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

We test the hypothesis that the barrier to a gas-phase radical-molecule rea ction 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 o verlap and energy difference on the delocalization energy of the transition state, comparing the reactions OH + ethane, OH + propane, and OH + cyclopr opane using experimental data and theoretical analysis. These reactions con stitute a homologous series in which the spatial extent and energy of inter acting orbitals change dramatically, providing for an examination of the re lative importance of energy sind overlap on barrier height control. In addi tion, contrasting pictures of barrier height control, either by molecular p roperties or by bond properties of the reactants and products, are evaluate d. Our kinetic data, obtained in a high-pressure flow system, cover a suppr essed temperature range (180 - 360 K) in order to isolate the lowest barrie r pathway. The results for ethane and propane are consistent with barrier h eight control by the singly occupied molecular orbital (SOMO) of the OH rad ical and the highest occupied molecular orbital (HOMO) of the molecule. The se are the historically defined frontier orbitals. The results for cyclopro pane, however, suggest that it is the interaction of the SOMO with the seco nd highest occupied molecular orbitals (SHOMOs) which controls barrier heig ht. The SHOMOs of cyclopropane are spatially extended relative to the HOMOs ; at the transition state the interaction between OH and the SHOMOs of cycl opropane overwhelms the interaction between OH and the HOMOs of cyclopropan e. We examine the competition between energy and overlap of two reacting sp ecies and present an alternative definition of the frontier orbitals not ne cessarily as the highest energy orbitals, but rather as the orbitals that d elocalize to the greatest extent at the transition state.