A complete set of tools?

We assume that the following tools allow us to build all the stiff hydrocarbons that are needed in a hydrocarbon assembler. This assumption requires further investigation, but appears plausible at the present time. Our objective is therefore to synthesize and refresh the following molecular tools from butadiyne:

• The hydrogen abstraction tool:

  

• Two carbon, a silicon and a tin radical:

  

• Carbenes:

  

• The dimer deposition tool:

  

• Positionally controlled transition metal(s):

  

• The hydrogen deposition tool:

  

The hydrogen abstraction tool has been investigated using high order ab initio quantum chemistry by (Musgrave et al., 1991); and with molecular dynamics by (Sinnott et al., 1994) and (Brenner et al. 1996) using a potential energy function (Brenner, 1990) able to accurately model a wide range of hydrocarbon structures including transition states (e.g., bond formation and bond breaking). All support the conclusion that this tool should be able to abstract hydrogen from a wide range of hydrocarbons. The molecular dynamics simulations performed at 300 K support the idea that reliable room temperature operation should be feasible.

The elevation of the ethynyl radical to the status of "the" hydrogen abstraction tool is based on two main factors. First, it has a higher affinity than almost any other structure for hydrogen. While the H-F bond is stronger than the H-C bond in H-C#C-H, it is unclear how to hold and position a fluorine atom while retaining its high hydrogen affinity. Second, the ethynyl radical has desirable steric properties: it is unencumbered by bulky side groups and and can more easily abstract hydrogens from somewhat less accessible locations.

Many other radicals exist. The group IV radicals -- carbon, silicon, germanium, tin and lead -- seem particularly useful as they can be bonded to three supporting atoms in their radical state. This provides high stiffness and permits relatively large forces to be applied if desired. Group IV radicals also permit selection of radical properties -- the ethynyl radical is the strongest, the phenyl radical -- C₆H₅ -- is next, followed by the sp³ carbon radical; and then the silicon, germanium, tin and lead radicals in order. Radical properties can be fine tuned by modifying the supporting structures.

Many of the following reactions use one or more radicals. It is often unclear which radical would best serve a particular function. As a consequence, the specific radicals illustrated should be viewed as suggestions: substitution of alternative radicals, particularly other group IV radicals, might well prove advantageous.

Carbenes have proven remarkably useful in the synthesis of organic compounds. Positionally controlled carbenes should be equally if not more useful.

The ability to add two carbon atoms in a single operation using a dimer deposition tool provides more options in adding carbon to a growing structure. (A "dimer" is simply two of something joined together -- in this context, the "dimer" refers to two carbon atoms that are joined together by a triple bond: -C#C- ). While the addition of larger molecular fragments (e.g., polyyne strands, small segments of graphite, etc) will likely prove desirable in many circumstances, the short length of the present paper imposes limits on what we can consider. Future proposals will no doubt consider the advantages of adding larger moieties during the synthesis of hydrocarbons.

The remarkable utility of transition metal catalysts in chemical synthesis rather strongly suggests that positionally controlled transition metals can play a useful role in the synthesis of hydrocarbons. While the present paper uses only a single transition metal it seems likely that more than one will prove useful, particularly as we consider catalyzing a wider range of reactions than the specific ones considered here.

No single proposal is at present accepted as archetypal for the hydrogen deposition tool. Tin is a plausible candidate, as it forms a weak bond to hydrogen. Lead has an even weaker bond to hydrogen and so might prove superior.

We assume that synthesis takes place in an inert environment (vacuum or a noble gas) and that positional control is used throughout. (As the tools are often highly reactive, positional control is essential to prevent undesired reactions). These assumptions permit the use of novel and relatively simple reaction pathways. While the ability to achieve an inert environment using present methods might lead to an attractive implementation pathway, the primary point of the present discussion is to establish that, given a suitable environment, a relatively simple set of reactions and a relatively simple set of molecular tools should be sufficient to let us make the class of stiff hydrocarbons. This class of materials, with a few additions, should be sufficient to let us build some simple assemblers. This class would also let us build environments that would be inert, and in which reactive tools could be deployed with low error rates.