What Do Transition States Look Like

A large body of experimental evidence confirms that covalent bonds have characteristic distances depending on bond type. Carbon-carbon single and double bond lengths are around 1.54A and 1.32A, respectively, while partial double bond distances, e.g., in benzene, are about 1.40A. 

Transition states, because they represent a molecule in which bonds are being made (or broken), necessarily contain partial bonds. There are no experimental data, however, that can tell us how long these bonds are, or whether partial bonds even have characteristic distances. 

Examine transition-state structures and bond density surfaces for the Diels-Alder, ene and Cope reactions. 

Bond density surface for transition state for ene reaction shows making and breaking of bonds. 

Draw the two resonance contributors that are needed to describe each transition state. Identify all partial carbon-carbon double bonds () and measure their distances. Are these values like that found in benzene, or do transition states have their own characteristic partial double bond 

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Experiments cannot tell us what transition states look like. The fact is that transition states cannot even be detected experimentally let alone characterized, at least not directly. While measured activation energies relate to the energies of transition states above reactants, and while activation entropies and activation volumes, as well as kinetic isotope effects, may be invoked to imply some aspects of transition-state structure, no experiment can actually provide direct information about the detailed geometries and/or other physical properties of transition states. Quite simply, transition states do not exist in terms of a stable population of molecules on which experimental measurements may be made. Experimental activation parameters provide some guide, but tell us little detail about what actually transpires in going from reactants to products. 

What do the molecular orbitals of the transition state look like To answer this question, we need only construct the molecular orbitals for the system of a hydrogen Ir orbital interacting with the n and 7t orbitals of an alkene. The combination of Is, n, and Jt wll yield three new molecular orbitals A, B, and C.They are constmcted in Figure 11.63. 

Reaction mechanism (Section 7.2) Loosely speaking. How does the reaction occur How do the reactants come together Are there any intermediates What do the transition states look like More precisely, a determination in terms of structure and energy of the stable molecules, reaction intermediates, and transition states involved in the reaction, along with a consideration of how the energy changes as the reaction progresses. 

When we translate these observations into Lagrangian wave speed, the data would look like that shown in the lower diagram of Fig. 7.11. The points e and q represent volume strains at whieh elastie-perfeetly-plastie release (e) and quasi-elastie release (q) would undergo transition to large-seale, reverse plastie flow (reverse yield point). The question is the following What is responsible for quasi-elastie release from the shoeked state, and what do release-wave data tell us about the mieromeehanieal response in the shoeked state ... 

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