March 29, 2024, 05:19:48 AM
Forum Rules: Read This Before Posting


Topic: A Mystery in Hexaiodobenzene  (Read 19633 times)

0 Members and 1 Guest are viewing this topic.

Offline Mitch

  • General Chemist
  • Administrator
  • Sr. Member
  • *
  • Posts: 5298
  • Mole Snacks: +376/-3
  • Gender: Male
  • "I bring you peace." -Mr. Burns
    • Chemistry Blog
A Mystery in Hexaiodobenzene
« on: October 07, 2007, 08:00:50 PM »
A mystery for the Chemical Illuminati.

Many years ago, I synthesized hexaiodobenzene as a reagent for some Sonogashira reactions I was exploring. After crystallizing the product out of solution, I began filtering the gorgeous orange-red crystals.  As my attention span is small, I also began refilling my manifold traps with liquid nitrogen. Some of the liquid nitrogen splashed onto the filtering hexaiodobenzene crystals and instantly turned yellow-orange. As the vial heats up it will reversibly return to its red-orange color. A video of the process is shown below: Or click me for direct link.


Color Change In Hexaiodobenzene

I've always wondered what caused the color change, as I'm unfamiliar if this is a common phenomenon. The only two-three reasonable explanations I can think of are:  1.) N2 is complexing into the aromatic ring somehow 2.) Some sort of solid-solid phase transition is occurring 3.) Changing temperatures caused the relative energies of it's Homo-Lumo gap to change by some mechanism.

If you have any thoughts, or thoughtful ideas on what can be done to sort out this mystery lets hear them. Perhaps the Carbon-Based Curiosities will enlighten me.

Mitch
« Last Edit: October 08, 2007, 11:32:33 PM by Mitch »
Most Common Suggestions I Make on the Forums.
1. Start by writing a balanced chemical equation.
2. Don't confuse thermodynamic stability with chemical reactivity.
3. Forum Supports LaTex

Offline enahs

  • 16-92-15-68 32-7-53-92-16
  • Retired Staff
  • Sr. Member
  • *
  • Posts: 2179
  • Mole Snacks: +206/-44
  • Gender: Male
Re: A Mystery in Hexaiodobenzene
« Reply #1 on: October 08, 2007, 02:55:18 PM »
Question:
If you got plenty around, after the color changes back to the red-orange, if you then submerge the vial in the liquid nitrogen to chill is back down without allowing the chemicals to directly contact, does it turn back yellowish?


Offline Mitch

  • General Chemist
  • Administrator
  • Sr. Member
  • *
  • Posts: 5298
  • Mole Snacks: +376/-3
  • Gender: Male
  • "I bring you peace." -Mr. Burns
    • Chemistry Blog
Re: A Mystery in Hexaiodobenzene
« Reply #2 on: October 08, 2007, 03:10:03 PM »
Yes it does. Since Nitrogen is a major component of air, it indirectly ends up being the same experiment.
Most Common Suggestions I Make on the Forums.
1. Start by writing a balanced chemical equation.
2. Don't confuse thermodynamic stability with chemical reactivity.
3. Forum Supports LaTex

Online Borek

  • Mr. pH
  • Administrator
  • Deity Member
  • *
  • Posts: 27634
  • Mole Snacks: +1799/-410
  • Gender: Male
  • I am known to be occasionally wrong.
    • Chembuddy
Re: A Mystery in Hexaiodobenzene
« Reply #3 on: October 08, 2007, 04:03:20 PM »
Yes it does. Since Nitrogen is a major component of air, it indirectly ends up being the same experiment.

Not necesarilly - if you use a stopper. Assuming 1:1 50mL of N2 chilled to 100K can react with 5 grams of hexaiodobenzene.

You can also try to flush nitrogen with argon.
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info

Offline excimer

  • Regular Member
  • ***
  • Posts: 23
  • Mole Snacks: +0/-0
    • Carbon-based curiosities
Re: A Mystery in Hexaiodobenzene
« Reply #4 on: October 08, 2007, 04:39:39 PM »
I'm for #2. I've never heard of an aromatic binding to N2 to the point where it's metastable like that. A good experiment to try would be to simply cool the crystals under vacuum and see if they change color like that. If so, it ain't nitrogen. It is possible that cooling it is changing the electronic structure of the bulk, but that can only happen through a change in crystal morphology. #3 suggests intermolecular charge transfer, but I don't know whether temperature affects CT in the solid state. My guess is yes, but, again, through a change in solid-state morphology. Does this color change happen in solution? If so, definitely not a solid-state issue.

Here's the crystal structure, if that helps. http://dx.doi.org/10.1107/S1600536807002279

Offline enahs

  • 16-92-15-68 32-7-53-92-16
  • Retired Staff
  • Sr. Member
  • *
  • Posts: 2179
  • Mole Snacks: +206/-44
  • Gender: Male
Re: A Mystery in Hexaiodobenzene
« Reply #5 on: October 08, 2007, 10:05:52 PM »
Yes it does. Since Nitrogen is a major component of air, it indirectly ends up being the same experiment.

Well, I mean, it is easy to pull a vacuum on the container or flush it:).

If you have the time and material, try pulling a vacuum or flushing and then chilling and see if the color change occurs.


