Testing frontier orbital control: Kinetics of OH with ethane, propane, and cyclopropane from 180 to 360K

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.