Calculated barriers to abstraction and exchange for CH/sub 4/+H
Saddle point geometries and barrier heights have been calculated for abstraction and exchange in CH/sub 4/+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 C/sub 3v/ symmetry (abstraction collinear with a CH bond). The calculated saddle point geometry is closer to products (CH/sub 3/+H/sub 2/) than to reactants (CH/sub 4/+H) and has the CH and HH bonds stretched by 0.38 A (35%) and 0.18 A (24%), respectively. The CH bond lengths of the remaining three CH bonds change only very slightly (<0.01 A) during the reaction, while at the saddle point each methyl CH bond is approx.12.4/sup 0/ out of the plane perpendicular to the C------H------H axis and containing the C atom (19.47/sup 0/ for CH/sub 4/ and 0/sup 0/ for CH/sub 3/). The calculated barrier height is 15.9 kcal/mole (using the largest basis set). Comparison to comparable calculations for H/sub 3/ indicates an error of approx.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 r/sub CH/=1.41 A (equal CH bond lengths), while the methyl CH bond lengths change only slightly (<0.01 A) 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.
- Research Organization:
- Theoretical Chemistry Group, Chemistry Division, Argonne National Laboratory, Argonne, Illinois 60439
- DOE Contract Number:
- W-31-109-ENG-38
- OSTI ID:
- 5486743
- Journal Information:
- J. Chem. Phys.; (United States), Vol. 72:9
- Country of Publication:
- United States
- Language:
- English
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