Limiting unimolecular mechanism

The pK values seen in log( K) profiles are those of groups involved in the rate-limiting unimolecular steps in the mechanism. In most cases these groups are involved in catalysis in some way, but the pK values are perturbed from where they were in free enzyme or substrate. It is also important to note which pAT... 

Much evidence has been obtained in support of the El mechanism. For example, El reactions show first-order kinetics, consistent with a rate-limiting, unimolecular dissociation process. Furthermore, El reactions show no deuterium isotope effect. Because rupture of the C-H (or C-D) bond occurs after the rate-limiting step rather than during it, we can t measure a rate difference between a deuterated and nondeuterated substrate. A final piece of evidence is that there is no geometric requirement on the El reaction because the halide and the hydrogen are lost in separate steps, unlike the E2 reaction, where anti periplanar geometry is required. 

Figure A3.4.9. Pressure dependence of the effective unimolecular rate constant. Schematic fall-off curve for the Lindemaim-FIinshelwood mechanism. A is the (constant) high-pressure limit of the effective rate constant...

Quack M 1984 On the mechanism of reversible unimolecular reactions and the canonical ( high pressure ) limit of the rate coefficient at low pressures Ber. Bunsenges. Phys. Chem. 88 94-100... 

Randall and Alberly (136) have studied the binding of various ligands to aquocobalamin using stopped flow techniques. This work suffers from the fact that it is not clear if the added ligand is displacing coordinated water or coordinated benzimidazole. One might be led to believe that the reaction studied in this work is in fact displacement of benzimidazole because the kinetics are at least inconsistent with a mechanism in which unimolecular dissociation of coordinated water is the rate limiting step. 

The basic classification of nucleophilic substitutions is founded on the consideration that when a new metal complex is formed through the breaking of a coordination bond with the first ligand (or water) and the formation of a new coordination bond with the second ligand, the rupture and formation of the two bonds can occur to a greater or lesser extent in a synchronons manner. When the mpture and the formation of the bonds occur in a synchronous way, the mechanism is called substitution nucleophilic bimolecular (in symbols Sn2). On the other extreme, when the rupture of the first bond precedes the formation of the new one, the mechanism is called substitution nucleophilic unimolecular (in symbols SnI). Mechanisms Sn2 and SnI are only limiting cases, and an entire range of intermediate situations exists. 

As carried out above for the Lindemann mechanism, application of the steady-state approximation gives the apparent unimolecular rate constant in Equation (24) where [Av] represents the IR photon density. Again two limits may be considered. 

An entire mechanism can be unimolecular, provided that the rate-limiting step is unimolecular. See also Chemical Kinetics, Molecularity Order of Reaction Elementary Reactions... 

In spreading the drop does not extend as a homogeneous oil phase in the form of a thin lamina but takes place by a process of superficial solution of the oil in a thin film (Hardy s primary layer) which we shall note is unimolecular in thickness consideration of the mechanism of such spreading will be deferred to a subsequent section. If the area of the water on which the drop is placed is limited in extent the oil continues to spread until the rate of superficial solution is balanced by the rate of return of molecules from the surface film into the lens. The oil of the lens may be regarded... 

One point of interest deriving from the equations of TST (and Arrhenius theory) is that the upper limit for the 298 K rate constant of a unimolecular reaction that takes place with zero activation energy (of whatever sort) is roughly 10 sec . This is, in some sense, a conceptually obvious result since that is on the order of a molecular vibrational frequency, which is thought of as the mechanism by which a transition state goes to its products. 

Pressure effects are also seen in a class of bimolecular reactions known as chemical activation reactions, which were introduced in Section 9.5. The treatment in that chapter was analogous to the Lindemann treatment of unimolecular reactions. The formulas derived in Section 9.5 provide a qualitative explanation of chemical activation reactions, and give the proper high- and low-pressure limits. However, that simple treatment neglected many quantum mechanical effects, namely the energy dependence of various excitation/de-excitation steps. 

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