Vibrational Spectroscopy Flashcards
How many degrees of freedom does an isolated atom have?
3
What are the three different types of motion for a diatomic molecule?
- Translation (In-phase displacement with not change in internuclear separation)
- Rotation (Change in orientation but no change in internuclear separation or centre of mass)
- Vibration (No change in centre of mass or orientation but a change in internuclear separation)
What is the equation for determining the number of modes of vibration for a LINEAR molecule?
3N - 5
Where N = number of molecules
What is the equation for determining the number of modes of vibration for a NON-LINEAR molecules?
3N - 6
Where N = number of molecules
What is a normal mode of vibration for a molecule?
The natural vibrations of a molecule:
What are the 3 things that happen to all nuclei during a normal mode?
- They undergo simple harmonic motion
- Have the same frequency of oscillation
- Move in phase (but with different amplitudes)
What is Hooke’s law in context of a simple harmonic oscillator?
F = - kx
Where: F is the restoring force
k is stiffness of the bond (force constant)
x is the displacement from equilibrium (= bond length - equilibrium bond length)
For an anharmonic oscillator, as k tends to 0 what happens to r?
r becomes larger and larger
For a harmonic oscillator where are nuclei post likely to be found and why?
At the end of the vibration as they travel more slowly
Are bending frequencies higher or lower than stretching frequencies?
Lower frequency
Does infrared spectroscopy produce a scattering or absorption spectrum?
Absorption
What is transmittance and how is it defined in an equation?
Transmittance is the fraction of incident light that successfully “passes” through a substance. Therefore:
T = I/Io
I = transmitted light
I = incident light
How does an IR spectrometer work?
Photons in a substance absorb incident polychromatic light when the energy of the incident photo exactly matches ΔE for excitation from v=0 to v=1. The transmitted light is then recorded to form an IR spectrum.
What are the two selection rules for harmonic oscillators for IR?
1) A transition is allowed only if the dipole moment changes with displacement
2) Transitions can only occur between adjacent levels
How do the selection rules between harmonic and anharmonic oscillators differ for IR?
In harmonic oscillators transitions can only occur between adjacent levels whereas in anharmonic oscillators transitions can occur to many levels (know as overtones)
How does the energy, frequency and wavelength change from the fundamental absorption to the first overtone?
The first overtone is twice the energy, twice the frequency and half the wavelength
Explain where would a v=1 to v=2 hotband appear compared to v=0 to v=1
The v=1 to v=2 band would appear slightly to the left of the v=0 to v=1 band as in an anharmonic oscillator the energy levels become closer together as they increase, meaning that they are closer in energy to each other so the overtone band would be of a lower energy
Why are the called hot bands?
As temperature increases the intensity of a hotband increases
Does the requirement for a changing dipole mean that only polar molecules display an infrared spectrum?
NO - asymmetric stretches or bendings induces a dipole
What did Raman discover?
That monochromatic scattered light produces both unaltered incident light and light of a lower frequency - light is scatted both elastically (Reyleigh) and inelastically (Stokes and Anti-Stokes)
What happens during Stokes scattering?
Inelastic scattering - photon loses energy by supplying energy to a molecule
What is polarisability?
A measure of the extent to which electrons in an atom or molecule can be displace relative to the nuclei by an electric field
What is the selection rule for Raman emission?
The molecular polarisability must change during the molecular vibration
What is a Fermi resonance?
Two Raman peaks can appear due to fundamental band and the overtone band are at the same wavenumber.
