Organic Chemistry 1.3 - Molecular Orbitals - Molecular Orbital Colour Theory Flashcards
Molecular Orbital (MO) colour theory can be used to explain why ___ molecules are colourless or coloured.
organic
The fact that bonding MOs have ___ ___ than antibonding MOs is the basis of this colour theory.
different energies
Bonding MOs have ___er energies than antibonding* MOs, and contain ___.
lower, electrons
Antibonding* MOs have ___ energies than bonding MOs, and do not contain ___.
higher, electrons
What is the highest energy bonding MO called?
What is the lowest energy antibonding MO?
HOMO.
LUMO.
*(The highest of the lowest and the lowest of the highest)
What does HOMO and LUMO stand for?
Highest Occupied MO
Lowest Unoccupied MO
*occupied with electrons
Which has higher energy: HOMO or LUMO?
LUMO
When is electromagnetic radiation absorbed?
whilst promoting electrons from HOMO to LUMO
Why do certain organic molecules appear colourless?
because no EMR from the visible region is absorbed (and therefore it is all reflected - and all colours together = clear)
The majority of organic molecules appear colourless because the ___ difference between ___ and ___ is so large that EMR from the __ region is absorbed.
energy, HOMO, LUMO, UV
Coloured molecules have a HOMO-LUMO energy difference which corresponds to…
visible light
The absorption of a wavelength of visible light means that the ___ ___ is observed/reflected.
complementary colour
*colour wheel in db page 19
Some organic molecules contain…
chromophores
What is a chromophore?
a group of atoms within an molecule that is responsible for the absorption of a photon/wavelength of light from the visible region of the EM spectrum
Light is absorbed when ___ in a ____ are promoted from HOMO to LUMO.
electrons, chromophore
Chromophores exist in molecules which have a…
conjugated system
What is a conjugated system?
a conjugated system is a chain of alternating double single bonds (with delocalised electrons).
c-c=c-c=c-c=c
Conjugated systems can only exist between atoms of which element?
carbon
A conjugated system is system of adjacent ___ p orbitals between carbon atoms which overlap __-on to form a ___ orbital which spans a number of atoms
unhybridised, side, molecular.
It is said that ‘molecules with ___ of alternating double-single bonds between carbon atoms have a ___ ___.’
chains, conjugated system.
The more carbon atoms in a conjugated system, the…
smaller the energy difference between HOMO and LUMO, therefore the lower energy photons of light absorbed
To reiterate: the larger the conjugated system, the ___ the energy of light absorbed.
lower
‘What structural feature allows molecule x to absorb visible light?’
a conjugated system made up of alternating double and single bonds
Explain how a conjugated system gives rise to the blue colour of molecule x.’
the wavelength of light which is complementary to blue is absorbed whilst promoting an electron from HOMO to LUMO.
or other words to that affect.
What are the key words in this answer?
‘the wavelength of light which is complementary to blue is absorbed whilst promoting an electron from HOMO to LUMO’
light, complementary, blue, absorbed, promoting, electron, HOMO, LUMO
What is the difference between a conjugated system and a chromophore??
a conjugated system is a series of alternating double and single bonds between carbon atoms
a chromophore is a group of atoms within a molecule that is capable of absorbing a photon of visible light.
Explain fully why this dye molecule will absorb a shorter wavelength of light than the other two dye molecules. (it is smaller than another ones and has less carbons in its conjusated system)
There is less conjugation
OR
a shorter sequence of alternating double and single bonds
OR
a smaller chromophore. (1)
Larger gap/greater energy
(absorbed). (1)
Smaller conjugated system = (smaller/larger) HOMO-LUMO energy difference.
LARGER