AB-INITIO PREDICTION OF THE ACTIVATION-ENERGY FOR THE ABSTRACTION OF A HYDROGEN-ATOM FROM METHANE BY CHLORINE ATOM

The geometry for the transition state for the reaction Cl. + CH4 --> CH3. + HCl has been predicted by using high-level ab initio molecular orbital theory at the MP2 level with a large basis set (TZ+2P). The classical barrier height was calculated at the QCISD(T) and CCSD(T) levels with a modified correlation consistent basis set. The classical barrier height is 8.90 kcal/mol, and the zero-point correction lowers the barrier height to 4.87 kcal/mol at the QCISD(T) level; the CCSD(T) results are within 0.1 kcal/mol of the QCISD(T) results. Transition state theory was used to predict activation energies and rate constants for comparison to experiment. The calculated activation energy is too high by about 1-1.3 kcal/mol. The calculated rates are too low as compared to the experimental rates at low temperature, but reasonable agreement is found for T > 300 K. Calculations at the density functional theory level were also performed with three differene nonlocal (gradient corrected) potentials. Only with the B3LYP potential could a transition state be located. Although the enthalpy of the reaction could be calculated reasonably accurately at the B3LYP level, the barrier height of 4.36 kcal/ mol (corrected for zero-point energy differences Delta E(a)(0) = 0.44 kcal/mol) is much too low.