Bimolecular event

Like any reaction, an exchange process does not necessarily occur by bimolecular events. We shall derive an expression giving the progress of the exchange in terms of the exchange rates, Rex. 

A treatment similar to that for unimolecular reactions is necessary for recombination reactions which result in a single product. An example is the possible termination step for the mechanism for decomposition of C Hg, H + CjH - (Section 6.1.2). The initial formation of ethane in this reaction can be treated as a bimolecular event. However, the newly formed molecule has enough energy to redissociate, and must be stabilized by transfer of some of this energy to another molecule. 

Most elementary chemical reactions can be categorized as unimolecular or bimolecular events. However, further phenomenological classification is useful for the development of detailed chemical kinetic models. This way, rate parameters for new reactions can be estimated rapidly and reliably by analogy to similar reactions in the same phenomenological class. In addition, the number of different elementary reactions that must separately be treated is reduced. It must be recognized, however, that exceptional cases... 

A multicomponent reaction (MCR) represents a sequence of bimolecular events leading to products that incorporate essentially all atoms of three or more starting materials. MCRs allow for the rapid and facile access to complex target structures... 

EPQ, and EQ. The central complexes are EAB and EPQ. The binary complexes EA and EQ are often referred to as Michaelis complexes in that they are generated by simple binding events but no chemistry occurs until one or more other reactants bind to the active site. Note that central complexes can only participate in unimolecular events whereas Michaelis complexes can participate in both unimolecular and bimolecular events. 

Coke formation is considered, with just cause, to be a malignant side reaction of normal carbonium ions. However, while chain reactions dominate events occurring on the surface, and produce the majority of products, certain less desirable bimolecular events have a finite chance of involving the same carbonium ions in a bimolecular interaction with one another. Of these reactions, most will produce a paraffin and leave carbene/carboid-type species on the surface. This carbene/carboid-type species can produce other products but the most damaging product will be one which remains on the catalyst surface and cannot be desorbed and results in the formation of coke, or remains in a noncoke form but effectively blocks the active sites of the catalyst. 

The foregoing procedure must be extended to incorporate bimolecular events. Two free radicals are involved in the termination step and the growth histories of both chains must be known to calculate the MWD. 

First Order Stoppage Alone. If stoppage is determined solely by a first order process, such as transfer, the foregoing analysis predicts a nearly exponential distribution function. The polydispersity index must then be very close to 2.00. The same result is obtained for bulk and solution polymerizations dominated by chain transfer. Compartmentalization thus has no major effect on the polydispersity of the polymer produced, as was recognized by Gerrens (11), if the stoppage process is dominated by chain transfer. This contrasts with the significant effects of compartmentalization if bimolecular events dominate termination. 

Bimolecular Termination Alone. The effect of compartmentalization in an emulsion polymerization is to broaden significantly the MWD of the polymer produced if termination is dominated by bimolecular events. This was clearly established by Katz,... 

One of the available ways for investigating intramolecular electron transfer is that of inducing a charge shift by bimolecular PET (Figure Ic). The feasibility of this relies on the fact that the bimolecular event producing the thermodynamically unfavored electronic isomer of the dyad is faster than the intramolecular electron transfer process to be observed. When this requirement is met, since the back recombination reaction is a bimolecular process, relatively slow intramolecular electron transfer processes can be studied. This type of process has been extensively used for studying electron transfer in proteins [206], but has not been frequently applied to dyads. In an early study [207], the Rh(III)-diquat dyad (28) was... 

Chemical reactions occur on the molecular level by a sequence of one or more steps known as a mechanism. Every step is either a unimolecular or a bimolecular event. Typically, a single-headed arrow (— ) is used to indicate a unimolecular or bimolecular event. 

Three examples of bimolecular events are shown below ... 

In this section we summarize methods for solution of the master equation, which couples the collisional relaxation of the highly excited unimolecular species with the microcanonical dissociation rates to determine, for a given temperature and pressure, the non-equilibrium probability distribution for the molecular population over energies and angular momenta, and thence the thermal rate coefficient k(T, P). The separability of molecular interactions in the gas phase into unimolecular events and bimolecular events enables the overall thermal dissociation process to be modeled by the two-dimensional master equation, expressed in continuum notation as... 

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