Core 3: Electronic, Vibrational and Rotational Spectroscopy

Question 1

List the parts of the EM spectrum and their spectroscopic effects.

Answer 1

Radiowaves→Change of spin, NMR/EPR

Microwaves→Rotational

IR→Vibrational

UV/Vis→Electronic (valence electron changes)

X-ray→Core electronic changes

γ-ray→Change of nuclear configuration

Question 2

What are the wavelengths of microwaves to x-rays?

Answer 2

Microwaves IR Vis UV X-ray

1cm 100μm 1μm 10nm

Question 3

What is the basic structure of an absorption spectrometer?

Answer 3

Radiation source→Monochrometer→Sample cell→Detector

Question 4

How can absorbance be represented?

Answer 4

A=log₁₀(I₀/I)=log₁₀(1/T)

Question 5

How is an emission spectrometer different to an absorbance spectrometer?

Answer 5

Emission spectrometers have the sample before the monochrometer.

Question 6

What decides if a transion occurs?

Answer 6

The selection rules.

Question 7

What decides how intense a band is?

Answer 7

The population of an energy level. When the differences between energy levels is large, the majority of the population occupies one level making the other transitions very weak.

Question 8

What is the difference between rotational energy levels?

What occurs to the molecule at the lowest energy level?

Answer 8

ΔE_0→1=2B where B is the rotational constant.

ΔE_1→2=4B, The energy of the transition increases by 2B.

At J=0, the molecule does not rotate at all.

Question 9

What effect does weight of atoms and bond length have on the energy gaps between rotational energy levels?

Answer 9

Lighter atoms and shorter bonds result in larger energy gaps between rotational energy levels.

Question 10

What are the gross and specific selection rules for rotational energy transitions?

Answer 10

Gross: Must be a change of dipole on rotation

Specific: Transitions only allowed ΔJ=±1

Question 11

Explain the appearence of rotational energy levels on spectra?

Answer 11

Many energy levels are occupied with the majority of the population not in the J=0 level meaning there is a rising curve of intensity of transitions before it declines over time.

Question 12

What can small peaks in the baseline of the rotational spectra be explained by?

Answer 12

The small number of isotopes which will have different rotational energy level changes.

Question 13

What can the rotationary peaks be used to find?

Answer 13

The distance between the peaks is 2B and B can therefore be found.

Question 14

How are non-rigid rotors different to rigid rotors?

Answer 14

Non-rigid rotors account for the fact that the bond will stretch as the molecule rotates faster. This lowers the energy at which the bond rotates as the moment of inertia changes.

Question 15

How many potential moments of inertia exist for any molecule? How can you decide which are zero, equal and different?

Answer 15

There is 3 potential rotations for each molecule, along each axis.

For any linear molecule, the intermolecular axis rotation is zero as there is no dipole change. If a dipole changes when a rotation occurs it will exist in the spectra.

Question 16

How are vibrational energy levels spaced?

Answer 16

They are moderately spaced with transitions in the IR region.

Question 17

For diatomic harmonic oscillators, what is the lowest vibrational energy level and the difference between energy levels?

Answer 17

The lowest vibrational energy level is ½ṽ_vib where ṽ_vib is the rotational energy constant. The difference between energy levels is ṽ_vib.

Question 18

What is the gross and specific selection rule for vibrational energy changes?

Answer 18

Gross: Change in dipole on vibration

Specifc: ΔV=±1

Question 19

What will be seen for the vibrational spectrum? What can this be used for?

Answer 19

Only one major transition is seen, ΔV_0→1 this can be used to calculate factors relating to bond strength.

Question 20

When are rotational changes permitted?

Answer 20

When the spectra is recorded in gas phase.

Question 21

What changes have to be made for diatomic anharmonic oscillators?

Answer 21

The bond lengthening has to be accounted for reducing the energy of the vibration.

The specific selection rule gets relaxed to ±1 or ±2 but +1 is still the strongest.

Question 22

How many vibrations are possible for linear and non-linear polyatomic molecules?

Answer 22

Linear=3N-5 (no rotation about nuclear axis)

Non-linear=3N-6

N is the number of atoms, the integer taken away is the degrees of freedom of the molecule (vibrations and translations).

Question 23

What types of vibrations are possible for a molecule?

Answer 23

Symmetric stretch, asymmetric stretch and bend.

Question 24

How molecules with similar or identical groups vibrate?

Answer 24

The groups either vibrate in phase or out of phase. All individual vibrations pass through the equlibrium point at the same time but different vibrations can occur at whatever frequency suits them best.

Question 25

How do dipole strengths affect bond width?

Answer 25

The larger the dipole change from vibration, the larger the band produced.

Question 26

What two types of vibration can occur for a molecule? What are their uses?

Answer 26

Skeletal vibrations and group vibrations. Group are specific and localised so are useful for identifiying functional groups, skeletal vibrations occur in the fingerprint region and can identify molecules.

Question 27

How do the frequency of bends compare to stretches?

Answer 27

Bends occur at lower frequencies(and wavenumbers) than stretches.

Question 28

What is the name for molecules that absorb in the UV/Vis region?

Answer 28

Chromophores.

Question 29

What is the gross selection rule for electronic transitions? Which transitions are most likely?

Answer 29

Gross: must be a change in dipole when the electron moves, large dipole changes means a stronger peak is shown.

Most common changes are π→π* then n(non-bonding)→π*. Other transitions require a vacuum so they are rarely studied.

Question 30

How does the wavelength of the π→π* absorbtion change as conjugation changes?

Answer 30

The wavelength gets longer as more high energy π orbitals and low energy π* orbitals become avaliable.

Question 31

What is the isobestic point?

Answer 31

When two species are interconverting, they will have the same ε value at this specific point.

Question 32

What scattering are we interested in for Raman spectroscopy? What are the Stokes and anti-Stokes lines?

Answer 32

Inelastic, where energy is exchanged. The Stokes line is where the photon loses energy, the anti-Stokes is where the photon gains energy.

Question 33

What kind of radiation source is used for Raman spectrometry? How does its energy compare to that of the transfers?

Answer 33

Visible laser light, its energy is much greater than the change in energy from scattering.

Question 34

How will scattering intensity depend on wavelength?

Answer 34

Short wavelength light is more scattered, scattering intensity∝1/λ⁴

Question 35

What are the selection rules for Raman scattering?

Answer 35

Gross: there must be a change of polarisablilty on rotation or vibration.

Specific: ΔJ=0, ±2 ΔV=±1

Question 36

Which molecules are not shown in Raman scattering?

Answer 36

Isotropic ellipsoids which have equal polarisability in all directions.

Question 37

What does a centre of inversion for a molecule mean for the raman and IR active modes?

Answer 37

If the molecule has a centre of inversion then its raman active modes are IR inactive. For example, for CO₂ the bend and asymmetric stretch have a change in dipole but symmetric stetch doesn’t. Therefore the only Raman active vibration is the symmetric stretch. This does not apply for molecules without a centre of inversion.