Calculated barrier to hydrogen atom abstraction from CH4 by O(3P)

Abstract

The barrier height and transition state geometry have been calculated for the reaction CH4+O(3P) →CH3+OH using POL–CI wave functions with a valence double zeta plus polarization basis set. The saddle point geometry is found to be of C3v symmetry with the CH and OH bonds stretched by ∼0.27 Å (25%) and ∼0.21 Å (21%), respectively. The CH bond lengths of the CH3 group change only very slightly (<0.01 Å) during the reaction, while at the transition state each CH bond is ∼13.1° out of the plane perpendicular to the CHO axis and containing the carbon atom (19.5° for CH4 and 0.0° for CH3). The calculated barrier height is 14.4 kcal/mole using a [3s2p1d/2s1p] basis set. From comparison to similiar calculations for O(3P)+H2→OH+H the basis set error in ΔEb is estimated to be ∼2.4 kcal/mole leading to a predicted barrier height of ∼12.0 kcal/mole. Including zero point corrections leads to estimated activation energies of 10.3 kcal/mole (3A′) and 10.1 kcal/mole (3A″) as compared to experimentally derived activation energies of 9.0–11.4 kcal/mole.