lecture 3

Question 1

how can a pi-system be called conjugated?

Answer 1

when the unhybridized p-orbitals are continuous, the pi-system is called conjugated.
a conjugated pi-system allows electron to be delocalized . this phenomenon is called resonance.

Question 2

classification of pi-system?

Answer 2

1. isolated pi-systems: <3continuous p-orbitals, all parallel
2. cumulated pi-systems: >= continuous p-orbitals, not all parallel
3. conjugated pi-system: >= continuous p-orbitals, all parallel

Question 3

how is electron delocalization related to stability?

Answer 3

the more electron delocalization, the better the stability

Question 4

how to compare the stability of contributing structure?

Answer 4

1. number of covalent bonds (more is better)
2. number of complete octets (more is better)
3. degree of charge separation (less formal charges is better)
4. hyper conjugation effects for cations (3>2>1)

Question 5

what is HOMO and LUMO?

Answer 5

LUMO: lowest, unoccupied molecular orbital
HOMO: highest, occupied molecular orbital
* more p-orbitals, smaller gaps between LUMO and HOMO

Question 6

relationship between wavelength and E (energy gap)?

Answer 6

1. non-conjugated pi-system: lamda<200nm, larger E
2. conjugated pi-system: lamda>200nm, smaller E
   * colored conjugated systems have small enough HOMO-LUMO energy gaps for visible light to cause electron excitation (400nm-700nm)

Question 7

the unique properties of conjugated system?

Answer 7

1. being colored
2. organic materials with small HOMO-LUMO energy gaps are also conducting materials, but unlike metals, they are flexible and can be much cheaper
3. conjugated organic compounds will be the electronic material of the future for flexible screens and devices