| Abstract
| - The temperature dependence of the dynamical bottlenecklocation for the prototype olefin additionprocess H + C2H4 has been studied byvariational transition state theory. In addition, amultidimensionaltunneling calculation has been carried out. To obtain a reliablepotential energy profile, a new way to extrapolateelectronic structure calculations to the limit of full configurationmixing and a complete electronic basis set isproposed. The method, called variable scaling of externalcorrelation (VSEC) uses a scale factor that varieswith geometry to combine a complete-active-space self-consistent-field(CASSCF) calculation with a calculationincluding an appreciable amount of the dynamical correlation energy.The parameters in the scaling factorare adjusted to experimental or high-quality theoretical data atcritical points. For the title reaction we findexcellent agreement with experiment at all temperatures both for therate constant of the association reactionand also for the rate constant of its reverse dissociation, and we usethe resulting model to infer critical detailsof the dynamics. In particular, we find that dynamical bottlenecksare ∼0.06−0.15 Å tighter than the saddlepoint, with bending frequencies 1.2−1.8 times higher than at thesaddle point. Furthermore, we conclude thattunneling effects account quantitatively for the curvature of theArrhenius plots.
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