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.