CARbon MAnifolds

Computing Precursors and Recipes for
Rational Synthesis of Fullerenes


Only a small handful of fullerenes have so far been realized experimentally. These are produced at several thousand Kelvin by vaporization of graphite followed by chromotography of the resulting soot, or by combustion of simple hydrocarbons in fuel-rich flames. While this can produce I$_h$-C$_{60}$ in kilogram quantities, it always produces the same few fullerenes, mainly I$_h$-C$_{60}$ and D$_{5h}$-C$_{70}$. There is little hope that specific fullerene structures found to have interesting properties can be produced by such methods. Gaining access to the full space of fullerene isomers will require methods for direct chemical synthesis.

Scott et al. pioneered rational synthesis of fullerenes (and geodesic polyarenes in general), successfully synthesizing I$_h$-C$_{60}$ directly by constructing, in a sequence of high-yield steps, a planar precursor that autoassembled to I$_h$-C$_{60}$ in flash vacuum pyrolysis (Scott et al. 2002, Scott et al. 2004). While yields of the final FVP stage were low (0.1-1%) because of the harsh reaction conditions, Otero et al. devised a highly efficient surface-catalysed cyclodehydrogenation process that improved the autoassembly stage to nearly 100% yield (Otero et al. 2008); and in Kabdulov et al. 2010, the precursor was improved with fluorine as radical promoter, which could be introduced in the most sterically impeded regions at key positions to control the folding stage.

Fig.1: Rational synthesis for $I_h$-C$_{60}$ (Scott 2004). Radical-controlled cage autoassembly through flash vacuum pyrolysis.

The planar precursor excellently matches the unfoldings to the Eisenstein plane. It is in fact a particular such unfolding with extra constraints: it should be symmetric (to allow parallel construction), the symmetric generators should correspond to stable molecules, and it must be able to ``fold up'' in a controlled manner, timed by radical-initiated aryl-aryl coupling reactions. Any success in exploiting this connection to aid in the discovery of precursors and retrosynthesis of arbitrary fullerenes would be groundbreaking. Task R below deals specifically with this.

Fig.2: Rational synthesis precursor for a drum-shaped $D_6$-C$_{120}$ fullerene found using my prototype.

In these tasks, we will exploit the relationship between the unfoldings and planar fullerene precursors. The goal is to build software that, given an individual fullerene isomer, aids in finding plausible precursors for its direct chemical synthesis. Subtasks include:

  1. Generate precursors using maximally symmetric unfoldings with geometric constraints determined by autoassembly requirements.
  2. Geometric analysis of folding and optimal placement of halogens (for low-temperature generation of radical sites and controlling folding reaction sequence).
  3. Systematize building-block reactions.
  4. Compute suggestions for reaction pathways by decomposition to chemically accessible fragments through a selection of synthetically feasible cuts of the carbon skeleton.

A fast but very approximate scheme can be used for screening, and a more exhaustive but slow scheme for aiding retrosynthesis of specific interesting fullerenes.

Scott's successful scheme for rational synthesis of I$_h$-C$_{60}$ was the culmination of decades of work, and there will be a great deal of know-how that must be incorporated into such a scheme.


Task G: Generating Precursor Molecules for Autoassembly

We take as input a bond graph for a particular fullerene isomer, and produce good candidates for planar precursor molecules that will autoassemble into the polyhedral structure via flash vacuum pyrolysis. The main task is to find and codify the constaints to an unfolding that produces a "good" planar precursor. See Fig.3A for an example.

Task U: Understanding and Modeling Fullerene Autoassembly

In order to discover good rules for generating planar precursors that fold up to polyhedral molecules, we must understand the autoassembly process involved. This task investigates the radical-induced cyclodehydrogenation process using standard quantum chemistry software.

Task A: Simulating Fullerene Autoassembly from Planar Precursor

Based on Task U, we will program (much faster) molecular dynamics simulations that can simulate autoassembly and assess fitness of precursor molecule candidates, produced by Task G, and find transition states and estimated required energy input. This then feeds back into Task G as constraints that guide the precursor generation.