As shown in Figure , the energy difference between exo-TCD-chair and exo-TCD-boat is 1.83 kcal/mol, the exo-chair isomer is more stable than the boat isomer, in agreement with previous studies . Such the energy difference also agrees well with 2.1 kcal/mol in using M06-2x/6-31++G(d,p) model, 2.06 kcal/mol using the M06-2X/6-311+G(2df,2p) model , and 3.81 kcal/mol using B3LYP/6-31G(d,p) . Note that the present study uses the B3LYP/cc-pVTZ model in the gas phase without taking into consideration of the zeropoint vibrational energy. Table compares the optimized structure of the exo-TCD-chair in the present study with literature values given by . As can be seen, the C-C bond lengths and dihedral angles are essentially identical. *There is not traceable information about how these parameters are obtained. The authors believe that the negative sign of dihedral angles ∠3-1-8-9 and ∠1-8-10-9 are misplaced. The atom labels in have been converted to the present labels.