emission wavelength 3. Lifetime

Question 34

List disadvantages of fluorescence over UV-VIS ## Footnote 5 points

Answer 34

- Intensity of fluorescence is not a direct measure of concentration - Intensity depends on experimental & instrumental factors - Intensity depends on quantum yield - Requires careful production of calibration curves - Ratiometric measurement advantageous

Question 35

What are the experimental and instramental factors that affect fluorescent intensity? ## Footnote 4 points

Answer 35

- Intensity of excitation source - Efficiency of collection of fluorescence - Inner filter effect - Reabsorption

Question 36

# Fluorescence What is quantum yield affected by? ## Footnote 2 points

Answer 36

1. Sample conditions (pH, temp, quenching) 2. Intermolecular interactions

Question 37

# IR How does a FTIR work? ## Footnote 4 points

Answer 37

1. Transmittance is measured (like UV-VIS) 2. An interferometer is used instead of monochromator 3. All IR frequencies incident on detector simultaneously 4. Fast spectrum acquisition

Question 38

# IR How does an interferometer work?

Answer 38

- Interferometer produced signal which has all the infrared frequencies encoded into it - Fourier transform converts intensity versus time to intensity versus frequency