Structure and Bonding

Question 1

What is the Born-Oppenheimer Approximation?

Answer 1

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

Question 2

When does the BOA breakdown?

Answer 2

• Fast Nuclei
• Rydberg States

Question 3

How do you classify an electronic state

Answer 3

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

Question 4

What is the use of photoelectron spectroscopy?

Answer 4

Used to probe binding energy and symmetry of orbitals

Question 5

For photo electron spectroscopy what pattern is expected for :

Bonding Orbital

Anti Bonding Orbital

Non Bonding Orbital

Answer 5

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

Question 6

What is the franck condon principle?

Answer 6

Nuclei do not move during transition,

Adding or removing electrons changes the bond length and spectroscopic constants

Question 7

What is the franck condon factor?

Answer 7

Used to calculate the transition moment (selection rule for vibronic transitions)

Represents the overlap of orbitals, if they dont overlap then transition cannot occur

Question 8

How do use a walsh diagram and what is it good for?

Answer 8

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

Question 9

What is huckel theory good for?

Answer 9

Predicting the energy of pi systems

Question 10

What are the coulomb and exchange integrals in huckel theory

Answer 10

H_ij = coulomb integral (hamiltonian)

S_ij = exchange integral (overlap)

Question 11

Write down the equation for Hij

Answer 11

Question 12

Write down the equation for Sij

Answer 12

Question 13

How do the Pz orbitals overlap in huckel theory

Answer 13

Normalized, interacts only with itself

Sij = 0

Sii = 1

Question 14

What are some flaws in huckel theory

Answer 14

• doesnt consider the difference between bond lengths and terminal/central carbon atoms
• only a 1st approximation

Question 15

How does huckel theory explain huckels rule?

Answer 15

For any aromatic molecule the electrons will completely fill all bonding pi interactions causing the bonding to be very strong.