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PIB and Highly Conjugated Molecules (14.7)PowerPoint Presentation

PIB and Highly Conjugated Molecules (14.7)

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PIB and Highly Conjugated Molecules (14.7)

- Conjugated molecules have alternating carbon-carbon single and double bonds
- π-electrons can move freely along all carbon atoms in the conjugated portion of the molecule
- The 1D PIB model can be crudely applied to the energy states of a polyene (e.g., β-carotene)

- Energy levels (and wavefunctions) can be predicted given some information
- Energy depends on quantum number, mass of particle (electron), and length of the box
- Number of nodes in the wavefunction can be predicted based on the quantum number

Electronic Spectroscopy of Highly Conjugated Molecules (14.7)

- Highly conjugated molecules often have color associated with them
- Absorb certain colors of visible light, so we see complimentary colors reflected back to us
- Light is absorbed by an electron that is promoted from one state to a higher energy state
- If PIB model holds, we can calculate the energy of the photon absorbed if we know two things: length of the box and the energy levels the electron starts in and ends in

- Length of the box can be approximated by summing up the carbon-carbon bond lengths in the conjugated part of the molecule
- Box is capped by double bonds, with single and double bonds alternating in the interior
- For β-carotene, we have 10 C-C single bonds (0.154 nm each) and 11 C-C double bonds (0.133 nm each)

- Energy levels are determined by number of double bonds in the system
- Electron is promoted from last occupied π-bond (HOMO) to an unoccupied π-bond (LUMO)
- For β-carotene: HOMO (n = 11) and LUMO (n = 12)

Electronic Spectroscopy of Highly Conjugated Molecules (Con’t)

- Energy of the photon absorbed can be predicted from formula for PIB energy levels
- Knowing the quantum numbers involved simplifies the formula (watch units!)

- We can also predict what happens when we change some of the conditions of the problem
- When conjugation is increased, the length of the box is increased (how does this affect energy of the electronic transition?)
- If conjugation increases, what else changes?

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