General principles of spectroscopy Flashcards by Izzy Katie

What is spectroscopy?

It is the study of electromagnetic radiation.

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Radiation in EMR can be…

Absorbed, emitted and scattered.

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What happens when radiation is absorbed?

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

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What happens when radiation is emitted?

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

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What can happen when a molecule absorbs a photon?

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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Can a molecule lose energy by interacting with EMR?

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

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What direction does electric and magnetic field oscillate in?

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

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What speed does the wave propagate at?

3.0E8 m/s

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

Speed of light = frequency * wavelength
C = vh

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What is the equation for the energy of one photon?

E = hv
Energy = Planck’s constant * frequency

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What energy should photon energy match?

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

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What is Beer-Lambert law equation?

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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What does intensity provide info on?

nature of the transition and the chromophore.

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

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

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

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

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

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

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).

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

Enough thermal energy to overcome ΔE.

What is kBxT?

k then B that is small and T

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

So what does ΔE/kB*T compare?

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.

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

Pes/Pgs = e^(-ΔE/kB*T)

E in J
and T in K

What is Pes?

Population at energy ΔE

What is Pgs?

Population at energy 0

What happens when ΔE&raquo_space; kB*T?

Pes is negligible (eg UV)

What happens when ΔE << kB*T?

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

Have you read the example questions for this?

Please check

What happens when EMR is turned off?

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.

Relaxation is efficient for?

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

What is relaxation not efficient for?

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.

d-d transitions and spin states are

... important for electronic absorption

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

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.

What does splitting allow for?

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.

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

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.

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

5/2 (=2.5)

Large Δoct values (strong ligands) prevent what?

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.

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

2S+1 S= spin =1/2 for each unpaired electron. so 1 is a singlet, 2 is doublet etc.