Solid state debromination of C60Br24 back to C60 with cesium iodide: kinetics, thermodynamics and mechanism

Abstract

The bromofullerene C60Br24 undergoes a complete and quantitative debromination to C60 when it is mixed and ground in the solid state with cesium iodide (CsI). The kinetics of this unique solid state debromination reaction was studied with FT-IR spectroscopy on C60Br24 embedded in CsI pellet. The debromination rate constant was measured k = 8.4 x 10−4 s−1 and found independent from the C60Br24 concentration. Chemical thermodynamics calculations show that the C60Br24 debromination in CsI matrix is characterized by a largely favorable free energy of reaction (ΔGr) and the reaction is exothermal. A debromination mechanism is discussed in terms of concerted elimination of the bromide anions and formation of carbocations on the fullerene cage in agreement with the E1 type elimination reaction mechanism. The carbocations sites are reduced to radicals and stabilized as trivinylmethyl radicals (the reduction occurs by the action of iodide ions of CsI which are oxidized to molecular iodine) and then a rearrangement of the double bonds leads back to C60 quantitatively.