A interpretation of stepwise bond dissociation energies of CH4

Composite ab initio methods and density functional theory methods were employed to calculate the stepwise bond dissociation energies of CH4. It is found that the bond dissociation energies, bond lengths and bond angles of CH4, CH3, CH2 and CH calculated by composite G3 method are in good agreement with literature values. The NBO analysis was carried out to explain the differences of stepwise bond dissociation energies of CH4. With the increased number of binding hydrogen atoms to carbon atom, the increasing promotion of 2s electrons (C) to 2p orbitals (C) gives rise to less screening effect, the more electron transfer of 1s electrons (H) to 2p orbital (C) increases electrostatic interaction between carbon atom and hydrogen atoms. The repulsion between C–H bond and C–H bond, C–H bond and unpaired p orbital in CH3 radical may be less than that of C–H bonds in CH4, resulting in shorter bond lengths of CH3 in contrast to bond lengths of CH4, so bond dissociation energy of CH3 is larger than that of CH4. In CH radical, the lone pair electron is localized in 2s orbital, resulting in the strongest screen effect and the longest C–H bond, so the CH bond dissociation energy is the smallest.