Did Smalley deliver a killer blow to Drexlerian MNT?

The most high profile opponent of Drexlerian nanotechnology (MNT) is certainly Richard Smalley; he’s a brilliant chemist who commands a great deal of attention because of his Nobel prize, and his polemics are certainly entertainingly written. He has a handy way with a soundbite, too, and his phrases “fat fingers” and sticky fingers” have become a shorthand expression of the scientific case against MNT. On the other hand, as I discussed below in the context of the Betterhumans article, I don’t think that the now-famous exchange between Smalley and Drexler delivered the killer blow against MNT that sceptics were hoping for.

For my part, I am one of those sceptics; I’m convinced that the MNT project as laid out in Nanosystems will be very much more difficult than many of its supporters think, and that other approaches will be more fruitful. The argument for this is covered in my book Soft Machines. But, on the other hand, I’m not convinced that a central part of Smalley’s argument is actually correct. In fact, Smalley‚Äôs line of reasoning if taken to its conclusion would imply not only that MNT was impossible, but that conventional chemistry is impossible too.

The key concept is the idea of an energy hypersurface embedded in a many-dimensional hyperspace, the dimensions corresponding to the degrees of freedom of the participating atoms in the reaction. Smalley argues that this space is so vast that it would be impossible for a robot arm or arms to guide the reaction along the correct path from reactants to products. This seems plausible enough on first sight – until one pauses to ask, what in an ordinary chemical reaction guides the system through this complex space? The fact that ordinary chemistry works – one can put a collection of reactants in a flask, apply some heat, and remove the key products (hopefully this will be your desired product in a respectable yield, with maybe some unwanted products of side-reactions as well) – tells us that in many cases the topography of the hypersurface is actually rather simple. The initial state of the reaction corresponds to a deep free energy minimum, the product of each reaction corresponds to another, similarly deep minimum, and connecting these two wells is a valley; this leads over a saddle-point, like a mountain pass, that defines the transition state. A few side-valleys correspond to the side-reactions. Given this simple topography, the system doesn’t need a guide to find its way through the landscape; it is strongly constrained to take the valley route over the mountain pass, with the probability of it taking an excursion to climb a nearby mountain being negligible. This insight is the fundamental justification of the basic theory of reaction kinetics that every undergraduate chemist learns. Elementary textbooks feature graphs with energy on one axis, and a “reaction coordinate” along the other; the graph shows a low energy starting point, a low energy finishing point, and an energy barrier in between. This plot encapsulates the implicit, and almost always correct, assumption that out of all the myriad of possible paths the system could take through the hyperspace of configuration space the only one that matters is the easy way, along the valley and over the pass.

So if in ordinary chemistry the system can navigate its own way through hyperspace, what’s different in the world of Drexlerian mechanochemistry? Constraining the system by having the reaction take place on a surface and spatially localising one of the reactants will simplify the structure of the hyperspace by reducing the number of degrees of freedom. This makes life easier, not harder – surfaces of any kind generally have a strong tendency to have a catalytic effect – but nonetheless, the same basic considerations apply. Given a sensible starting point and a sensible desired product (i.e. one defined by a free energy minimum) chemistry teaches us that it is quite reasonable to hope for a topographically straightforward path through the energy landscape. As Drexler says, if the pathway isn’t straightforward you need to choose different conditions or different targets. You don’t need an impossible number of fingers to guide the system through configuration space for the same reason that you don’t need fingers in conventional chemistry, the structure of configuration space itself guides the way the system searches it.

This is a technical and rather abstract argument. As always, the real test is experimental. There’s some powerful food for thought in the report on a Royal Society Discussion Meeting “‘Organizing atoms: manipulation of matter on the sub-10 nm scale'” which was published in the June 15 issue of Philosophical Transactions. Perhaps the most impressive example of a chemical reaction induced by physically moving individual reactants into place with an STM is the synthesis of biphenyl from two iodobenzene molecules (Hla et al, PRL 85 2777 (2001)). To use their concluding words “In conclusion, we have demonstrated that by employing the STM tip as an engineering tool on the atomic scale all
steps of a chemical reaction can be induced: Chemical reactants can be prepared, brought together mechanically, and finally welded together chemically. ” Two caveats need to be added: firstly, the work was done at very low temperature (20 K) presumably so the molecules didn’t run around too much as a result of Brownian motion. Secondly, the reaction wasn’t induced simply by putting fragments together into physical proximity; the chemical state of the reactants had to be manipulated by the injection and withdrawal of electrons from the STM tip.

Nonetheless, I rather suspect that this is exactly the sort of reaction that one would say wasn’t possible on the basis of Smalley’s argument.

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