Calculated barriers to abstraction and exchange for CH4+H

Abstract

Saddle point geometries and barrier heights have been calculated for abstraction and exchange in CH4+H using POL–CI wave functions with basis sets up to triple zeta valence with double zeta polarization on C and single zeta polarization on H. The saddle point for the abstraction reaction is found to have C3v symmetry (abstraction collinear with a CH bond). The calculated saddle point geometry is closer to products (CH3+H2) than to reactants (CH4+H) and has the CH and HH bonds stretched by 0.38 Å (35%) and 0.18 Å (24%), respectively. The CH bond lengths of the remaining three CH bonds change only very slightly (<0.01 Å) during the reaction, while at the saddle point each methyl CH bond is ∼12.4° out of the plane perpendicular to the C–––H–––H axis and containing the C atom (19.47° for CH4 and 0° for CH3). The calculated barrier height is 15.9 kcal/mole (using the largest basis set). Comparison to comparable calculations for H3 indicates an error of ∼2.4 kcal/mole due to the POL–CI approximation leading to 13.5 kcal/mole as the best estimate of the barrier height. The saddle point for the exchange reaction has a trigonal bipyramidal geometry. The calculated saddle point geometry has rCH=1.41 Å (equal CH bond lengths), while the methyl CH bond lengths change only slightly (<0.01 Å) during the reaction. The calculated barrier height is 36.9 kcal/mole (for the largest basis set). Using the same estimate for the error due to the POL–CI approximation as for the abstraction reaction leads to 34.5 kcal/mole as the best estimate of the barrier height for the exchange reaction.