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My teacher told me that the HOMO–LUMO gap in solid iodine is smaller than that in gaseous iodine.

When I asked about the reason, he told me that it is due to higher interactions in solid iodine as compared to gaseous iodine, which I did not understand. How can the physical state of a compound affect the HOMO–LUMO gap, even though the chemical formula is the same?

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    $\begingroup$ Chemical formula of solid Na and gaseous Na is also the same: Na. But I suppose you aren't surprised that their HOMO/LUMO stuff is quite different. $\endgroup$ – Ivan Neretin Sep 6 '17 at 8:46
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I will broadly mention several mechanisms that may influence color of a solid vs. gas, not just the HOMO-LUMO transition. Without going into the literature and/or performing (extensive) calculations, I cannot confirm or reject your teacher explanation.

  • In the solid state, you may find intermolecular MO interactions that will affect orbital energies. Depending on the strength, you may even have to abandon the concept of molecular orbitals in favor of crystal orbitals. Quantum dots and their dependence of color on size are somewhat of an example.
  • Molecules may alter their shape in solid state compared to gaseous state. This is less of an issue in $\ce{I2}$.
  • In the solid state, intermolecular excitations are more likely (i.e. going beyond HOMO-LUMO transitions). This is especially true if you do not have a pure substance, think doped solids.
  • Note that supermicroscopic spatial distribution of a substance will have influence on the color as well: finely distributed silver or mercury (as in calomel) may appear as black rather than shiny.
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  • $\begingroup$ What are quantum dots ? $\endgroup$ – P Sriram Goutam Sep 7 '17 at 3:04

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