A transition states

In the statistical description of ununolecular kinetics, known as Rice-Ramsperger-Kassel-Marcus (RRKM) theory [4,7,8], it is assumed that complete IVR occurs on a timescale much shorter than that for the unimolecular reaction [9]. Furdiemiore, to identify states of the system as those for the reactant, a dividing surface [10], called a transition state, is placed at the potential energy barrier region of the potential energy surface. The assumption implicit m RRKM theory is described in the next section. 

Chesnavich W J, Bass L, Su T and Bowers M T 1981 Multiple transition states in unimolecular reactions a transition state switching model. Application to C Hg" J. Chem. Rhys. 74 2228—46... 

The reaction path is defined by Fukui [83] as the line q(.s) leading down from a transition state along the steepest descent direction... 

A. (The gas phase estimate is about 100 picoseconds for A at 1 atm pressure.) This suggests tliat tire great majority of fast bimolecular processes, e.g., ionic associations, acid-base reactions, metal complexations and ligand-enzyme binding reactions, as well as many slower reactions that are rate limited by a transition state barrier can be conveniently studied with fast transient metliods. 

Figure 39. Benzene to benzvalene reaction, (a) Assuming that the prebenzvalene structure is a transition state. The two benzvalene isomers are anchors, (b) Assuming that prebenzvalene is an intermediate, A two-anchor loop results, compare Figure 12.

The eigenvalues (coa of the mass weighted Hessian matrix (see below) are used to compute, for each of the 3N-7 vibrations with real and positive cOa values, a vibrational partition function that is combined to produce a transition-state vibrational partition function ... 

The effect of aromatic substrates on the formation of N02" is shown in the considerably increased substrate selectivity over that obtained with NO2+ salts. On the basis of the experimental data it is suggested that in these nitrations a weaker nitrating species than NO2+ must be involved in the primary interaction with the aromatic substrates. This incipient nitronium ion then attaches itself to the aromatics in a step giving high substrate selectivity. Whether the incipient nitronium ion is the nitracidium ion (H2NO3+), protonated acetyl nitrate (CH3COO—HN02 ) or probably a transition state of any of those unstable species to N02, in which water is loosened, but not yet completely eliminated, is difficult to say and no direct physical evidence is available. 

For electrophilic substitutions in general, and leaving aside theories which have only historical interest, two general processes have to be considered. In the first, the 5 3 process, a transition state is involved which is formed from the aromatic compound, the electrophile (E+), and the base (B) needed to remove the proton ... 

However, the electronic theory also lays stress upon substitution being a developing process, and by adding to its description of the polarization of aromatic molecules means for describing their polarisa-bility by an approaching reagent, it moves towards a transition state theory of reactivity. These means are the electromeric and inductomeric effects. 

Consideration of (i), as in the work of Ridd and his co-workers, would constitute a transition state theory of the substituent effects. (2) alone would give an isolated molecule description, and (3), in so far as the charge on the electrophile was considered to modify those on the... 

A different explanation of the high 0 -ratios is based on the view, for which there is some evidence, that in a transition state for substitution which resembles the Wheland intermediate in structure there is a larger positive charge at the - than at the o-position. Substituents of the present type would therefore stabilise the transition state more from the 0-than from the -position. ... 

Another difficulty is that the extent to which hydrogen bonded association and ion-pairing influence the observed kinetics has yet to be determined. However the high order of the reaction in the stoichiometric concentration of nitric acid would seem to preclude a transition state composed only of a nitronium ion and an aromatic molecule. 

Toth et al. have thoroughly studied the rearrangement their kinetic determinations suggest a general acid-catalyzed mechanism (Scheme 115) (1578). Some points remain unclear, however why is the intermediate (181) written as a transition state when it is known that a tetrahedral intermediate (181b or 181b ) could as well be postulated How does this... 

A potential energy diagram for nng inversion m cyclohexane is shown m Figure 3 18 In the first step the chair conformation is converted to a skew boat which then proceeds to the inverted chair m the second step The skew boat conformation is an inter mediate in the process of ring inversion Unlike a transition state an intermediate is not a potential energy maximum but is a local minimum on the potential energy profile... 

Inferring the structure of a transition state on the basis of the reactants and prod nets of the elementary step m which it is involved is a time honored practice m organic chemistry Speaking specifically of transition states George S Hammond suggested that... 

---