Lecture 3 - vibrational spectroscopy Flashcards
How does the harmonic oscillator work for a single moving particle?
-This describes the vibration of a molecule, two nuclei held together by a spring. Only 1 is moving away while the other particle is stationary.
- The force of each nuclei is given by F = -kx
Where k is a constant and x is displacement.
X is measured by the difference between the spring when its at rest and when its stretched, or the difference between when its rested and compressed.
The allowed energy levels are found by using Schrodinger’s equations with the potential energy equation.
Which gives Ev = (v + 1/2)hcv
How does the harmonic oscillator work for a 2 particle system?
- The relative motion of the 2 nuclei is measured, and depends on x relative to the centre of the mass of the molecule.
So the reduced mass is used, u = m1m2/m1+m2
-The harmonic oscillator approach believes that as you pull the molecules further apart, the energy just keeps going, doesn’t take into account that the molecule will break.
This only works for small intermolecular distances.
What is the morse curve?
-This shows that as you pull the molecules apart, the energy will increase but will eventually level of as the molecule will be broken apart.
-This called anharmonicity, and shows that the energy gaps now get closer together as you get closer to the breaking point (no longer harmonic)
-There is a morse curve for each electronic level. Which has rot and vib levels also inside.
What is Hooke’s Law?
-A heteronuclear diatomic molecule can absorb or emit radiation. The photon energy corresponds to the change in vibrational energy, because of DeltaV = +/- 1
-The corresponding photon energy hv gives the radiation a frequency equal to the classical frequency of vibration of the atoms. This gives the wavenumber (v-hat).
- This law also allows the prediction of the position of a particular peak in a vibrational spectrum for a particular bond of the mass of the atoms is known.
-Shows that stronger bonds absorb at higher frequencies.
What is IR spectroscopy?
-Absorption of IR frequencies due to oscillating dipole moment of the molecule (Molecules interact with electromagnetic radiation absorbing or emitting photons if they possess an oscillating dipole)
-Bond requires a dipole and a change in dipole during vibration to be observed (HCl)
What is Raman spectroscopy?
-Scattering of incident radiation one vibrational unit different from the incident excitation due to a change in the molecular polarizability.
-Does not require a change in dipole- light induces a dipole in molecule, strength depends on µ = αE, polarizability and strength of field.
What are the different types of energy?and how d0es 3N-6/5 arise?
Rot, vib and electronic.
There are a total of 3N degrees of freedom in a molecule.
3 come from translational, 2/3 from rotational based on if the diatomic molecule is linear.
This leaves vibrational to be;
3N-5 if the molecule is
linear
3N-6 if the molecule is non-linear
The 3N is from the DOF.
In a non-linear molecule three of these independent motions will move the whole molecule along X,Y and Z axis. Another 3 rotating around the axis.
Which gives 3N-6.
In a linear molecule, rotation around the X axis (IA) will effectively be 0 and wont change the configuration of the molecule, so 3N-5 modes aloud for vibrational change.
How would you work out how many vibrations a molecules can have?
For non-linear 3N-6 gives the number.
For linear 3N-5 gives the number.
What types of vibrations is there?
-Stretching (asymmetric and symmetric)
-Bending (scissoring, waging, rocking, twisting)
Summary of IR.
- IR is a vibrational spectroscopy since the spectra arise from transitions between vibrational energy levels of a molecule.
-Infrared involves absorption of light
-For a particular transition a molecule absorbs a discrete amount of energy: one quantum.
-A quantum of energy (E) is given by:
E = hv
i.e. E = hv = hv1 – hv0
-The selection rule is Δv = +/-1
How does IF work for polyatomic molecules?
-Possible vibrations but wont all be IR active - need to look at each molecule separately.
-Example, in a C-C bond there will be no band, but if a C-H bond in the same molecule will.
-C-C give weak bands, C-H give a great intense peak.
What are group frequences?
- For groups like CH2, CH3 and benzene.
-If a bond strength of two bonds (Like a C-H and C-H) which are very similar, they both have an effect on each other.
-These are treated together as a unit and give group frequencies.
For example, there will be peak in the IR for aromatic ring stretches alone - wouldn’t be able to tell which is from each C.
