Transition state loose, tight

The results pertaining to cesium polystyrene imply a contraction of volume in the transition state of tight pairs to be greater than in their conversion into loose pairs, i.e. AV > AV,i, or an expansion of the volume in the transition state of the loose pair, i.e. AV/ > 0... 

Fig. 16 A schematic diagram showing the two transition state model of the HO2 + HO2 reaction. The reaction is barrierless so TSl is a loose transition state. The tight transition state is submerged (i.e. lower that the entrance channel). The species H2O4 and H2O4 are accessible only by third body collision.

Variational RRKM calculations, as described above, show that a imimolecular dissociation reaction may have two variational transition states [32, 31, 34, 31 and 36], i.e. one that is a tight vibrator type and another that is a loose rotator type. Wliether a particular reaction has both of these variational transition states, at a particular energy, depends on the properties of the reaction s potential energy surface [33, 34 and 31]- For many dissociation reactions there is only one variational transition state, which smoothly changes from a loose rotator type to a tight vibrator type as the energy is increased [26],... 

CO2 channel dominates as it is spin allowed and occurs via a loose transition state. In contrast, production of VO is spin forbidden and goes via a tight transition state that lies higher in energy than W" + CO2. 

What is a tight or a loose transition state How can one infer the nature of a transition state from the value of the pre-exponential factor ... 

Figure 23. Plot of experimental ( ) and theoretical three-body rate constants as a function of cluster size for the clustering of one CO molecule to copper clusters, Cun. Note the dramatic increase in reactivity (almost four orders of magnitude) within the first seven atom additions to the clusters. The overall trend represents a transition from termolecular to effective bimolecular behavior. The solid line (theory) was obtained assuming a loose transition state while the dotted line shows the results for a tight transition state for monomer and dimer only (upper limit). Taken with permission from ref. 155.

The tight and loose transition state hypothesis. Intramolecular proton transfer 99... 

THE TIGHT AND LOOSE TRANSITION STATE HYPOTHESIS. INTRAMOLECULAR PROTON TRANSFER... 

The tight and loose transition-state hypothesis is in contrast with the assumption that there is extensive cancellation of contributions due to chemical change in the entropic component of the EM (p. 81). Indeed, the uniform behaviours displayed by 0AS-data for reactions widely differing in nature (Figs 5, 23, and 24) clearly shows that no matter how loose a transition state or product is, the entropy contribution from such looseness will be cancelled out extensively by virtue of the operator 0. 

Marcus and Rice6 made a more detailed analysis of the recombination from the point of view of the reverse reaction, the unimolecular decomposition of ethane, C2Ha - 2CH3. By the principle of microscopic reversibility the transition states must be the same for forward and reverse paths. Although they reached no definite conclusion they pointed out that a very efficient recombination of CH3 radicals would imply a very high Arrhenius A factor for the unimolecular rate constant of the C2H6 decomposition which in turn would be compatible only with a very "loose transition state. Conversely, a very low recombination efficiency would imply a very tight structure for the transition state and a low A factor for the unimolecular decomposition. 

The alkali metals share many common features, yet differences in size, atomic number, ionization potential, and solvation energy leads to each element maintaining individual chemical characteristics. Among K, Na, and Li compounds, potassium compounds are more ionic and more nucleophilic. Potassium ions form loose or solvent-separated ion pairs with counteranions in polar solvents. Large potassium cations tend to stabilize delocalized (soft) anions in transition states. In contrast, lithium compounds are more covalent, more soluble in nonpolar solvents, usually existing as aggregates (tetramers and hexamers) in the form of tight ion pairs. Small lithium cations stabilize localized (hard) counteranions (see Lithium and lithium compounds). Sodium chemistry is intermediate between that of potassium and lithium (see Sodium and sodium alloys). 

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