Lecture 9 - Electronic spectroscopy Flashcards
How does electronic absorbance work?
- The photon (energy) is absorbed and goes to a higher energy level.
- In terms of MO’s, electron goes from a HOMO to a LUMO.
How does electronic emission work?
- The electron goes from an excited state back down to the state before and emits energy (photons)
- In MO’s term, electron goes from LUMO to HOMO.
What does the UV-vis show us about the molecule?
- Shows the absorbance between the E1 and E2 which is the maximum absorbance on the spectrum - shows where the most absorbance happens
How are chromophores involved?
Photon absorption usually involved the movement of the whole molecule but mostly attributed to the chromophore of a molecule - most absorption usually comes from this.
How are electronic transitions classified?
- By orbital transitions
Can be from pi to pi, n to sigma or n to pi* (star is antibonding) - Conjugation gives longer wavelengths ie molecules with high conjugation are strong chromophores and absorb strongly
What are examples of each transition class?
Examples of n-> sigma* are lone pair atoms like N, but also C=O, usually lowest nm.
An example of n-> pi* is also C=O (double bond) this has the highest value of nm.
Examples of pi-> pi* is like C=C and aromatic rings. Middle nm.
What are examples of solvents used for UV-vis?
Water
Chloroform
Methanol
What happens to electrons when they are excited? (selection rules)
- Electrons are paired in lower energy states - one electron is facing up and one facing down, when one electron from a pair gets excited the electron stays whatever way it was facing (known as spin (S)) so is conserved (S=0)
- So S in unchanged for an ALLOWED transition.
- When the spin of an electron changes upon being excited, and doesn’t retain its spin then this is a FORBIDDEN transition (S=0 -> T1)
- This is known as an excited triplet state.
- These are very rare as the selection rule of S=0 is strictly adhered to for organic molecules.
What are the orbital symmetry selection rules?
- Transition depends on orbitals involved and the way they overlap.
- For n -> pi* for C=O groups, symmetry forbidden.
- For pi -> pi* for benzene derivatives is forbidden.
- For pi->pi* for isolated double bond is allowed.
What is the absorption selection rule?
- Based on a molar absorption coefficient (dm3 mol-1 cm-1)
- This can be calculated from the Beer-lambert law which is
log10 (I0/I) = coeff x c x l
log10(I0/I) = A (absorbance)
A = coeff x c l
c = concentration in dm3 mol-1
l - path length is cm-1
Coeff is large for allowed transitions.
How and why is UV-vis used in quantitative analysis?
- Non destructive, simple, small volumes and low concs.
- Background is subtracted as reference used.
- Ref sample must contain sample conc of solvent as sample as the solvent will absorb strongly (means you can pick out sample peak)
- Quartz cuvette used for UV, plastic can be used if in visible region.
What are the red and blue shifts?
- In a blue shift (also knwon as hypsochromic), the peak will shift to lower wl - higher energy.
- In a red shift (also known as bathochromic) the peak will shift to higher wl - lower energy. This is caused by an “auxochrome” which is a group joined to a chromophore.
What happens in electronic transitions for metal complexes?
What are the 3 types of electronic transitions?
- Any involving n, pi and signa electrons - ex in ligand based transitions, usually shifted to higher E.
- Transitions involving charge transfer electrons - metal to ligand or ligand to metal.
- Transitions involving d and f electrons - metal based d-d, weak.
What is involved in the ligand field theory?
Electronic transitions in metals take place between occupied and empty orbitals which depends on the interaction between the metal and ligand and the transitions that can occur between them.
