electronic spectroscopy

Question 1

what type of EM radiation is used in electronic spectroscopy?

Answer 1

UV-Vis

Question 2

why can UV-Vis radiation be used for electronic spectroscopy?

Answer 2

electronic energy levels have the largest energy gaps ~30 000 cm^-1, and UV-Vis photons are high enough in energy ~50-500 kJ mol^-1 to excite molecules to higher energy levels

Question 3

why is radiation above UV-Vis not used for spectroscopy?

Answer 3

this radiation would be too high in energy, X-ray radiation can eject electrons out of atoms, and even UV-Vis can trigger bond breaking

Question 4

what kind of molecules can electronic spectroscopy be used on?

Answer 4

most molecules, including single atoms, unlike rotational / vibrational spectroscopy, this is called colourimetry

Question 5

what are the 2 types of electronic spectroscopy?

Answer 5

atomic absorption/transmission and atomic emission

Question 6

what is electronic atomic absorption/transmission spectroscopy?

Answer 6

photons cause electrons to be promoted to a higher energy level as the missing/absorbed light is measured

Question 7

what is electronic atomic emission spectroscopy?

Answer 7

electrons are already excited within the molecules, they jump down to a lower energy level and release a photon of a particular wavelength

Question 8

why are only photons with specific wavelengths absorbed or emitted?

Answer 8

because electronic energy levels are quanitsed

Question 9

give 4 useful purposes of electronic atomic spectroscopy

Answer 9

• testing atomic models + their prediction of energy levels
• producing coloured light in fireworks
• lights
• element identification via flame tests

Question 10

what range of wavelengths is the UV region?

Answer 10

~ 100-400 nm

Question 11

what range of wavelengths is the visible region?

Answer 11

~ 400-700 nm

Question 12

what are the axes of a UV-Vis spectra?

Answer 12

x axis = wavenumber
y axis = absorbance

Question 13

why do some chemicals/substances appear coloured?

Answer 13

when these compounds interact with light, some is transmitted and some is absorbed, we see light that is transmitted, we can measure what colours are absorbed and find the colours transmitted, which are the opposite colours
energy of absorption depends on differences in electronic energy levels in molecular orbitals of compounds

Question 14

what are the 2 types of UV-Vis spectrometer?

Answer 14

single beam and double beam

Question 15

how does a single beam UV-Vis spectrometer work?

Answer 15

beam of EM radiation is emitted from the source and passes through a monochromator, which selects specific wavelengths of EM, which then pass through the sample which absorbs some wavelengths and transmits others, intensity of radiation is measured by a detector, the spectra is then processed + displayed on a computer

Question 16

how does a double beam UV-Vis spectrometer work?

Answer 16

beam of EM radiation is emitted from the source and passes through a monochromator, which selects specific wavelengths of EM, which then pass through a beam splitter and gets split into 2 identical beams, one beam passes through the sample which absorbs some wavelengths and transmits others, the other beam passes through a reference (e.g. solvent) intensity of radiation of both beams is measured by a detector, the spectra is then processed + displayed on a computer

Question 17

why is electronic spectroscopy a much more quantitative technique?

Answer 17

UV-Vis spectrometry only measures 1 type of transition (electronic), making it much more quantitative - this also means it is much more affected by concentration of substance

Question 18

why are some peaks more intense than others?

Answer 18

intensity of peaks depends on how many protons are absorbed, which depends on the amount of material light passes through (affected by path length or concentration) and probability of absorption (absorption coefficient)

Question 19

how do you calculate transmittance?

Answer 19

It / Io
It = intensity of light transmitted
Io = intensity of light incident to sample/that passes through sample

Question 20

what is the relationship between absorbance and transmittance?

Answer 20

absorbance = log10(transmittance)
*remember, when you log a fraction it flips!

Question 21

what is molar absorption coefficient , ε ?

Answer 21

an intrinsic property of chemical species that is dependent on their chemical position + structure

Question 22

what is the relationship between absorbance and concentration?

Answer 22

they are proportional to each other, so as concentration increases, absorbance decreases
- this also means it is inversely proportional to transmittance

Question 22

when might the beer-lambert law not be accurate?

