The synchronous-transit method for determining reaction pathways and locating molecular transition states

In the synchronous-transit method, a model linear synchronous transit pathway is first constructed and is then refined by optimizing one or more intermediate structures subject to the constraint that the optimized structure retain the same relative position along the path (orthogonal optimization). The method yields a series of energy estimates which progressively bound the energy of the transition state from above and from below. High computational efficiency is attainable, and sufficient flexibility is provided to deal with asynchronous processes. Comparisons are made to the alternative “reaction-coordinate” approach, which is shown to be subject to several serious deficiencies. The method is applied to a model two-dimensional energy surface and to the allowed electrocyclic interconversions of the cyclopropyl and allyl cations and of cyclobutene and cis-butadiene.