Unimolecular process/reaction/step

The number of molecules that participate as reactants in an elementary step defines the molecularity of the step. If a single molecule is involved, the reaction is unimolecular. The rearrangement of methyl isonitrile is a unimolecular process. Elementary steps involving the collision of two reactant molecules are bimolecular. The reaction between NO and O3 (Equation 14.22) is bimolecular. Elementary steps involving the simultaneous collision of three molecules are termolecular. Termolecular steps are far less probable than unimolecular or bimolecular processes and are rarely encountered. The chance tiiat four or more molecules will collide simultaneously with any regularity is even more remote consequently, such collisions are never proposed as part of a reaction mechanism. 

The first step (loss of the leaving group) is the rate-determining step, much Uke we saw for SnI processes. The base does not participate in this step, and therefore, the concentration of the base does not affect the rate. Because this step involves only one chemical entity, it is said to be uiumolecular. Unimolecular elimination reactions are called El reactions, where the 1 stands for unimolecular. ... 

In a termolecular reaction, three chemical species collide simultaneously. Termolecular reactions are rare because they require a collision of three species at the same time and in exactly the right orientation to form products. The odds against such a simultaneous three-body collision are high. Instead, processes involving three species usually occur in two-step sequences. In the first step, two molecules collide and form a collision complex. In a second step, a third molecule collides with the complex before it breaks apart. Most chemical reactions, including all those introduced in this book, can be described at the molecular level as sequences of bimolecular and unimolecular elementary reactions. 

In a unimolecular reaction, a molecule fragments into two pieces or rearranges to a different isomer, hi either case, a chemical bond breaks. For example, in the fragmentation of bromine molecules, breaking a ffbond gives a pair of bromine atoms Bf2 2 Br Recall that this unimolecular process is the first step of the reaction between molecular hydrogen and molecular bromine to give HBr. 

In this part of the chapter, we will briefly outline the main types of CL reactions which can be functionally classified by the nature of the excitation process that leads to the formation of the electronically excited state of the light-emitting species. Direct chemiluminescence is the term employed for a reaction in which the excited product is formed directly from the unimolecular reaction of a high-energy intermediate that has been formed in prior reaction steps. The simplest example of this type of CL is the unimolecular decomposition of 1,2-dioxetanes, which are isolated HEI. Thermal decomposition of 1,2-dioxetanes leads mainly to the formation of triplet-excited carbonyl compounds. Although singlet-excited carbonyl compounds are produced in much lower yields, their fluorescence emission constitutes the direct chemiluminescence emission observed in these transformations under normal conditions in aerated solutions ... 

Rate constants of unimolecular processes can be obtained from spectral data and are useful parameters in photochemical kinetics. Even the nature of photoproducts may be different if these parameters change due to some perturbations. In the absence of bimolecular quenching and photochemical reactions, the following reaction steps are important in deactivating the excited molecule back to the ground state. 

For a photoexcited molecule, the time allowed for a reaction to occur is of the order of the lifetime of the particular excited state, or less when the reaction step must compete with other photophysical processes. The photoreaction can be unimolecular such as photodissociation and photo isomerization or may need another molecule, usually unexcited, of the same or different kind and hence bimolectdar. If the primary processes generate free radicals, they may lead to secondary processes in the dark. 

The El mechanism has, as the rate-determining step in solution, the ionisation of the reactant forming a carbonium ion which then decomposes rapidly. For heterogeneous catalytic reactions, the important features are the occurrence of the reaction in two steps and the presence on the solid surface of carbonium ions or species resembling them closely. Again, the kinetic characterisation by way of an unimolecular process is of little value. Even the relative rates of the two steps may be reversed on solid catalysts. A cooperation of an acidic and a basic site is also assumed, the reaction being initiated by the action of the acidic site on the group X. 

Isobutyl species formed on the catalyst in the initiation steps do not react in unimolecular processes since such reactions (e.g., the formation of n-butyl species) involve primary carbenium ion transition states. Therefore, propagation steps in the conversion of isobutane include oligomerization reactions... 

Photochemical initiation has often been used as an excellent method of studying radical and chain reactions.1 2 The primary step in many systems is followed by a sequence of steps, which may include conventional unimolecular processes of species having known or calculable energy. Examples are numerous and well known. In order to understand such systems, whether reaction is initiated photochemical ly or thermally, the typical characteristics of unimolecular reactions and their dependence on the energy parameters of the systems and on molecular structure must be clarified. This is the purpose of the present chapter, which will deal principally with the smaller hydrocarbon species below C6. 

Rate laws are employed to evaluate reaction mechanisms in soil-water systems. To accomplish this, kinetics are used to elucidate the various individual reaction steps or elementary reactions. Identifying and quantifying the elementary steps of a complex process allow one to understand the mechanism(s) of the process. For example, unimolecular reactions are generally described by first-order reactions bimolecular reactions are described by second-order reactions,... 

