Structure and Bonding Flashcards
What is the Born-Oppenheimer Approximation?
Electrons move very quickly compared to nuclei.
This means that during an electronic transition the nuclei will not change very much allowing for the calculation of just the electronic wavefunctions
When does the BOA breakdown?
- Fast Nuclei
- Rydberg States
How do you classify an electronic state
For Λ
s contributes 0
p contributes +/- 1
Λ=0, Sigma state
Λ=1, Pi state
The only way to get a - state is by having parallel electrons in a pi orbital
What is the use of photoelectron spectroscopy?
Used to probe binding energy and symmetry of orbitals
For photo electron spectroscopy what pattern is expected for :
Bonding Orbital
Anti Bonding Orbital
Non Bonding Orbital
Loss/Gain of (anti)bonding electron changes bond strength and spectroscopic constants. This changes the energy levels causing a change in vibrational structure.
BO - closely spaced vibrational structure.
ABO - widely spaced vibrational structure
NBO - Single peak, note that bond length doesnt change
What is the franck condon principle?
Nuclei do not move during transition,
Adding or removing electrons changes the bond length and spectroscopic constants
What is the franck condon factor?
Used to calculate the transition moment (selection rule for vibronic transitions)
Represents the overlap of orbitals, if they dont overlap then transition cannot occur
How do use a walsh diagram and what is it good for?
Good for predicting bond angles and how they change as the molecule is ionised
Sum valence electron and fill orbitals, balance so that the lowest energy is made, angle is on the x axis
What is huckel theory good for?
Predicting the energy of pi systems
What are the coulomb and exchange integrals in huckel theory
Hij = coulomb integral (hamiltonian)
Sij = exchange integral (overlap)
Write down the equation for Hij
Write down the equation for Sij
How do the Pz orbitals overlap in huckel theory
Normalized, interacts only with itself
Sij = 0
Sii = 1
What are some flaws in huckel theory
- doesnt consider the difference between bond lengths and terminal/central carbon atoms
- only a 1st approximation
How does huckel theory explain huckels rule?
For any aromatic molecule the electrons will completely fill all bonding pi interactions causing the bonding to be very strong.