Answer 22

when concentration of solution/absorbance is so high, the sample is likely to deviate from the law
- this is why it is good to check if a linear relationship is expressed in your data

Question 23

how do you plot the beer-lambert law linearly?

Answer 23

A = εc where A = absorbance, this is what is measured c = concentration, controlled and ε = molar absorption coefficient, constant l = path length, constant at 1cm usually so can be ignored essentially, or would be part of gradient term with ε

Question 24

give 2 limitations of electronic spectroscopy

Answer 24

- minimum absorbance that can be accurately measured ~0.01 - compounds with a high ε (aka highly coloured) will have a much lower practical detection limit than compounds with a low ε (weak absorbers)

Question 25

how can weakly absorbing samples be adapted for use for electronic spectroscopy?

Answer 25

a complexing agent can be added to create a coloured complex that is more strongly absorbing to increase ε so lower concentrations can be measured

Question 26

give 2 selection rules that govern electronic spectroscopy for transition metals

Answer 26

the spin rule/ΔS = 0 - electron must retain their spin when promoted to a higher energy level the orbital rule/ΔL = +/-1 - for molecules with a centre of symmetry, transitions between a given set of orbitals are forbidden, e.g. p->p, s->s

Question 27

what is vibronic coupling?

Answer 27

complexes can become asymmetric if bond lengths are uneven, this means π-acceptors/ donor ligands can mix with d-orbitals so transitions aren't purely d->d, meaning electrons undergo these transitions form ligands to empty orbitals on metal, as it isn't directly d->d - a way around the orbital rule

Question 28

how are electronic energy levels determined for (conjugated) organic molecules?

Answer 28

electrons are treated as waves, which are only stable is they are exact full or 1/2 wave cycles - each successive energy level is +1/2 of the wave, this allows delocalised π electrons, which are free to move along the organic molecule, to be assigned energy levels - must consider 1 electron per C atom, and electrons fill up energy levels like shells, paired from lowest to highest energy

Question 29

how does chain length impact radiation absorption?

Answer 29

longer conjugated chains absorb larger wavelengths, causing absorption to shift from UV to visible region

Question 30

how do electronic transitions affect bonds?

Answer 30

often electronic transitions move electrons from bonding to antibonding orbitals, weakening the bond

Question 31

what are the consequences of electronic transitions weakening bonds?

Answer 31

weakening bonds changes the force constant and vibrational energy of the bond - electronic transitions can also include changes in vibrational and even rotational energy levels, which are nested inside electronic energy levels

Question 32

what implications does bond weakening caused by electronic transitions have?

Answer 32

electronic transitions are very fast - if an electron moves from a bonding to an antibonding orbital, the bond MUST get weaker, however for this to occur at the same speed as the transition, the nuclei would have to move instantly, which is not possible because nuclei are much heavier than electrons therefore move much slower

Question 33

what is the frank-condon principle?

Answer 33

states that since electronic transitions are so fast that nuclei don't have time to move, the most likely transitions are those where the initial and final nuclear configurations have greatest overlap in their vibrational functions - essentially so nuclei don't need to move at all, just gains vibrational energy - this is why UV-Vis spectra appear as broad curves, as its a combination of the many vibratoinal transitions that occur alongside the electronic transitions

Question 34

how can vibrational transitions be seen during electronic spectroscopy?

Answer 34

at a high resolution peaks under envelope/ UV-Vis main peaks give information on vibrational energy levels

Question 35

how does fluorescence spectroscopy work?

Answer 35

involves an electronic excitation (+ vibrational energy change alongside), but observed over a long period of time, molecule relaxes from excited vibrational state to the lowest vibrational state of the excited electronic state, resulting in the emission of a photon at lower energy = fluorescence

Question 36

what happens to vibrational energy that gets absorbed in electronic spectroscopy?

Answer 36

relaxation to the lowest vibrational state (of the excitation electronic state)

Question 37

what is measured in fluorescence spectroscopy?

Answer 37

the emitted photon

Question 38

how long does the fluorescence last?

Answer 38

~10^-10 - 10^-8 seconds it stops when the exciting light is taken away

Question 39

how does fluorescence relate to concentration?

Answer 39

intensity of emitted fluorescence is proportional to concentration, this means that intensity of fluorescence is a linear function of concentration up to a certain point