Lecture 19 Flashcards
1
Q
light and energy
A
- when something gets very hot, it emits light
- when you shine light on something, it heats up
2
Q
light
A
- form of energy transfer, interacts with matter, can be measured
- light is electromagnetic radiation and occupies a very narrow range of wavelengths
- light can be described as both a wave and a particle (photon)
3
Q
wavelength
A
- frequency, defines the energy of a photon
- energy is quantised so one photon delivers one equivalent of energy
4
Q
absorption
A
- molecules absorb light corresponding to their HOMO- LUMO gap and reflect less
- energy is dissipated as heat
5
Q
HOMO- LUMO gap
A
- smallest gap between occupied and empty orbital, it gives the lowest energy absorption possible
6
Q
MO energy
A
MO energy is quantised, only specific wavelength/frequencies are absorbed
7
Q
example (absorption and color)
A
(real molecules have multiple full and empty MOs)
- HOMO-LUMO is n is pi*
-next smallest is pi is pi *
8
Q
formalydehyde
A
- UV is colourless because the gap is too large, absorbs in UV
9
Q
energy selection rule
A
- in order for a molecule to absorb a photon of light, the frequency of the light must match an energy gap for a transition from an occupied orbital to an unoccupied orbital
- transitions are quantised and only some photons can be absorbed
10
Q
spin selection
A
- photons don’t have a spin, electrons do
- since photons don’t have a spin, they can’t cause the spin of a molecule to change
- they can’t flip electrons
check notes
11
Q
laporte selection rule
A
- photos must have angular momentum, so absorbing a photon involves a change in angular momentum
12
Q
centrosymmetric species
A
atoms, molecules which can have u or g orbitals, transitions must involve a change in parity (angular momentum)
13
Q
for atoms
A
means that change in 1 = +/- spin
s to s is forbidden but s to p or p to d is allowed
14
Q
for molecules
A
only u to g, g to u are permitted
handy because its simplifies spectra
15
Q
anisotropic charge displacement rule
A
- light consists of waves of oscillating electric and magnetic fields, so it interacts with dipoles
- for a transition to be allowed, the dipole of the molecule must change
- electron must move from one part of the molecule to another
check notes
