Delivered by Ingenta to: Guest User IP : 70.231.169.111 Tue, 04 Apr 2006 20:25:38 RESEARCH ARTICLE Copyright © 2005 American Scientific Publishers All rights reserved Printed in the United States of America Journal of Computational and Theoretical Nanoscience Vol. 2, 45–55, 2005 Design and Analysis of a Molecular Tool for Carbon Transfer in Mechanosynthesis Damian G. Allis 1 and K. Eric Drexler 2 * 1 Syracuse University, Center for Science and Technology, Syracuse, NY 13224, USA 2 Foresight Institute, 1455 Adams Drive, Menlo Park, CA 94025, USA Mechanosynthesis of a target class of graphene-, nanotube-, and diamond-like structures will require molecular tools capable of transferring carbon moieties to structures that have binding ener- gies in the range of 1.105 to 1.181 aJ per atom (159 to 170 kcal mol -1 ). Desirable properties for tools include exoergic transfer of moieties to these structures; good geometrical exposure of moieties; and structural, electronic, and positional stability. We introduce a novel carbon-transfer tool design (named by us “DC10c”), the first predicted to exhibit these properties in combination. The DC10c tool is a stiff hydrocarbon structure that binds carbon dimers through strained  -bonds. On dimer removal, diradical generation at the dimer-binding sites is avoided by means of -delocalization across the binding face of the empty form, creating a strained aromatic ring. Transfer of carbon dimers to each of the structures in the target class is exoergic by a mean energy >0.261 aJ per dimer (>38 kcal mol -1 ); this is compatible with transfer-failure rates of ∼10 -24 per operation at 300 K. We present a B3LYP/6-31G(d,p) study of the geometry and energetics of DC10c, together with discussion of its anticipated reliability in mechanosynthetic applications. Keywords: Quantum Chemistry, Mechanosynthesis, Graphene, Graphite, Diamond, Nanotube, Productive Nanosystems, Molecular Manufacturing, Nanotechnology. 1. INTRODUCTION Mechanosynthesis exploits mechanical positioning to direct reactive moieties to specific reactive sites on target structures. This mechanism of control contrasts with that of conventional synthesis techniques, in which solution- phase diffusion produces undirected molecular encounters. Despite this lack of direct positional control, diffusion- based synthesis techniques can achieve considerable site specificity by seeking reaction sequences in which each distinct reactive site, at each step, differs from the rest in its reactivity. This strategy for structural control becomes more difficult as structures grow larger and more com- plex, due to the proliferation of similar reactive sites. Mechanosynthetic techniques, in contrast, can perform different synthetic operations on target sites of similar reactivity that are distinguished solely by their structural position. This means of control is essentially indepen- dent of product scale and complexity and can be quite * Author to whom correspondence should be addressed. reliable. Diffusion-based synthesis techniques have been under development for more than a century and have achieved striking results. Mechanosynthetic techniques are rudimentary today, but their further development promises to greatly expand the scale, diversity, and complexity of products made by structurally precise molecular synthesis. The following discussion addresses operations suitable for an advanced class of mechanosynthetic systems that work in a “machine phase” characterized by strict con- straints on the motions and encounter geometries of all reactive moieties involved. This entails rigorous exclu- sion of unconstrained molecules. In this regard, the mac- hine phase resembles the “inner phase” of molecular containers; 1 they are similar in their ability to stabilize a range of structures that would otherwise essentially behave as short-lived reactive intermediates. The dynam- ical behavior of molecules in a machine-phase system is qualitatively different from that of the immobile or dif- fusing molecules found in solid-, liquid-, and gas-phase systems, or at their interfaces. (Mechanosynthesis is, of course, not synonymous with advanced machine-phase J. Comput. Theor. Nanosci. 2005, Vol. 2, No. 1 1546-198X/2005/2/045/011/$17.00+.25 doi:10.1166/jctn.2005.003 45