1. Electronic Spectra: Ultraviolet and Visible Spectroscopy 14-11 Ultraviolet and visible light give rise to electronic excitations. Spectroscopy of organic molecules is possible because the absorption of radiation is restricted to quanta having energies corresponding to specific molecular excitations: Ultraviolet (200-400 nm) and visible (400-800 nm) spectroscopy are particularly useful for investigating the electronic structures of unsaturated molecules and for measuring their extent of conjugation. UV and visible spectroscopic samples are usually dissolved in solvents having no absorption peaks above 200 nm (ethanol, methanol, cyclohexane).

2. Ultraviolet and visible light give rise to electronic excitations. In the ground electronic state of a molecule, all electrons (except for lone pairs) occupy bonding molecular orbitals. The absorption of a photon of UV or visible light transfers an electron from its ground electronic state in a bonding orbital, to an excited electronic state in an antibonding orbital. The dissipation of this absorbed energy can be in the form of a chemical reaction, the emission of light (fluorescence, phosphorescence) or as heat.

3. The energy gap between the bonding and antibonding molecular orbitals of organic  bonds is too large to allow excitation by visible or UV photons. The bonding, non-bonding, and antibonding molecular orbitals of organic  bonds are much closer together, however. Excitation of electrons in  orbitals leads to    * transitions. Excitation of electrons in non-bonding orbitals occurs even more readily leading to n   * transitions. The number of  molecular orbitals in a molecule is equal to the number of component p orbitals. As a result, extended conjugation leads to very complex spectra.

4. A typical UV spectra is that of 2-methyl-1,3-butadiene (isoprene). A peak is identified by the wavelength at its maximum, max (in nm), and its height, reported as the molar extinction coefficient or molar absorptivity (  ). The molar absorptivity is defined as the peak height (absorbance A) divided by the concentration of the sample (assuming a 1 cm standard cell length).

5. Electronic spectra tell us the extent of delocalization. The more double bonds there are in conjugation, the longer the wavelength is for the lowest energy excitation and the more peaks will appear in the spectrum. Ethene max =171 nm. 1,4-Pentadiene (unconjugated) max =178 nm. 1,3-Butadiene (conjugated) max = 217 nm.

6. Beer- Lambert Law Absorbance is directly proportional to concentration

7. Beyond 400 nm, molecules become colored (yellow, then orange, red, violet, then blue-green). The electronic diagrams of ethene, 2-propenyl radical, and butadiene show why larger conjugated  systems have more readily accessible and lower-energy excited states.

8. Conjugation in cyclopolyenes is governed by a separate set of rules to be discussed later. Benzene is colorless, while azulene is a deep blue color: