Electronic absorbance Flashcards
What determines the intensity of a transition?
Depends in the symmetry of the 2 orbitals as ground and excited have different symmetries.
Symmetric orbitals are…
… EVEN
Asymetric orbitals are…
… ODD
Non-bonding orbitals are considered?
EVEN
What orbitals are EVEN?
S, d and pi*
What orbitals are ODD?
p and pi
What are the intensity and symmetry selection rule?
Go from one point to the other side if same charge orbital is EVEN, if opposite then it is ODD.
What transition are allowed?
Between ODD and EVEN are allowed.
ODD <-> ODD and EVEN <-> EVEN were formally forbidden but do occasionally happen. (why copper sulphate is blue).
pi –> pi*
What transition states are forbidden?
n –> pi*
When do pi –> pi* transitions occur?
In molecules with 1 or more double bonds. e = 10^2 – 10^6 M-1 cm-1
What happens in charge transfer transitions (CT)?
An e- moves btwn orbitals on diff atoms (and then rapidly returns, losing energy as heat). eg ligand-to-metal (LCMT) transition or vice versa (MLCT). also strong allowed transitions.
d-d transitions occur when?
an e- moves btwn orbitals on transition metal ions. AKA ligand-field transitions. d-d should technically be forbidden but it can happen at low intensity so is called formally forbidden.
Spin is magnetic or electronic?
Magnetic so should not change during transition.
Lowest energy electronic absorption band of a pi-conjugated system shifts to?
Longer wavelength as the no. of pi bonds increases.
What does HOMO stand for
Highest occupied molecular orbital.
