Effect of Charge State on Polymeric Bonding Geometry: The Ground State ofNa2RbC60

Theoretical calculations have predicted all fullerene polymers to have interfullerene connections via $[2+2]$ cycloaddition. We find that the ground state of ${\mathrm{Na}}_2{\mathrm{RbC}}_{60}$ is a one-dimensional polymer of fullerene molecules connected by single carbon-carbon bonds. We discuss and exclude possible steric and kinetic reasons for this discrepancy. Our results imply that it is the charge state of the fullerene molecule that drives the bonding mechanism, causing $({\mathrm{C}}_{60}^{1-}{)}_n$ and neutral $({\mathrm{C}}_{60}{)}_n$ to favor cycloaddition and $({\mathrm{C}}_{60}^{3-}{)}_n$ to favor single carbon-carbon bonds.