Fascination with nano-objects, molecules which resemble every day devices, is increasing. Thus the world’s smallest car has just been built[1]. The mechanics of such a device can often be understood in terms of chemical concepts taught to most students. So I thought I would have a go at this one!
The car comprises a single (relatively small) molecule, shown above as the authors represented it. The motion along a surface comprised of copper atoms is driven by light as fuel coupled with encouragement from an STM probe. The distance travelled in a straight line was about 6nm in ten steps (note the nanodistance), although the average speed for the complete journey is not recorded. It is probably safe to say it was not recorded using a speed camera!
The chemistry is shown below. The car has four wheels (the fluorene units) which rotate about an C=C double bond axle using light as the fuel (a configurational change). The component labelled helix inversion can also be described by the chemical name atropisomerism, a topic I dealt with earlier with the example of Taxol and which is a conformational change.
These two processes are used to rotate the wheels in the sequence shown below (after which the wheels return to their starting point).
I set out to build the car by optimising the 3D geometry of the molecule. This so that I could view the device from any direction (not just the one represented in the diagrams above). I also felt it important to estimate the change in energy of the car as the wheels rolled (something not touched upon in the original article). A good place to start would be to raid the supplementary information associated with the article. This comprises a PDF document and four movies. As it happens, none of these contain 3D coordinates for the molecule. Well, in truth this is not unusual, and I am used to such absence by now. Ah well, I would start from the top diagram, which is a schematic 2D representation of the molecule. As you can read in this post, such representations can often be illusory, or even contradictory. One is indeed lucky if they are free of ambiguity. Thus:
Well, it is possible to build a 3D model armed with these instructions (although it has to be done visually, with constant comparisons with the space fill representations in the article).
Postscript: The optimised ωB97XD/6-31G(d) geometries for the two poses of the car are to be found at 10042/to-10227 and 10042/to-10219 The total energy difference is 15.5 kcal/mol (compared with 8 at the PM6 level).
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I remember reading this in Chemistry World.
Thanks for the extra explanations, I'll be checking out the .fchk file in Gaussian if I have time and computing power.
Ben Feringa, the principle author of the article above, is one of the recipients of the 2016 Nobel prize for chemistry. See http://www.kva.se/en/pressroom/2016/the-nobel-prize-in-chemistry-2016/