Loose transition states

The relationship between secondary a.-1 H/2H KIEs and transition state looseness has been investigated by MP2/6-31 ++G(d,p) calculations on three identity X- + CH3X 5k2 reactions.69 The secondary KIE increases with increasing transition structure looseness. It was shown that looseness is best defined as a A-elongation of the C—X bond lengths, where the reference state can be either the isolated reactants or the ion-dipole complex. 

Glad SS, Jensen F (1997) Transition state looseness and secondary kinetic isotope effects. J. Am. Chem. Soc. 119 227-232... 

The smallest change that appears to be of some interest is isotopic substitution in the methyl group. The interest is in the isotope effect (for example, for CD3 transfer) to mesh our identity rate work with Schowen and co-workers (28) isotope effect criterion of transition-state looseness. Clearly, the error of the fit to the Marcus treatment (as shown in Figure 1) is well outside any conceivable isotope effect, but isotope effects should nev-... 

The transition states loose ion-pairs in so far as they involve charge-delocalized anions, thereby enhancing polarity compared with the ground states (in which the ion-pairs are tighter), because of an increase in anionic dissociation as the more bulky product anion is formed. As a consequence, specific microwave effects, directly connected to polarity enhancement, should depend on the structure of reactive ion-pairs in the GS ... 

The secondary a- and perdeutero-deuterium KIEs E2 S 2 ratios reaction efficiencies transition-state looseness parameters and the relative basicities of the nucleophiles in the gas-phase reactions between methyl, ethyl, i-propyl, and f-butyl iodides and SH , Cr, and CN have been used to show that (i) the ethyl and i-propyl iodides react mainly by an Sf 2 mechanism, while the i-butyl iodide reacts by an E2 mechanism, (ii) that the E2 barrier is more sensitive to basicity of the nucleophile than the 5 2 barrier, and (iii) that stronger bases promote more 2 elimination. The KIEs in the ethyl iodide reactions indicate that the transition state becomes looser as the nucleophile changes from Cl to CN to SH. Larger (more normal) deuterium KIEs and looser S 2 transition states are found when more alkyl groups are added to the C of the substrate. 

The secondary a- and perdeutero-deuterium KIEs EllSf l ratios reaction efficiencies transition-state looseness parameters and the relative basicities of the nucleophiles in the gas-phase reactions between methyl, ethyl, i-propyl, and /-butyl iodides and SH , Cl , and are discussed earlier. 

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],... 

Wardlaw D M and Marcus R A 1984 RRKM reaction rate theory for transition states of any looseness Chem. Rhys. Lett. 110 230-4... 

Nevertheless, the reaction in region B has other unusual features, notably in the pattern of substituent effects for 4-substituents this is the reverse of that observed in region A. From these and other considerations, Ridd s group (Challis and Ridd, 1962 de Fabricio et al., 1966) concluded that the nitrosating agent attacks the pro-tonated amine and is loosely associated with the aromatic ring in the transition state. 

Protonatioil of l.he epoxide gives cation (33) which reacts utcr a loose transition state so that a partial posit ve charge appears on the carbon atom under attack. The more stable partial cation is therefore formed. 

In either case, abstraction mechanisms are direct (no long-lived collision complex is formed), have small entropy costs ( loose transition states), and typically deposit large amounts of vibrational energy in the newly formed bond while the other bonds in the system act largely as spectators. 

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 ... 

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