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 sp(2) or sp(3) 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.
Temelso
Berhane
B
Center for Computational Molecular Science and Technology, School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, USA.
Sherrill
C David
CD
Merkle
Ralph C
RC
Freitas
Robert A
RA
D003198:Computer Simulation; D004583:Electrons; D005609:Free Radicals / Q000737:chemistry; D006838:Hydrocarbons / Q000737:chemistry*; D006859:Hydrogen / Q000737:chemistry*; D008958:Models, Molecular; D008968:Molecular Conformation; D013237:Stereoisomerism; D013816:Thermodynamics
D005609:Free Radicals; D006838:Hydrocarbons; D006859:Hydrogen