B - Spectroscopy - UV-vis

Question 1

Why are spectroscopic techniques useful for biology and medicine?

Answer 1

They:
• are non-destructive
• can be used in-situ
• mean the sample can be retrieved
• allow small or large samples to be used

Question 2

How do electric and magnetic fields travel in space?

Answer 2

They fluctuate phase: both in time and in the propagation direction. These are perpendicular to the direction of travel

Question 3

What is the speed of light in a vacuum?

Answer 3

c = λ x ν = 2.998 x 10^8 m/s

Question 4

What is the range of the visible spectrum in nm?

Answer 4

740nm (red) to 390nm (purple)

Question 5

What is the order of the EM spectrum?

Answer 5

Radio waves, microwaves, infrared, visible light, UV, X-rays, Gamma rays

Question 6

What is the equation for the energy of one photon?

Answer 6

E = h x ν = h x (c/λ) where h = Planck’s constant, 6.626 x 10^-34 J s )

Question 7

What kind of electrons does UV-vis absorption correspond to?

Answer 7

Valence electrons

Question 8

What happens when a molecule absorbs light in the UV-vis range?

Answer 8

It causes an electron to be promoted to an anti-bonding orbital

Question 9

What type of molecules absorb in the visible range?

Answer 9

Unsaturated organic compounds ie. chromophores
• conjugated polyunsaturated systems
• polyaromatic systems
• systems with C=O, C=S, C-P etc
• molecules with transition metal ions

Question 10

Where do π to π* and π to σ* transitions lie?

Answer 10

• UV (200nm - 300nm)

* lie at too high an energy to be detected in UV-vis

Question 11

What is the effect of increasing the number of π bonds in a conjugated system?

Answer 11

The gap between bonding and anti-bonding levels decreases. A lower energy and therefore longer wavelength is needed for the transition, hence it is called a red shift. Required wavelength is increased by 30nm for each additional C=C bond

Question 12

What kind of transitions occur in aromatic ring systems, and what is their λmax?

Answer 12

π to π* = 203nm

Additional groups change λmax to 203nm - 295nm

Question 13

What wavelengths do polycyclic aromatics (such as chlorophyll) absorb?

Answer 13

400 - 450nm (blue - violet) and 600 - 700nm (red)

This results in the reflection of green - yellow light -> plant leaves

Question 14

What is the Beer-Lambert law?

Answer 14

Absorbance, A = log (Io/It) =  ε c l
where  ε = molar absorptivity, dm^3 mol^-1 cm^-1
c = concentration, mol dm^-3
l = sample thickness, cm
A has no units

Question 15

If there is no UV-vis spectrum, what kind of molecule is it?

Answer 15

There are no:
• multiple bonds
• aromatics
• O, N or S containing groups
• transition metal ions

-> likely a fully saturated hydrocarbon

Question 16

Why are small, unsaturated hydrocarbons not coloured?

Answer 16

They have their absorptions entirely in the UV range, lambda

Question 17

What are spikes on the peaks of UV-vis spectra due to?

Answer 17

Vibrational transitions occurring alongside electronic transitions

Question 18

Where does absorption occur for heteroatoms, and how strong is it?

Answer 18

200 - 400nm (n to π*)

Usually 10 - 100 times weaker than π to π* transitions

Question 19

How is fluorescence imaging different to UV-vis?

Answer 19

UV-vis detects the absorption of incident light. In fluorescence imaging, after radiation, the excited molecule re-emits light as the electron goes back to its ground state.

Question 20

How is phosphorescence different to fluorescence?

Answer 20

Fluorescence emits light for a short period of time. Phosphorescence emits light for a long period of time, usually because the electron drops down several intermediate levels.

Question 21

What is the de Broglie relationship?

Answer 21

It expresses the particle-wave duality of light. Each ‘particle’ has a pilot wave, termed a photon, with a wavelength related to the mass and particle velocity.

λ = h / p = h / mv

Question 22

Which electronic transitions involve valence electrons, tightly bound electrons and weakly bound electrons?

Answer 22

valence = UV-vis
tightly bound = X-Ray transitions
weakly bound = IR transitions

Question 23

What is UV-vis useful for studying?

Answer 23

π - π* transitions in aromatic organic molecules
determining chain length in conjugated hydrocarbons
studying C=C vs C=O unsaturated species
determining the presence of chromophores
determinging the presence of organometallic, transition metal compounds