Relative stability of polymerized phases of C60: Depolymerization of a tetragonal phase

Differential scanning calorimetry and IR-spectroscopy have been used to study the depolymerization of the 2D tetragonal (T) polymerized phase of C60 at p = 1 atm. The depolymerization enthalpy obtained was 17.7 ± 1.7 kJ per mole of C60. Experimental enthalpies of depolymerization along with the lattice energies calculated by the atom-atom potential method were combined into the thermochemical cycles to account for the trends in the relative stability of the polymerized phases of C60. Surprisingly low depolymerization enthalpy of 2D rhombohedral (R) phase compared to 1D orthorhombic (O) and 2D T-phases was explained by the unfavorable packing energy of the isolated rhombohedral layers into R-crystal lattice, though the averaged C60 = C60 bond energy was also lower for R- than for O- and T- phases, being 31, 42 and 43 kJ/mol, respectively. Results of DFT quantum chemical calculations of the energetics of C60 polymers were in qualitative agreement with this trend. The mechanism of depolymerization appeared to be significantly different for R-phase compared to other polymers. While decomposition of O- and T-phases occurred in one step without any IR-detectable intermediate species, depolymerization of R-phase was found to be at least a two-step process. Comparison of experimental and DFT simulated IR-spectra suggested that intermediate species were cyclic trimers or similar oligomers.