(Alpha testing)
(Alpha testing)
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The Outline of the Chemical Question
Briefing: Many years ago, Mitch synthesized hexaiodobenzene as a reagent. After crystallizing the product out of solution, he began filtering the orange-red crystals. As his attention span is small, he also began refilling his manifold traps with liquid nitrogen. Some of the liquid nitrogen splashed onto the filtering hexaiodobenzene crystals and instantly turned yellow-orange. The color then returned to orange-red as the crystals heated back towards room temperature. A video of the process is shown below. Color Change In Hexaiodobenzene Hypotheses: A.) N2 is complexing into the aromatic ring somehow. B.) Some sort of solid-solid phase transition is occurring. C.) Changing temperatures caused the relative energies of it's Homo-Lumo gap to change. D.) Impuity in the crystals is causing something weird. Tasks Needed for Immediate Testing the Hypotheses: A.) Perform same experiment in a none Nitrogen environment.B.) No idea.C.)
We may now require computational models of the two most common pi stacking formations which I am doing as of Nov 21st. D.) Sublime product for higher purity crystals.All.)
Links to relevant papers/files.
Contact If you would like a sample of hexaiodobenzene please send a request to  . |
Comments and Discussion
26.) Daniel: November 21st 2007 10:13:33 AM Hello everyone ! I have added some computational models and insight on the outline. Please tell me what you think about them. Remember that you can always download gabedit or molden to visualize the files I uploaded 25.) Lukas: November 11th 2007 12:07:05 PM I agree with Felix, that totally reminds me of the HgI2 phase transition. I think the pi stacking mode in the C6I6 xtal structure could affect the energy levels significantly... but I have no idea how to corroborate that theory easily. Incidentally, does the color of C6I6 change if only you cool it down to -78C? 24.) Bobby Baum: October 25th 2007 10:38:10 PM Try cooling the hexaiodobenzene slowly. See if the change occurs suddenly at a certain temperature or gradually. A gradual change should argue against B while a sudden change should argue against C. 23.) Kyle Finchsigmate: October 18th 2007 01:23:02 PM Mitch, Take some iodobenzene and place it in an NMR tube and evacuate/fill with argon and cap it. You should be able to submerse it in liquid nitrogen and see the color change. But make sure the you use dry argon... It may also be worth considering that the solid state structure is far better defined at low temperatures. It would be interesting to get a xrd of the crystals at 100K and at 250 or 300K. 22.) Felix: October 17th 2007 02:09:45 AM I can tell you something about the excitations in benzene and Iodobenzene. what I did is compute the excited states at the TDDFT/B3LYP/TZVP level. but maybe you need a multireference method for a system with degenerate HOMO and LUMO? iodobenzenes 3 lowest energy states are: a2g 613.7nm e1g 446.9nm e1u 419.8nm only the last one is symmetry allowed (character table) but even it has a very small oscillator strength. my idea is that with distortions in the crystal some of these states change their oscillator strengths and therefore the color changes. the problem with this interpretation is that the state that should be active in benzene with experimental absorption at 256nm should be the b2u (228,0 nm) but it is symmetry forbidden. the allowed states e1u and a2u are much higher in energy with (174.3nm and 159.0nm) here you go. something to think about 21.) Kyle Finchsigmate: October 15th 2007 07:44:19 PM Well... since the air is already 70% nitrogen, I don't think that's going to be your likely cause - especially since you're now doing the rxn at 77K. You probably froze ambient water to the outside of the crystals. 20.) Mitch: October 15th 2007 04:40:09 PM To Quinny: There is absolutely no way I could use CO at LBL. Would you want some crystals to do the CO study with? Mitch 19.) Quinny: October 15th 2007 12:42:05 PM I'd have to agree that it's most likely due to a difference in solid state packing, but that's a bold statement at this point in time. Anyways, cooling down the C6I6 in the absence of N2, whether it be vacuum, or using a different gas (Ar or maybe CO, being isoelectronic) might shed some light on it. Is it possible to do a low T x-ray while continusously cooling with liq. N2? 18.) Mitch: October 14th 2007 09:44:30 PM I doubt hexaiodobenzene has an ionic component though. Perhaps an impurity might cause the color change? I'll add it to the list of hypotheses. 15.) Felix: October 11th 2007 01:58:30 AM this totally reminds me of HgI2 which turns yellow when you sublimate it. there it is because the red ionic structure becomes a yellow molecular structure. but well, this does not make sense here 12.) AWK: October 10, 2007 09:58:46 PM 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. 11.) ?*?: October 9, 2007 09:39:58 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. 10.) AWK: October 9, 2007 09:39:58 PM eg; pi-stacking or not 9.) ?*?: October 9, 2007 08:36:51 PM A: Sounds unlikely if not impossible. B: Plausible. I'm not exactly sure how changes in the crystal packing would affect the color. C: 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. 8.) AWK: October 9, 2007 01:03:52 AM I think it is a phase transition. A high pressure metallic phase of C6I6 is known. 7.) Noel: October 9, 2007 12:24:48 AM I should give it a try. 6.) Mitch: October 08, 2007 08:36:23 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? 5.) enahs: October 08, 2007 07:05:52 PM 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. 4.) excimer: October 08, 2007 01:39:39 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. Doi Link 3.) Borek: October 08, 2007 01:03:20 PM 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. 2.) Mitch: October 08, 2007 12:10:03 PM Yes it does. Since Nitrogen is a major component of air, it indirectly ends up being the same experiment. 1.) enahs: October 08, 2007 11:55:18 AM 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? |
