Summary
Highlights
Different molecules absorb different wavelengths of light. UV/Vis spectrophotometers measure these absorptions, typically from 200 to 800 nanometers. An absorption spectrum shows which wavelengths are most strongly absorbed. For 1,3-Butadiene, the strongest absorption (lambda max) is at 217 nanometers, which is in the UV region, making it colorless.
1,3-Butadiene has four sp2 hybridized carbons, each with a p orbital. These four atomic orbitals combine to form four molecular orbitals: two bonding and two antibonding. Bonding orbitals are lower in energy, and antibonding orbitals are higher. Butadiene has four pi electrons, which occupy the two lowest energy bonding molecular orbitals in its ground state.
When 1,3-Butadiene absorbs light, a pi electron is promoted from the Highest Occupied Molecular Orbital (HOMO) to the Lowest Unoccupied Molecular Orbital (LUMO). This transition requires a specific amount of energy, which corresponds to a particular wavelength of light. The energy of a photon (E) is related to its wavelength (lambda) by E = hc/lambda, meaning energy and wavelength are inversely proportional. The absorption at 217 nm for butadiene corresponds to this HOMO-LUMO energy gap.
Ethanal, with two pi electrons, also undergoes a pi to pi star transition (from a bonding pi orbital to an antibonding pi* orbital). This transition typically corresponds to an absorption wavelength of approximately 180 nanometers, which is often below the detection range of standard UV/Vis spectrophotometers.
Ethanal also has lone pairs on oxygen, which occupy a non-bonding (n) orbital, which is higher in energy than bonding pi orbitals but lower than antibonding pi* orbitals. This allows for an n to pi star transition. This energy difference is smaller than the pi to pi star transition, resulting in the absorption of light at a longer wavelength, approximately 290 nanometers. A smaller energy difference always corresponds to a longer absorbed wavelength.