Testing frontier orbital control: Kinetics of OH with ethane, propane, and cyclopropane from 180 to 360K. Clarke, J., S., Kroll, J., H., Donahue, N., M., & Anderson, J., G. J. Phys. Chem. A, 102:9847-9857, 1998.
abstract   bibtex   
We test the hypothesis that the barrier to a gas-phase radical-molecule reaction is controlled by an avoided curve crossing of ground and ionic states of the reactants and products. We focus on the competing role of orbital overlap and energy difference on the delocalization energy of the transition state, comparing the reactions OH + ethane, OH + propane, and OH + cyclopropane using experimental data and theoretical analysis. These reactions constitute a homologous series in which the spatial extent and energy of interacting orbitals change dramatically, providing for an examination of the relative importance of energy sind overlap on barrier height control. In addition, contrasting pictures of barrier height control, either by molecular properties or by bond properties of the reactants and products, are evaluated. Our kinetic data, obtained in a high-pressure flow system, cover a suppressed temperature range (180 - 360 K) in order to isolate the lowest barrier pathway. The results for ethane and propane are consistent with barrier height control by the singly occupied molecular orbital (SOMO) of the OH radical and the highest occupied molecular orbital (HOMO) of the molecule. These are the historically defined frontier orbitals. The results for cyclopropane, however, suggest that it is the interaction of the SOMO with the second highest occupied molecular orbitals (SHOMOs) which controls barrier height. The SHOMOs of cyclopropane are spatially extended relative to the HOMOs; at the transition state the interaction between OH and the SHOMOs of cyclopropane overwhelms the interaction between OH and the HOMOs of cyclopropane. We examine the competition between energy and overlap of two reacting species and present an alternative definition of the frontier orbitals not necessarily as the highest energy orbitals, but rather as the orbitals that delocalize to the greatest extent at the transition state. C1 Harvard Univ, Dept Chem & Chem Biol, Cambridge, MA 02138 USA.
@article{
 title = {Testing frontier orbital control: Kinetics of OH with ethane, propane, and cyclopropane from 180 to 360K},
 type = {article},
 year = {1998},
 pages = {9847-9857},
 volume = {102},
 id = {7e0ce051-6f7e-3968-8530-398f38444689},
 created = {2014-10-08T16:28:18.000Z},
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 profile_id = {363623ef-1990-38f1-b354-f5cdaa6548b2},
 group_id = {02267cec-5558-3876-9cfc-78d056bad5b9},
 last_modified = {2017-03-14T17:32:24.802Z},
 read = {false},
 starred = {false},
 authored = {false},
 confirmed = {true},
 hidden = {false},
 citation_key = {Clarke:JPCA:1998a},
 source_type = {article},
 private_publication = {false},
 abstract = {We test the hypothesis that the barrier to a gas-phase
radical-molecule reaction is controlled by an avoided curve
crossing of ground and ionic states of the reactants and products.
We focus on the competing role of orbital overlap and energy
difference on the delocalization energy of the transition state,
comparing the reactions OH + ethane, OH + propane, and OH +
cyclopropane using experimental data and theoretical analysis.
These reactions constitute a homologous series in which the spatial
extent and energy of interacting orbitals change dramatically,
providing for an examination of the relative importance of energy
sind overlap on barrier height control. In addition, contrasting
pictures of barrier height control, either by molecular properties
or by bond properties of the reactants and products, are evaluated.
Our kinetic data, obtained in a high-pressure flow system, cover a
suppressed temperature range (180 - 360 K) in order to isolate the
lowest barrier pathway. The results for ethane and propane are
consistent with barrier height control by the singly occupied
molecular orbital (SOMO) of the OH radical and the highest occupied
molecular orbital (HOMO) of the molecule. These are the
historically defined frontier orbitals. The results for
cyclopropane, however, suggest that it is the interaction of the
SOMO with the second highest occupied molecular orbitals (SHOMOs)
which controls barrier height. The SHOMOs of cyclopropane are
spatially extended relative to the HOMOs; at the transition state
the interaction between OH and the SHOMOs of cyclopropane
overwhelms the interaction between OH and the HOMOs of
cyclopropane. We examine the competition between energy and overlap
of two reacting species and present an alternative definition of
the frontier orbitals not necessarily as the highest energy
orbitals, but rather as the orbitals that delocalize to the
greatest extent at the transition state. C1 Harvard Univ, Dept Chem
& Chem Biol, Cambridge, MA 02138 USA.},
 bibtype = {article},
 author = {Clarke, J S and Kroll, J H and Donahue, N M and Anderson, J G},
 journal = {J. Phys. Chem. A}
}
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