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UV-Vis Spectroscopy of Conjugated Alkenes quiz

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  • What happens to an electron when UV or visible light energy is applied to a molecule?
    The electron is promoted from a lower energy molecular orbital (HOMO) to a higher energy molecular orbital (LUMO).
  • What does HOMO stand for in molecular orbital theory?
    HOMO stands for Highest Occupied Molecular Orbital, which is the highest energy orbital containing electrons.
  • What does LUMO stand for in molecular orbital theory?
    LUMO stands for Lowest Unoccupied Molecular Orbital, which is the lowest energy orbital without electrons.
  • How does UV radiation affect the molecular orbitals in 1,3-butadiene?
    UV radiation promotes an electron from the HOMO to the LUMO, changing which orbitals are occupied and unoccupied.
  • What is lambda max in UV-Vis spectroscopy?
    Lambda max is the wavelength at which maximum absorption occurs during electronic transitions.
  • How does the energy gap between HOMO and LUMO change with increased conjugation?
    The energy gap decreases as the number of conjugated double bonds increases.
  • What is the relationship between energy and wavelength in UV-Vis spectroscopy?
    Energy and wavelength are inversely related; as energy decreases, wavelength increases.
  • What is the lambda max for 1,3-butadiene?
    The lambda max for 1,3-butadiene is 217 nanometers.
  • What is the lambda max for 1,3,5-hexatriene?
    The lambda max for 1,3,5-hexatriene is 258 nanometers.
  • Why does 1,3,5-hexatriene have a higher lambda max than 1,3-butadiene?
    Because 1,3,5-hexatriene has more conjugation, which lowers the energy gap and increases the lambda max.
  • What type of electronic transition is observed in UV-Vis spectroscopy of conjugated alkenes?
    A pi to pi* (pi star) electronic transition is observed.
  • How does increasing conjugation affect the absorption spectrum of a molecule?
    Increasing conjugation shifts absorption to longer wavelengths, potentially moving from the UV to the visible spectrum.
  • What causes visible colors in many plants according to UV-Vis spectroscopy?
    Highly conjugated compounds in plants absorb visible light, resulting in observable colors.
  • What equation relates energy and wavelength in UV-Vis spectroscopy?
    Energy equals Planck's constant times the speed of light divided by wavelength (E = h*c/λ).
  • What happens to lambda max as the energy gap between HOMO and LUMO decreases?
    Lambda max increases as the energy gap decreases.