Both these processes can be considered to occur in several distinct stages as follows (i) formation of precursor state where the reacting centers are geometrically positioned for electron transfer, (ii) activation of nuclear reaction coordinates to form the transition state, (iii) electron tunneling, (iv) nuclear deactivation to form a successor state, and (v) dissociation of successor state to form the eventual products. At least for weak-overlap reactions, step (iii) will occur sufficiently rapidly (< 10 16s) so that the nuclear coordinates remain essentially fixed. The "elementary electron-transfer step associated with the unimolecular rate constant kel [eqn. (10)] comprises stages (ii)—(iv). 

The great majority of radical cascades involve sequences of intramolecular steps where the overall propagation is a unimolecular process (with the exclusion of the initiation and termination steps), and recent overviews are given in Refs 1-5. However, meanwhile many tandem reactions involving both intra- and intermolecular steps, as well as multicomponent tandem reactions, have been reported in the literature, which are compiled in recent reviews. " ... 

The various detailed theories which have been proposed for unimolecular reactions predict that, at sufficiently low total concentrations, all unimolecular processes should show a falling off of their apparent first-order constants with decreasing pressure. Convincing experimental evidence on this point is at present rather meager. The reasons for the sparsity of data arise from at least three sources. First and most difficult to deal with is the fact that the decompositions of most molecules ai e not simple but involve complex intermediate steps. Secondly, the over-all major reaction is seldom quantitative but usually involves varying amounts of side reac-... 

If we overlook the processes occurring in the region x < 0 in Figure 7.1, then the illustrated burning mechanism of the solid closely resembles that of a premixed gaseous laminar flame. To proceed with an analysis, let us assume for illustrative purposes that the gaseous reaction is the one-step unimolecular process... 

The methyl-radicals are p radicals, and if their recombination were the predominant chain-ending step the kinetics would be three-halves order (cf. Table 11). On the other hand, if reaction (9) were predominant the recombination would be of the Pfi type, and the overall kinetics would be first order. The experimental result that the order is between unity and three-halves - is thus explained if both of these chain-ending steps play some part, with perhaps some smaller contribution from reaction (10). The result that the order is closer to at lower pressures and higher temperatures, and closer to unity at higher pressures and lower temperatures, is also explicable on this basis. Since methyl radicals are removed bimole-cularly and propyl radicals by unimolecular processes the ratio [CH3]/[C3H7] will be higher at lower pressures, so that reaction (8), leading to f-order kinectics, will... 

The last step here is apparently a unimolecular process and depends only on the concentration of H atoms. This is a step that occurs on the surface of the reaction vessel and the above is a simplification of the rate expression for such a process. 

A different model [11] that can be used to obtain the kinetics equation for a pyrolytic reaction is adapted from the theory developed for the kinetics of heterogeneous catalytic reactions. This theory is described in literature for various cases regarding the determining step of the reaction rate. The case that can be adapted for a pyrolytic process in solid state is that of a heterogeneous catalytic reaction with the ratedetermining step consisting of a first-order unimolecular surface reaction. For the catalytic reaction of a gas, this case can be written as follows ... 

Although in all recorded TPO spectra of "hard" coke CO evolution is represented by a single Gaussian curve, reaction steps (4) and (5) suggest two peaks should be observed. The contribution of step (5), however, is expected to be small as such unimolecular desorption processes have been shown to continue, in the absence of oxygen, at temperatures well beyond those required for complete conversion during TPO [16]. Despite this, a small peak. 

The E2 mechanism is a concerted one-step process in which a nucleophile abstracts a hydrogen ion from one carbon while the halide is leaving from an adjacent one. The Ei mechanism is two-steps and involves a carbocation intermediate formed upon departure of the halide ion in the first step. E2 reactions are bimolecular and the reaction rate depends on the concentrations of both the alkyl halide and nucleophile. E1 reaction rates depend on the slowest step, formation of the carbocation, and are influenced only by the concentration of the alkyl halide the reaction is unimolecular. E2 reactions involve anti elimination and produce a specific alkene, either cis or trans. E1 reactions involve an intermediate carbocation and thus give products of both syn and anti elimination. 

Fluorescence correlation spectroscopy analyses the temporal fluctuations of the fluorescence intensity by means of an autocorrelation function from which translational and rotational diffusion coefficients, flow rates and rate constants of chemical processes of single molecules can be determined. For example, the dynamics of complex formation between /3-cyclodextrin as a host for guest molecules was investigated with singlemolecule sensitivity, which revealed that the formation of an encounter complex is followed by a unimolecular inclusion reaction as the rate-limiting step.263... 

The radiolysis of the protiated and the deuterated form of n-hexane and mixtures thereof has been examined as a function of temperature and at one N20-concentration. A strong isotope effect can be observed for fragmentation giving hexene and hydrogen by a unimolecular process, whereas the other product-distribution is not much affected by deuteration. N20 does not decrease the primary formation of C6Dt 3-radicals in mixtures. A novel reaction type is proposed for these systems giving hexene and water in a unimolecular step. 

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