The total energies of these hydrocarbons above NBA (gas phase at 0 K) are given by 33.75 eV, 36.23 eV, 67.75 eV and 68.75 eV for NBN, PPL, NBD and QC, accordingly, in agreement with 33.09 eV, 35.56 eV, 66.42 eV and 67.41 eV using G4 calculations of Rablen [37]. The energy difference between NBD and QC isomers is approximately 96.49 kJ/mol (~ 1 eV in gas phase at 0 K), in good agreement with the measured enthalpy of the same isomerisation of 92.05±1.18 kJ/mol using closed-bomb reaction calorimeter. The energy difference between NBN and PPL is calculated as 239.28 kJ/mol in the present study.