Final answer:
The statement that in UV-Vis spectroscopy electrons are excited from the LUMO to the HOMO is false. Instead, electrons are excite from the HOMO to the LUMO when UV or visible light is absorbed, correlating to the energy gap between these two orbitals.
Step-by-step explanation:
The statement in UV-Vis spectroscopy that electrons are excited from the LUMO to the HOMO is false. In UV-Vis spectroscopy, which is a technique used to observe the electronic structure of molecules, electrons are excited from a lower energy state to a higher one. This generally means electrons are promoted from the Highest Occupied Molecular Orbital (HOMO) to the Lowest Unoccupied Molecular Orbital (LUMO). An example of an electronic transition is a σ- σ* transition, where an electron in a bonding molecular orbital (σ) is excited to an antibonding molecular orbital (σ*) when the molecule absorbs UV light with the requisite energy, corresponding to the HOMO-LUMO gap.
The absorption of light in the UV or visible range results in an electron transition from the HOMO to the LUMO, and the specific wavelengths of light absorbed are dictated by the energy difference between these orbitals. For instance, 1,3-butadiene absorbs UV light at 217 nm due to its smaller HOMO-LUMO gap compared to isolated pi bonds. Additionally, molecules with extended pi systems, like beta-carotene, absorb light in the visible range and this absorption is responsible for their color; beta-carotene appears orange because it absorbs blue light and transmits red and yellow light.