Center for Computational and Molecular Science and Technology, School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, USA.
J Phys Chem A. 2013 Aug 15;117(32):7553-60. doi: 10.1021/jp402322h. Epub 2013 Jul 2.
The rates of chemical reactions (or any activated process) are by definition determined by the flux of reactants (or initial states) that end up as products (or final states). The forward flux through any surface that divides reactants from products is a constant as long as only those trajectories that were reactants in the infinite past and products in the infinite future are included in the flux once and only once. Transition state theory (TST) ignores this last clause, thereby overestimating the rate if any of the trajectories recross the dividing surface. However, its advantage is that it replaces a dynamical calculation with a statistical integral over the TST geometry. The recent identification of roaming trajectories-those that persist for a long time as neither reactant nor product without ever visiting near the col on the energy landscape-apparently challenges the dogma that TST's only error lies in the omission of recrossing trajectories. This question is investigated using the isomerization reaction of ketene in which the experimental values are verified to be in reasonable agreement with both the exact and TST values. We have found two trajectories for the ketene isomerization that carry the signature of roaming, but their effect on the calculation of the reaction rate constant using classical transition state theory is small. Indeed, the existence of roaming trajectories is seen to impose a limitation on which dividing surfaces are appropriate for the calculation of either exact or approximate TST rates, but in this case, they do not unseat the existence of dividing surfaces that can be used safely to calculate TST rates.
化学反应（或任何激活过程）的速率从定义上讲是由反应物（或初始状态）的通量决定的，这些反应物最终会成为产物（或最终状态）。只要仅包括那些在无限过去是反应物且在无限未来是产物的轨迹，那么将反应物与产物分开的任何表面上的正向通量都是恒定的。一旦且仅一次。过渡态理论（TST）忽略了最后一条条款，从而高估了任何轨迹重新穿越分隔表面的速率。但是，它的优点是它用 TST 几何形状的统计积分代替了动力学计算。最近发现了漫游轨迹-那些在很长一段时间内既不是反应物也不是产物，而从未在能量景观上的 col 附近停留过的轨迹-显然挑战了 TST 的唯一错误在于忽略了重新穿越轨迹的教条。使用乙烯酮的异构化反应来研究这个问题，实验值被证明与精确值和 TST 值非常吻合。我们已经找到了乙烯酮异构化的两条具有漫游特征的轨迹，但是它们对使用经典过渡态理论计算反应速率常数的影响很小。实际上，漫游轨迹的存在对计算精确或近似 TST 速率的合适分隔表面施加了限制，但是在这种情况下，它们并没有取代可以安全地用于计算 TST 速率的分隔表面的存在。
