High-Level ab Initio Studies of Hydrogen Abstraction from Prototype Hydrocarbon Systems

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.