Final answer:
Carbonyl compounds absorb light in the UV region predominantly due to the polarity of the carbonyl bond, which causes strong infrared absorption. Molecules with conjugated systems have smaller energy gaps between their HOMO and LUMO, leading to visible and UV light absorption, which can be observed in substances like beta-carotene and 4-methyl-3-penten-2-one.
Step-by-step explanation:
Carbonyl compounds are able to absorb light in the UV region due to their electronic structure. The carbonyl bond, being very polar, has a significant difference in the electronegativity between the carbon and oxygen atoms, which leads to a strong absorption of infrared light. This absorption corresponds to the vibrational excitation of the bond. Carbonyl compounds have a characteristic strong peak in their IR spectrum within the 1650-1750 cm¹ region, and this key absorption peak is a clear indication of the presence of a carbonyl group.
When looking at molecules with conjugated systems, like beta-carotene with its system of 11 conjugated double bonds, the energy gap between the Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO) is smaller, causing light in the visible rather than UV region to be absorbed. This relates to the 'color' of compounds, for example, resulting in the characteristic orange color of carrots. Molecules like 4-methyl-3-penten-2-one show strong UV absorbance due to a pi to pi* transition, as well as an absorbance due to the transition of a non-bonding electron on oxygen to a pi* antibonding Molecular Orbital (MO).
In general, molecules that absorb light strongly in the UV-Vis region are known as chromophores. For proteins, this typically involves the aromatic amino acids such as tryptophan, phenylalanine, and tyrosine, which have the ability to absorb UV light due to the presence of chromophores.