In the previous post, I wrote about the processes that might be involved in a molecular wheel rotating. A nano car has four wheels, and surely the most amazing thing is how the wheels manage to move in synchrony. This is one hell of a tough problem, and I do not attempt an answer here, but simply record an odd observation.
To rotate the wheel, the axle has to rotate, and this can be made to happen by electronically exciting it to decouple the two π-electrons. To study the excited state computationally requires a technique known as CASSCF, and the molecule above is far too big for such a method. But a crude approximation would be to decouple the spins of the two π-electrons by computing the triplet state. One can in fact decouple progressively by computing respectively the triplet, quintet, septet and nonet states. To ensure equality, these calculations were all started from the optimised singlet geometry, and allowed to evolve. One might imagine for example that the triplet state might end up delocalized over the entire molecule, weakening all four axles at the same time, although of course for this to happen, the nitrogens would have to participate.
The animation below shows the progress of the septet state. Watch the four axles individually, and notice how from the very start, three of them start to rotate, whilst the fourth (bottom left) stays rock steady!The really interesting one is the quintet state, where the green and magenta axles rotate, and the red and blue ones do not. The triplet state reserves rotation for just one (red) axle, but the connecting chassis of the car also buckles. The last state, the nonet, shows all four wheels rotated. The relative energies of the five states are 0.0, 32.6, 60.0, 107.7 and 125.3 kcal/mol. It seems there is not much electronic communication between the axles of the four wheels, possibly because to do so, they would have to disrupt the aromaticity of the intervening “Clar” benzo group adjacent to each axle.
Which makes the ability of this nano car to coordinate the motions of its wheels when travelling even more interesting!