ABINITIO MECHANISTIC STUDY OF RADICAL REACTIONS - TRANSITION-STATES AND REACTION BARRIERS FOR THE REACTION OF ATOMIC-HYDROGEN WITH ACETYLENE

Ab initio molecular orbital methods have been applied to the addition and abstraction reactions CH≡CH + H →CH2̿ĊH and CH≡CH + H → CH≡Ċ + H2. The reaction pathways and geometries for the transition state are investigated within the unrestricted Hartree-Fock SCF theory. The addition reaction proceeds with trans stereochemistry via an “early and loose” transition state, while the abstraction reaction prefers a “late and tight” transition state. The respective barrier heights are calculated to be 6.2 and 32.4 kcal/mol. Both reaction barriers originate primarily from the exchange repulsion interaction and from intramolecular deformation. It is also found that forward and then back charge-transfer interactions occur as the reaction proceeds. The formation and weakening of chemical bonds at the transition states are also discussed. © 1979, American Chemical Society. All rights reserved.