Electronic transitions of organic compounds

This transition appears in the far UV since the promotion of an electron from a cr bonding MO to a a anti-bonding MO requires a significant energy. This is the reason for saturated hydrocarbons that only contain this type of bonding being transparent in the near UV. 

Cyclohexane and heptane are used as solvents in the near UV. At 200 nm the absorbance of a 1 cm thickness of heptane is equal to 1. Unfortunately, the solvation power of these solvents is insufficient to dissolve many polar compounds. 

The promotion of an n electron from an atom of O, N, S, Cl to an MO a leads to a transition of moderate intensity located around 180 nm for alcohols, near 

This transition of low intensity results from the passage of an n electron (engaged in a non-bonding MO) to an anti-bonding ir orbital. This transition is usually observed in molecules containing a hetero atom carrying lone electron pairs as part of an unsaturated system. The best known is that corresponding to the carbonyl band, easily observed at around 270 to 295 nm. The molar absorption coefficient for this band is weak. 

Example ethanal A = 293nm(e = 12, with ethanol as solvent). 

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