High-level ab initio studies of hydrogen abstraction from prototype hydrocarbon systems
Temelso, B.; Sherrill, C. D.; Merkle, R. C.; Freitas, R. A. High-Level Ab Initio Studies of Hydrogen Abstraction from Prototype Hydrocarbon Systems. J. Phys. Chem. A 2006, 110 (38), 11160-11173.
Symmetric and nonsymmetric hydrogen abstraction reactions are studied using state-of-the-art ab initio electronic structure methods. Second-order Møller-Plesset perturbation theory (MP2) and the coupled-cluster singles, doubles, and perturbative triples [CCSD(T)] methods with large correlation consistent basis sets (cc-pVXZ, where X = D,T,Q) are used in determining the transition-state geometries, activation barriers, and thermodynamic properties of several representative hydrogen abstraction reactions. The importance of basis set, electron correlation, and choice of zeroth-order reference wave function in the accurate prediction of activation barriers and reaction enthalpies are also investigated. The ethynyl radical (·CCH), which has a very high affinity for hydrogen atoms, is studied as a prototype hydrogen abstraction agent. Our high-level quantum mechanical computations indicate that hydrogen abstraction using the ethynyl radical has an activation energy of less than 3 kcal mol-1 for hydrogens bonded to an sp2 or sp3 carbon. These low activation barriers further corroborate previous studies suggesting that ethynyl-type radicals would make good tooltips for abstracting hydrogens from diamondoid surfaces during mechanosynthesis. Modeling the diamond C(111) surface with isobutane and treating the ethynyl radical as a tooltip, hydrogen abstraction in this reaction is predicted to be barrierless.
The Journal of Physical Chemistry A