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Biophysical Chemistry > General principles of spectroscopy > Flashcards

General principles of spectroscopy Flashcards

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1
Q

What is spectroscopy?

A

It is the study of electromagnetic radiation.

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2
Q

Radiation in EMR can be…

A

Absorbed, emitted and scattered.

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3
Q

What happens when radiation is absorbed?

A

Molecules in a low energy level absorb energy from the irradiating EMR and move into a higher energy level.

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4
Q

What happens when radiation is emitted?

A

Molecules in a higher energy level are stimulated to emit energy by EMR. (ie stimulated, not spontaneous).

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5
Q

What can happen when a molecule absorbs a photon?

A

They go into an excited state. They may then occasionally return to ground state by emitting a photon. But this is a spontaneous emission and is not caused by EMR. eg Fluorescence.

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6
Q

Can a molecule lose energy by interacting with EMR?

A

Yes it can lose energy as a photon. Important in EPR and NMR.

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7
Q

What direction does electric and magnetic field oscillate in?

A

They are perpendicular to each other and both are perpendicular to the direction of propagation of the wave.

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8
Q

What speed does the wave propagate at?

A

3.0E8 m/s

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9
Q

What equation links the speed of light, wavelength and frequency?

A

Speed of light = frequency * wavelength
C = vh

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10
Q

What is the equation for the energy of one photon?

A

E = hv
Energy = Planck’s constant * frequency

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11
Q

What energy should photon energy match?

A

It must match the difference in energy between the two states involved (eg ground and excited)

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12
Q

What is Beer-Lambert law equation?

A

A = log I0/I = ecl

A = Absorbance
I0 = intensity of incident radiation.
I = intensity after passing through the sample.
c = conc on absorbing species (units M)
l = pathlength (cm)
e = extinction coefficient (intrinsic intensity) (M-1 cm-1)

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13
Q

What does intensity provide info on?

A

nature of the transition and the chromophore.

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14
Q

Why is a population difference between ground and excited state critical?

A

Because when sample is irradiated, molecules in both states are stimulated (w/ equal probabilities for each direction) to jump between the 2 states. Spectroscopy measures the net absorption of energy.

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15
Q

Can molecules (electrons I assume) see a drop down when irridiated?

A

Yes they can jump up by absorbing radiation and also drop down and increase the intensity of EMR.

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16
Q

Can molecules (electrons I assume) see a drop down when irradiated?

A

Yes they can jump up by absorbing radiation and also drop down and increase the intensity of EMR.

17
Q

What does a higher temperature allow for in regards to population difference?

A

More thermal energy available allowing a larger fraction of the population to exist in excited state at any one moment (eg dynamic equilibrium w/ molecules moving between the 2 states).

18
Q

What does a molecule need to get to the excited state?

A

Enough thermal energy to overcome ΔE.

19
Q

What is kBxT?

k then B that is small and T

A

It is the Boltzmann’s constant = 1.381 E-23 J/K

20
Q

So what does ΔE/kB*T compare?

A

Energy needed to bridge the gap compared to the thermal energy available. Appears in the equation for ratio of the populations at a particular temp.

21
Q

What is the equation for the ratio of the populations at a particular temperature?

A

Pes/Pgs = e^(-ΔE/kB*T)

E in J
and T in K

22
Q

What is Pes?

A

Population at energy ΔE

23
Q

What is Pgs?

A

Population at energy 0

24
Q

What happens when ΔE&raquo_space; kB*T?

A

Pes is negligible (eg UV)

25
Q

What happens when ΔE &laquo_space;kB*T?

A

e^(-ΔE/kB*T) = E^(0) = 1
i.e. Pes = Pgs
(eg radio freq, NMR)

26
Q

Have you read the example questions for this?

A

Please check

27
Q

What happens when EMR is turned off?

A

Excess energy has to be lost as heat to the surroundings (the lattice) by relaxation mechanisms in order to re-establish the correct Boltzmann distribution for T.

The irradiation (trying to equalise the populations) and the relaxation mechanisms (trying to re-establish Boltzmann distributions) are effectively in competition.

28
Q

Relaxation is efficient for?

A

For electronic absorption at high-energy UV-vis wl.
Easily dissipated excess energy so return to ground state quickly.

29
Q

What is relaxation not efficient for?

A

EPR and NMR (micro- and radio waves), relaxation is inefficient and it is easy, by too intense irradiation, to lose the population difference and therefore the signal = saturation.

30
Q

d-d transitions and spin states are

A

… important for electronic absorption

31
Q

In a transition metal free-ion the five d-orbitals are?

A

degenerate. In a ligand field of 6 equivalent ligands (ML6, octahedral), these orbitals split into 2 groups (eg and t2g). because 2 orbitals interact directly w/ the ligands, the lobes of the other 3 “point” between the ligands. Δoct is the energy gap.

32
Q

What does splitting allow for?

A

Spectroscopic transition in which an electron is promoted to the higher orbitals: known as ligand-field or d-d transitions. Strong ligands = large splittings = high energy = transition at short wavelengths.

33
Q

A transition has a spin, S, which depends on?

A

The no. of unpaired electrons. How many are unpaired depends on how individual electrons (each w/ S = 1/2) are arranged in the d orbitals. Arrangement is critically influenced by strength of the ligands and so the magnitude of Δoct.

34
Q

If there is 1 e- in each orbital (5 orbitals), what is the total spin?

A

5/2 (=2.5)

35
Q

Large Δoct values (strong ligands) prevent what?

A

Prevents the higher occupancy of higher orbitals: e- must pair to occupy the lower orbitals. If there are 5 spins, four cancel out leaving 1 unpaired e- so S = 1/2. This is the low spin state.

36
Q

What is the equation to find the multiplicity of the molecule?

A

2S+1
S= spin =1/2 for each unpaired electron.

so 1 is a singlet, 2 is doublet etc.

Biophysical Chemistry (2 decks)

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