I have suspicions it has little to do with the nitrogen and just the temperature.


Offline Mitch

  • General Chemist
  • Administrator
  • Sr. Member
  • *
  • Posts: 5298
  • Mole Snacks: +376/-3
  • Gender: Male
  • "I bring you peace." -Mr. Burns
    • Chemistry Blog
Re: A Mystery in Hexaiodobenzene
« Reply #6 on: October 08, 2007, 11:36:23 PM »
It might be easy for you to flush a vial, in this lab it would take several days to set that experiment up. A nuclear chem lab just isn't equipped with any of the right stuff. But, I'll try and set it up this week. Unless someone wants some to play with it too?
« Last Edit: October 09, 2007, 04:45:15 AM by Mitch »
Most Common Suggestions I Make on the Forums.
1. Start by writing a balanced chemical equation.
2. Don't confuse thermodynamic stability with chemical reactivity.
3. Forum Supports LaTex

Offline Noel

  • New Member
  • **
  • Posts: 7
  • Mole Snacks: +0/-0
Re: A Mystery in Hexaiodobenzene
« Reply #7 on: October 09, 2007, 03:24:48 AM »
I should give it a try.

Offline AWK

  • Retired Staff
  • Sr. Member
  • *
  • Posts: 7979
  • Mole Snacks: +555/-93
  • Gender: Male
Re: A Mystery in Hexaiodobenzene
« Reply #8 on: October 09, 2007, 04:03:52 AM »
I think it is a phase transition. A high pressure metallic phase of C6I6 is known.
AWK

Offline Ψ×Ψ

  • Chemist
  • Full Member
  • *
  • Posts: 263
  • Mole Snacks: +8/-4
  • ooh, shiny!
    • carbon-based curiosities
Re: A Mystery in Hexaiodobenzene
« Reply #9 on: October 09, 2007, 11:36:51 PM »
1: Sounds unlikely if not impossible.
2: Plausible.  I'm not exactly sure how changes in the crystal packing would affect the color.
3: Something like that...the higher vibrational levels of the ground state will be less populated as the temperature decreases.  If it's red-orange, it's probably absorbing around, IDK, 520ish to 550ish nm?  ...Yellowish probably corresponds to absorption somewhere more like 400, 450ish nm.  400ish nm corresponds to higher energy light than 500ish nm--what you'd expect if you knocked out the lower-energy transitions from higher vibrational levels.

Of course, I'm not a laser jock, so this amounts to speculation.  Ask a physical chemist--she/he might actually know what's going on.

Offline AWK

  • Retired Staff
  • Sr. Member
  • *
  • Posts: 7979
  • Mole Snacks: +555/-93
  • Gender: Male
Re: A Mystery in Hexaiodobenzene
« Reply #10 on: October 10, 2007, 12:39:58 AM »
2: Plausible.  I'm not exactly sure how changes in the crystal packing would affect the color.
eg; pi-stacking or not
AWK

Offline Ψ×Ψ

  • Chemist
  • Full Member
  • *
  • Posts: 263
  • Mole Snacks: +8/-4
  • ooh, shiny!
    • carbon-based curiosities
Re: A Mystery in Hexaiodobenzene
« Reply #11 on: October 10, 2007, 06:33:12 PM »
That occurred to me when I read the suggestion in the OP, but I have yet to see it in action.  I don't think I've seen vividly different colors in the solid state from very similar functionalized acenes with dramatically different crystal packing (believe me, we make enough of 'em)--you'd notice a difference in the absorption spectrum, sure...but significant enough to account for a color change like the one in the video?  I don't know.

Offline AWK

  • Retired Staff
  • Sr. Member
  • *
  • Posts: 7979
  • Mole Snacks: +555/-93
  • Gender: Male
Re: A Mystery in Hexaiodobenzene
« Reply #12 on: October 11, 2007, 12:58:46 AM »
Many years ago I got yellow and green crystals (different in shape) of a nickel complex during crystallization from water. When I solved their crystal structures I realized the only difference was a mutual orientation and  intermolecular interactions.
Other example comes from organic chemistry - glyoxal (OCH-CHO) is blue as vapor and yellow as a liquid.
AWK

Online Borek

  • Mr. pH
  • Administrator
  • Deity Member
  • *
  • Posts: 27634
  • Mole Snacks: +1799/-410
  • Gender: Male
  • I am known to be occasionally wrong.
    • Chembuddy
Re: A Mystery in Hexaiodobenzene
« Reply #13 on: October 11, 2007, 03:20:40 AM »
Mercury (II) iodide.
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info

Offline Mitch

  • General Chemist
  • Administrator
  • Sr. Member
  • *
  • Posts: 5298
  • Mole Snacks: +376/-3
  • Gender: Male
  • "I bring you peace." -Mr. Burns
    • Chemistry Blog
Re: A Mystery in Hexaiodobenzene
« Reply #14 on: October 11, 2007, 03:58:19 AM »
Topic Locked Pending Release of Something Super Awesome. Stay Tuned To This Blog For a Massive New Project Shortly!
Most Common Suggestions I Make on the Forums.
1. Start by writing a balanced chemical equation.
2. Don't confuse thermodynamic stability with chemical reactivity.
3. Forum Supports LaTex

Sponsored Links