Meditations, and a Paper

So I taught my first organic workshop today…it was a little weird, but things seemed to work out in the end. The first problem set was on Lewis structures, which I’ll always believe are somewhat arbitrary. It’s like, after you figure out the sigma bonds in something like, say, ONNO2, how do you know where to throw the rest of the valence electrons without bringing in MO theory and a bunch of extra crap? How do you make it feel non-arbitrary without drowning the kids in theory? I figured out about halfway through the class to just shut my trap and let them do their own thing on the problem set. Things seemed to end up well.

Anyway, all of that aside, I found an interesting paper today in Chemistry of Materials. Vaidyanathan et al. look at the field-effect mobilities of substituted [5,5′]-bisphenyl-[2,2′]-bithiophenes (PTTPs…phenyl, thiophene, thiophene, phenyl) as a function of their solubility in THF. Interestingly, increased solubility corresponds to decreased mobility, with also correlates with substituent size. The authors speculate (and support, with SAXS data) that the mobility decrease is due to structural disorder caused by steric hindrance. Sayonara, pi-stacking, adios mobility.

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9 Comments

  1. You’re teaching workshop? Fun. I did it last year, but didn’t get a section this time. (If you want them, I was ambitious enough to make a few handouts for 295. Also, if you ever need someone to cover your section, find me…)
    Thanks for pointing out the paper. I’ll check it out whenever I have time.

    Reply

  2. I suspect that silylethynyl pentacenes might deal with entropy by the disorder of the alkyl groups on the silicon atoms. So, by holding these out to the side, you get somewhere to ‘put entropy’ while allowing really good pi stacking. Counter-entropic stacking forces are not very big in organic crystals. I think it makes sense to make molecules with places to put entropy that won’t disrupt the stacking too much.

    In the examples in the paper, I think that the entropic tendencies/solubilities are too coupled with pi stacking for good solubility and transport properties to peacefully coexist.

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  3. “The first problem set was on Lewis structures, which I’ll always believe are somewhat arbitrary. … How do you make it feel non-arbitrary without drowning the kids in theory?”

    All science education is progressive de-lying. You can’t tell them everything at once, as it would be unmanageable. Certainly you can see this looking back on your own education.

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  4. All science education is progressive de-lying.

    Yeah, this is true. I just feel like there should be another way. Starting with quantum chemistry, for example, and building from there feels more solid than starting with seemingly random rules and filling in the “whys” later. I have heard of some schools starting freshmen with quantum chemistry, and I wish UK would follow suit, but what do I know?

    Ψ*Ψ, if I run into trouble I’ll let you know but for now…don’t delete the handouts :-).

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  5. All science education is progressive de-lying.

    I remember when I first understood group theory enough to recognize that all the orbitals in methane couldn’t be equivalent, and my simple valence-bond world was thus rendered void. I felt sort of dirty. Like finding out my favorite uncle was a pervert or something.

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  6. Quantum for freshmen sounds…rough. Thermo for freshmen would be much easier, but then again it’s definitely not the most interesting class.* I’d settle for gen chem & organic sections just for majors, though.

    *i hate to admit this but i kinda liked it except for all the homework (pleasedon’tkillme)

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  7. “Quantum for Freshmen”: UK’s route to the top 20! 🙂

    I didn’t like thermo myself, but it’s one of those things that comes into play at random moments in other classes and I say to myself, “should’ve paid more attention in thermo.” I don’t blame you. After all, I love group theory but people complain about that all the time.

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  8. I loved thermo. I think that it is a subject that a lot of chemists would warm up to (sorry) if they didn’t spend so damned much time teaching about dumb model systems that are meaningless to us (I haven’t done many gas phase reactions). They could spare some of the mathematics at first to get better models implanted in students- then the math would be motivated by something one could imagine.

    And I think the tradition of teaching thermo without appealing to molecular arguments is fucking crazy. I could give a crap that Carnot developed thermo without any idea of what was going on microscopically. We know now, and it is retarded to ignore that.

    Reply

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