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Mechanosynthesis in chemistry is any chemical synthesis that takes place by mechanical friction alone.
In conventional chemical synthesis or chemosynthesis, reactive molecules encounter one another through random thermal motion in a liquid or vapor. In a hypothesized process of mechanosynthesis, reactive molecules would be attached to molecular mechanical systems, and their encounters would result from mechanical motions bringing them together in planned sequences, positions, and orientations. It is envisioned that mechanosynthesis would avoid unwanted reactions by keeping potential reactants apart, and would strongly favor desired reactions by holding reactants together in optimal orientations for many molecular vibration cycles. Mechanosynthetic systems would be designed to resemble some biological mechanisms.
While the description of mechanosynthesis given above has not yet been achieved, primitive mechanochemistry has been performed at cryogenic temperatures using scanning tunneling microscopes). So far, such devices provide the closest approach to fabrication tools for molecular engineering.
Broader exploitation of mechanosynthesis awaits more advanced technology for constructing molecular machine systems - including a molecular assembler or precursors thereof.
It has been suggested, notably by K. Eric Drexler, that mechanosynthesis will be fundamental to molecular manufacturing based on nanofactories capable of building macroscopic objects with atomic precision. The potential for these has been disputed, notably by Nobel Laurate Richard Smalley, leading to a famous dispute between the two of them - see nanotechnology.
In part to resolve this and related questions about the dangers of industrial accidents and runaway events equivalent to Chernobyl and Bhopal, and the more remote issue of ecophagy, grey goo and green goo (various potential disasters arising from runaway replicators, which could be built using mechanosynthesis) the UK Royal Society and UK Royal Academy of Engineering in 2003 commissioned a study to deal with these issues and larger social and ecological implications, led by mechanical engineering professor Ann Dowling. This was anticipated by some to take a strong position on these problems and potentials - and suggest any development path to a general theory of so-called mechanosynthesis.
However, the Royal Society's nanotech report did not address molecular manufacturing at all, except to dismiss it along with gray goo.
There is some peer-reviewed theoretical work on synthesizing diamond by mechanically depositing carbon atoms (a process known as diamond mechanosynthesis or DMS).     The most recent paper in this continuing research effort by Freitas, Merkle and their collaborators reports that the most-studied mechanosynthesis tooltip motif (DCB6Ge) successfully places a C2 carbon dimer on a C(110) diamond surface at both 300K (room temperature) and 80K (liquid nitrogen temperature), and that the silicon variant (DCB6Si) also works at 80K but not at 300K. These tooltips are intended to be used only in carefully controlled environments (e.g., vacuum). Maximum acceptable limits for tooltip translational and rotational misplacement errors are reported in paper III -- tooltips must be positioned with great accuracy to avoid bonding the dimer incorrectly. Over 100,000 CPU hours were invested in this latest study. The DCB6 tooltip motif, initially described at a Foresight Conference in 2002, was the first complete tooltip ever proposed for diamond mechanosynthesis and remains the only tooltip motif that has been successfully simulated for its intended function on a full 200-atom diamond surface.
Further research  to consider alternate tips will require time-consuming computational chemistry and difficult laboratory work.
- Bibliography by Robert Freitas
- ^ Theoretical Analysis of a Carbon-Carbon Dimer Placement Tool for Diamond Mechanosynthesis. Merkle and Freitas, J. Nanosci. Nanotech. 2003, Vol. 3, No. 03.
- ^ Theoretical Analysis of Diamond Mechanosynthesis. Part I. Stability of C2 Mediated Growth of Nanocrystalline Diamond C(110) Surface. Peng, Freitas and Merkle. J. Comput. Theor. Nanosci. Vol. 1, No. 1 2004.
- ^ Theoretical Analysis of Diamond Mechanosynthesis. Part II. C2 Mediated Growth of Diamond C(110) Surface via Si/Ge-Triadamantane Dimer Placement Tools. Mann, Peng, Freitas and Merkle. J. Comput. Theor. Nanosci. Vol. 1, No. 1 2004.
- ^ Theoretical Analysis of Diamond Mechanosynthesis. Part III. Positional C2 Deposition on Diamond C(110) Surface using Si/Ge/Sn-based Dimer Placement Tools. Peng, Freitas, Merkle, Von Ehr, Randall and Skidmore. J. Comput. Theor. Nanosci. Vol. 3, pages 28-41, 2006.
- ^ www.foresight.org Link