Calculated barrier to hydrogen atom abstraction from CH4by Olpar;3Prpar;

摘要

The barrier height and transition state geometry have been calculated for the reaction CH4+Olpar;3Prpar; rarr;CH3+OH using POLndash;CI wave functions with a valence double zeta plus polarization basis set. The saddle point geometry is found to be ofC3vsymmetry with the CH and OH bonds stretched by sim;0.27 Aring; lpar;25percent;rpar; and sim;0.21 Aring; lpar;21percent;rpar;, respectively. The CH bond lengths of the CH3group change only very slightly lpar;0.01 Aring;rpar; during the reaction, while at the transition state each CH bond is sim;13.1deg; out of the plane perpendicular to the CHO axis and containing the carbon atom lpar;19.5deg; for CH4and 0.0deg; for CH3rpar;. The calculated barrier height is 14.4 kcal/mole using a lsqb;3s2p1d/2s1prsqb; basis set. From comparison to similiar calculations for Olpar;3Prpar;+H2rarr;OH+H the basis set error in Dgr;Ebis estimated to be sim;2.4 kcal/mole leading to a predicted barrier height of sim;12.0 kcal/mole. Including zero point corrections leads to estimated activation energies of 10.3 kcal/mole lpar;3Aprime;rpar; and 10.1 kcal/mole lpar;3APrime;rpar; as compared to experimentally derived activation energies of 9.0ndash;11.4 kcal/mole.