Finding Transition Structures

A transition structure is the molecular species that corresponds to the top of the potential energy curve in a simple, one-dimensional, reaction coordinate diagram. The energy of this species is needed in order to determine the energy barrier to reaction and thus the reaction rate. A general rule of thumb is that reactions with a barrier of 21 kcal/mol or less will proceed readily at room temperature. The geometry of a transition structure is also an important piece of information for describing the reaction mechanism. 

Short of determining an entire reaction coordinate, there are a number of structures and their energies that are important to defining a reaction mechanism. For the simplest single-step reaction, there would be five such structures  

It has been possible to determine transition structures computationally for many years, although not always easy. Experimentally, it has only recently become possible to examine reaction mechanisms directly using femtosecond pulsed laser spectroscopy. It will be some time before these techniques can be applied to all the compounds that are accessible computationally. Furthermore, these experimental techniques yield vibrational information rather than an actual geometry for the transition structure. 

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Many techniques for finding transition structures are discussed above. The following is a listing of each of these starting with those that are easiest to use and most often successful. In other words, start with number 1 and continue down the list until you And one that works. 

Once you are experienced at finding transition structures for a particular class of reactions, you will probably go directly to the technique that has been most reliable for those reactions. Until that time, the checklist above is our best advice for finding a transition structure with the least amount of work for the researcher and the computer. Regardless of experience, it is common to experience quite a bit of trial and error in finding transition structures. Even experienced researchers find that the way they have been regarding a reaction is often much more simplistic than the molecular motions actually involved. 

Fig. 4. The linear synchronous transit (LST) and quadratic synchronous transit (QST) methods for finding transition structures R, reactants P, prc ucts TS, true transition structure 1, maximum on LST path (full curveV, 2, minimum perpendicular to LST path 3, maximum on QST path (broken curve). The model surface is constructed from two...

Methods for Finding Local Minima Methods for Finding Transition Structures Related Articles References... 

Sierka M and Sauer J (2000), Finding transition structures in extended systems a strategy based on a combined quantum mechanics-empirical valence bond approach , J. Chem. Phys., 112(16), 6983-6996. 

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