Web Release Date: September 7,
High-Level ab Initio Studies of Hydrogen Abstraction from Prototype Hydrocarbon Systems
and
Center for Computational Molecular Science and Technology, School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, College of Computing, Georgia Institute of Technology, Atlanta, Georgia 30332-0280, and Institute for Molecular Manufacturing, 555 Bryant Street, Suite 354, Palo Alto, California 94301
Received: March 23, 2006
In Final Form: July 19, 2006
Abstract:
Symmetric and nonsymmetric hydrogen abstraction reactions are studied using state-of-the-art ab initio
electronic structure methods. Second-order M
ller-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.
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