TY - JOUR
T1 - What is the origin of the contrathermodynamic behavior in methyl radical addition to alkynes versus alkenes?
AU - Gómez-Balderas, Rodolfo
AU - Coote, Michelle L.
AU - Henry, David J.
AU - Fischer, Hanns
AU - Radom, Leo
PY - 2003/8/7
Y1 - 2003/8/7
N2 - High-level ab initio calculations of the barriers, enthalpies, and rate constants have been performed for methyl radical addition to ethyne, propyne, ethene, and propene. We find that addition to alkenes is kinetically favored over addition to alkynes, despite the larger exothermicity of the alkyne addition reactions. The results have been rationalized using the curve-crossing model. To this end, the singlet-triplet gaps and charge-transfer energies in the reactants, and the extent of charge separation in the transition structures, have been calculated. It is concluded that the greater barrier for addition to alkynes is primarily the result of the larger singlet-triplet gap in the substrate. This barrier-raising effect dominates the barrier-lowering effect of the reaction exothermicity.
AB - High-level ab initio calculations of the barriers, enthalpies, and rate constants have been performed for methyl radical addition to ethyne, propyne, ethene, and propene. We find that addition to alkenes is kinetically favored over addition to alkynes, despite the larger exothermicity of the alkyne addition reactions. The results have been rationalized using the curve-crossing model. To this end, the singlet-triplet gaps and charge-transfer energies in the reactants, and the extent of charge separation in the transition structures, have been calculated. It is concluded that the greater barrier for addition to alkynes is primarily the result of the larger singlet-triplet gap in the substrate. This barrier-raising effect dominates the barrier-lowering effect of the reaction exothermicity.
UR - http://www.scopus.com/inward/record.url?scp=0042232501&partnerID=8YFLogxK
U2 - 10.1021/jp035042z
DO - 10.1021/jp035042z
M3 - Article
SN - 1089-5639
VL - 107
SP - 6082
EP - 6090
JO - Journal of Physical Chemistry A
JF - Journal of Physical Chemistry A
IS - 31
ER -