Electronic Spectroscopy Flashcards
Where do electronic spectroscopic transitions occur?
For organic molecules: UV and vacuum UV region of the spectrum.
For coloured complexes: the visible region.
What happens when a UV-Vis photon is absorbed?
Results in an electronic transition; electrons are promoted from ground state to higher electronic states.
What is meant by the term “chromophore”?
The parts of the molecules, in a polyatomic molecule, associated with the absorption of light.
What does an electronic transition involve?
The promotion of an electron between molecular orbitals, from the HOMO to the LUMO.
State the link between conjugation and wavelength for organic molecules.
The longer the conjugated system, the longer the wavelength of the absorption.
What is the most common electronic transition for organic molecules?
Most common electronic transitions involve electrons in pi orbitals.
State the TWO types of transitions for inorganic complexes.
- d-d transitions: an electron is excited from a d-orbital to a d-orbital of higher energy.
- charge-transfer transitions: electrons migrate from the ligands into the metal’s d-orbitals or vice versa i.e. LMCT or MLCT.
Account for the colours of compounds in terms of the wavelength of the radiation absorbed.
The colour absorbed from white light by the compound is NOT the colour reflected. Instead it is a mixture of the remaining colours. See colour wheel
State the Beer-Lambert law.
A = -log(it/io) = ecl
What do the e values of the Beer-Lambert equation tell us?
Tell us whether the transition is forbidden or not.
How would transitions be forbidden?
Forbidden by spin or by symmetry.
State the values that would allow and forbid transitions.
e < 1: strongly forbidden
e 10-100: weakly allowed
e > 500: strongly allowed
State what is on the x and y axis of an electronic spectrum.
x-axis (horizontal): wavelength/nm
y-axis (vertical): absorbance A
