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| | From:  MikeKL5 (Original Message) | Sent: 5/15/2005 2:31 AM |
Hey Steve, How have you been? The semester just ended (thank God!), and I can finally put all of my attention into my reserach, instead of having it spread out over research and classes. I attended a seminar at GSU yesterday about liquid crystal chemistry. During the seminar the presenter (who is developing liquid crystal sensors for nerve agents) was proving some of his structures using IR spec, and two professors kept asking him why they weren't seeing back-bonding in the spec. What is back-bonding? What significance does that have in terms of IR spec? I have one more question about something that I heard my professor mention the other day. He was saying that the DNA double-helix is mainly stabilized by pi stacking. This is another term that I've never heard of before. Could you tell me what it is? I really appreciate your help. Thanks in advance, and take care. MikeKL5 P.S. You had mentioned before that you were interested in seeing my research once I got it online. It is now up and running. My website is: kanyschem.info If you go there, and click on the purple Research book (you'll see what I mean when you get there), you can see all of my research notes for both my PET tracer project, and the plant vaccinogen project. Might be an interesting diversion for you. :-) |
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| | | From: ·Steve· | Sent: 5/17/2005 6:33 PM |
Hi Mike, sorry I didn't get back to you sooner, I've been caught up in end-of-semester madness for the last few days. By now you've probably got a good idea what "back bonding" and "pi stacking" are, but I included a few links anyway. I'm only familiar with the back bonding concept in transition metal complexes, particularly those that have carbonyl ligands (CO molecules). Carbon monoxide molecules can complex with transition metals by donation of the lone pair on the carbon to the metal. However, it is observed in the infrared spectrum that the CO stretch is significantly lowered after complexation, indicating a weaker C-O bond. This is explained by electron donation from filled d-orbitals on the transition metal "back" to an empty pi antibonding molecular orbital on the CO molecule. This adds another bonding component to the metal to ligand interaction, while lowering the C-O bond order (recall the calculation for bond order, which decreases when more electrons occupy antibonding MOs).
I'm not familiar with the details of pi stacking, but I assume that it arises mainly from London interactions between the aromatic groups. This is the main type of attractive interaction that occurs between nonpolar molecules.
I started to browse your web site - very nice! I got hooked on the list of quotations on the main page.
Have a good, research-filled summer, Mike!
Steve
Back Bonding
http://dlottgroup.scs.uiuc.edu/group/Mbecho/LANLmb/sld011.htmCO bonding to Fe ion in a porphyrin ring: Carbonyl lone pair on carbon donates to Fe ion via empty Fe dz2 orbital. Meanwhile, occupied "dp" orbitals of iron donate electron density "back" to empty p* molecular orbitals of the CO molecule. The result is a lowering of bond order in the carbonyl ligand, evidenced by a lowering of stretching frequency in the IR spectrum.
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| | | From: ·Steve· | Sent: 5/17/2005 9:48 PM |
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There are also links to similar examples involving carbonyl and phosphine ligands. Enjoy!
Steve
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| | | From: MikeKL5 | Sent: 5/18/2005 11:59 PM |
Hey Steve, You explanation of back-bonding makes perfect sense in the context where I heard the term. The seminar, as I think I said before, was concerning using liquid crystals as sensors for nerve gases. What they would do is to take a gold film, and fix an ~8 carbon long alkyl chain to the gold via a sulfur bridge. The other end of the alkyl chain had carboxyl groups. This film was treated with Cu2+ ion, which was chelated by the carboxyl groups. On top of this was places a film of liquid crystal (the ovals in the picture I attached to this posting). When organophosphates bind to the copper ions, the cause the orientation of the liquid crystal molecules to change from vertical (like in the picture) to horizontal. When the LC molecules are vertical, they appear black. When horizontal, they scatter light, and look like (in my opinion) a solid version of they way oil on water scatters light. The way the presenter (an asian gentleman from University of Wisconsin - Madison, I don't remember his name) proved that the organophosphate compound was binding to the copper ion, and the way he proved the copper ion was binding to the carboxyl groups of the alkyl chains by showing IR specs taken before and after the addition of copper, or organophosphate. However, these IR specs did not show any back-bonding, and that is what threw the professors into a ruckus of question asking. We undergrads, being ignorant of the phenomena, tried to be quiet and unobtrusive. (lol.) Both the back-bonding and pi-stacking explanations make sense. I will check out the websites you recommended. Thank you for helping me sort this out. I really appreciate it. I'm glad you like the quotes on my website. Not many people look at it, so it's always nice to hear stuff back about it. I showed two fellow researchers in my lab my back-up lab notebook today. One was shocked, and impressed, and the other asked me if I was neurotic. lol. I pointed out to them that I, like everyone else in the lab, has all my research and notes in one notebook. If something ever happened to that notebook, we'd be so far up the creek that we'd be walking on dry land. It just makes sense to back it up on computer. Take it easy, bud, and thanks again for your help. MikeKL5 |
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