Core 3: Electronic, Vibrational and Rotational Spectroscopy Flashcards
List the parts of the EM spectrum and their spectroscopic effects.
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
What are the wavelengths of microwaves to x-rays?
Microwaves IR Vis UV X-ray
1cm 100μm 1μm 10nm
What is the basic structure of an absorption spectrometer?
Radiation source→Monochrometer→Sample cell→Detector
How can absorbance be represented?
A=log10(I0/I)=log10(1/T)
How is an emission spectrometer different to an absorbance spectrometer?
Emission spectrometers have the sample before the monochrometer.
What decides if a transion occurs?
The selection rules.
What decides how intense a band is?
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.
What is the difference between rotational energy levels?
What occurs to the molecule at the lowest energy level?
ΔE0→1=2B where B is the rotational constant.
ΔE1→2=4B, The energy of the transition increases by 2B.
At J=0, the molecule does not rotate at all.
What effect does weight of atoms and bond length have on the energy gaps between rotational energy levels?
Lighter atoms and shorter bonds result in larger energy gaps between rotational energy levels.
What are the gross and specific selection rules for rotational energy transitions?
Gross: Must be a change of dipole on rotation
Specific: Transitions only allowed ΔJ=±1
Explain the appearence of rotational energy levels on spectra?
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.
What can small peaks in the baseline of the rotational spectra be explained by?
The small number of isotopes which will have different rotational energy level changes.
What can the rotationary peaks be used to find?
The distance between the peaks is 2B and B can therefore be found.
How are non-rigid rotors different to rigid rotors?
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.
How many potential moments of inertia exist for any molecule? How can you decide which are zero, equal and different?
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.
How are vibrational energy levels spaced?
They are moderately spaced with transitions in the IR region.
For diatomic harmonic oscillators, what is the lowest vibrational energy level and the difference between energy levels?
The lowest vibrational energy level is ½ṽvib where ṽvib is the rotational energy constant. The difference between energy levels is ṽvib.
What is the gross and specific selection rule for vibrational energy changes?
Gross: Change in dipole on vibration
Specifc: ΔV=±1
What will be seen for the vibrational spectrum? What can this be used for?
Only one major transition is seen, ΔV0→1 this can be used to calculate factors relating to bond strength.
When are rotational changes permitted?
When the spectra is recorded in gas phase.
What changes have to be made for diatomic anharmonic oscillators?
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.
How many vibrations are possible for linear and non-linear polyatomic molecules?
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).
What types of vibrations are possible for a molecule?
Symmetric stretch, asymmetric stretch and bend.
How molecules with similar or identical groups vibrate?
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.
How do dipole strengths affect bond width?
The larger the dipole change from vibration, the larger the band produced.
What two types of vibration can occur for a molecule? What are their uses?
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.
How do the frequency of bends compare to stretches?
Bends occur at lower frequencies(and wavenumbers) than stretches.
What is the name for molecules that absorb in the UV/Vis region?
Chromophores.
What is the gross selection rule for electronic transitions? Which transitions are most likely?
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.
How does the wavelength of the π→π* absorbtion change as conjugation changes?
The wavelength gets longer as more high energy π orbitals and low energy π* orbitals become avaliable.
What is the isobestic point?
When two species are interconverting, they will have the same ε value at this specific point.
What scattering are we interested in for Raman spectroscopy? What are the Stokes and anti-Stokes lines?
Inelastic, where energy is exchanged. The Stokes line is where the photon loses energy, the anti-Stokes is where the photon gains energy.
What kind of radiation source is used for Raman spectrometry? How does its energy compare to that of the transfers?
Visible laser light, its energy is much greater than the change in energy from scattering.
How will scattering intensity depend on wavelength?
Short wavelength light is more scattered, scattering intensity∝1/λ4
What are the selection rules for Raman scattering?
Gross: there must be a change of polarisablilty on rotation or vibration.
Specific: ΔJ=0, ±2 ΔV=±1
Which molecules are not shown in Raman scattering?
Isotropic ellipsoids which have equal polarisability in all directions.
What does a centre of inversion for a molecule mean for the raman and IR active modes?
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.
