Reaction monomolecular

Classic examples are the spontaneous emission of light or spontaneous radioactive decay. In chemistry, an important class of monomolecular reactions is the predissociation of metastable (excited) species. An example is the fonnation of oxygen atoms in the upper atmosphere by predissociation of electronically excited O2 molecules [12, 13 and 14] ... 

Excited O2 molecules are fomied by UV light absorption. Monomolecular reactions (e.g., c = [Oi]) show a first-order rate law ... 

Experimentally, one finds the same first-order rate law as for monomolecular reactions, but with an effective rate constant /rthat now depends on [M],... 

Rule G6 A monomolecular reaction is electrophobic for an electron donor adsorbate and electrophilic for an electron acceptor adsorbate. 

Tables 6.2 to 6.9 summarize all local (Table 6.2), global (Tables 6.3 to 6.7), fundamental (Table 6.8) and practical (Table 6.9) promotional rules. Tables 6.6 and 6.7 provide some obvious extensions to monomolecular reactions, also in good agreement with experiment. All the rules can be summarized by the inequalities 6.11, 6.12 and 6.13 ...

Monomolecular reactions Two case have to be examined here ... 

Figure 6.21. Model predicted electrochemical promotion behaviour for a monomolecular reaction (a) electrophobic (b) electrophilic.

For a monomolecular reaction, such as the cracking of hydrocarbons by protonic zeolites, the rate expression is very similar to the one in Eq. (1.5). The rate of the reaction is now proportional to the concentration of molecules at the reaction center, the proton of the zeolite, Eq. (1.22a). 

The speed with which taste stimulation occurs, coupled with the fact that stimulation with toxic substances does no damage to the receptors, led Beidler to suggest that taste stimulus need not enter the interior of the taste cell in order to initiate excitation. Because a taste cell has been shown to be sensitive to a number of taste qualities, and to a large number of chemical stimuli, he and his coworkers concluded that a number of different sites of adsorption must exist on the surface of the cell. Therefore, they assumed that taste response results from adsorption of chemical stimuli to the surface of the receptor at given receptor sites. This adsorption is described by a monomolecular reaction similar to that assumed by Renqvist, Lasareff, and Hahn, but with a difference. From the fact that each type of chemical-stimulus compound has a unique level of saturation of the taste receptor, it was concluded that the magnitude of the response is dependent on the initial reaction with the receptor, and not on other, subsequent receptor-reactions that are common to all types of receptor stimulation. Therefore, it was assumed that the magnitude of neural response is directly proportional to the number of sites filled, the maximum response occurring when all of the sites are filled. Beidler derived a fundamental... 

Another monomolecular reaction that competes with the isomerization in collisionless condition is dissociation. Dissociation is found to be the... 

Perrin s theory was flawed both in his failure to clearly express the radiation hypothesis in quantum terms and in his concrete examples of monomolecular reactions. Thomas Martin Lowry, recently appointed to a new chair of physical chemistry at Cambridge University, argued that Perrin s choices of chemical examples were unfortunate. 

The only clear case of a monomolecular reaction is in radioactivity.99... 

Ionization of 1,5-hexadiene in fluorochloroalkane matrix (Scheme 2.43) represents cation-radical monomolecular reactions. The initially formed cation-radical collapses to the cyclohexane cation-radical, that is, spontaneous cyclization takes place (Williams 1994). Zhu et al. (1998) pointed out that the ring formation from the excited valence isomer in the center of Scheme 2.43 is easier than in the corresponding ground-state dienes. Notably, tandem mass spectrometry revealed the same transformation of 1,5-hexadiene in the gas phase too. This provides ns with a hint that mass spectrometry can serve as a method to express predictions of monomolecnlar transformation of cation-radicals in the condensed phase. A review by Lobodin and Lebedev (2005) discnsses this possibility in more detail. 

Elementary reactions (also termed monomolecular reactions) that involve only a single entity in the formation of an activated complex. Unimolecular rate constants, k, are concentration-independent and are typically expressed in units of sUnimolecular reactions are expected to be first order (i.e., -dc/dt = kc where c is the concentration and t is time). Examples of unimolecular processes include radioactive disintegrations, isomeriza-tions, disassociations, and decompositions. Reactions in solution are unimolecular only if the solvent is not covalently incorporated into the product(s). 

The importance of time, temperature and acid concentration in the hydrolysis of cellulose with dilute acid was recognized by early investigators and applied in the investigations of Simonsen in 1898. Further study was made by Kressman and reported in U. S. Department of Agriculture Bulletin No. 983. Reviews of the quantitative aspects have been made by Doree. Liiers pointed out that the conversion of cellulose dextrin to D-glucose by dilute sulfuric acid was a monomolecular reaction. The constants of the hydrolysis of wood cellulose have been determined by Saeman. The reaction rate (A ) was found to be expressed by the following equation ... 

Because a is a parameter that cannot be calculated from first principles. Equation 1-95 cannot be used to calculate reaction rate constant k from first principles. Furthermore, the collision theory applies best to bimolecular reactions. For monomolecular reactions, the collision theory does not apply. Tr3dng to calculate reaction rates from first principles for all kinds of reactions, chemists developed the transition state theory. 

In this section, we consider a general network of linear (monomolecular) reactions. This network is represented as a directed graph (digraph) vertices correspond to components A edges correspond to reactions A, Aj with kinetic constants fc >0. For each vertex. A,-, a positive real variable c, (concentration) is defined. A basis vector e corresponds to A,- with components ej — Sjj, where is the Kronecker delta. The kinetic equation for the system is... 

The map (p determines discrete dynamical system on a set of components V — A,. We call it the auxiliary discrete dynamical system for a given network of monomolecular reactions. Let us decompose this system and find the cycles Cy and their basins of attraction, Att(Cy). 

An oriented cycle of the length more than two is not solvable. For each number of vertices one can calculate the set of all maximal solvable mechanisms. For example, for five components there are two maximal solvable mechanisms of monomolecular reactions ... 

The reaction scheme in Fig. 26 gives a detailed picture of the main reactions initiated by the excitation of the dye molecule at the surface of an organic crystal. The rate constants connecting the reactants with the products are given in the dimensions s 1 for monomolecular reactions and in the dimensions cm3 s 1 for biomolecular reactions. Going from left to right in the first line a J describes the... 

According to A. J. B. Robertson [23] decomposition of octogen at temperatures above 280°C occurs as a monomolecular reaction. Activation energy =52.7 kcal, log = 19.7. 

At RT it decomposes at a rate characteristic of monomolecular reactions. Decompn is catalyzed in the dry state, but not in aq solos,... 

In conclusion, extensive research has revealed that the Lewis and Brpnsted acid sites on the promoted sulfated zirconia catalysts are not necessarily stronger acids than the corresponding sites in zeolites, but sulfated zirconia circumvents the energetically unfavorable monomolecular reaction path by following a bimolecular mechanism. The question of superacidity of sulfated zirconia, however, is still debated.312... 

The reaction of (alkyl)chlorosilanes with a silica surface has been discussed and reviewed in great detail in literature [10], Although 5 different reactions are possible with di-, tri- or tetrachlorosilanes, basically two important surface species are created. The first is a monodentate silyl group, created by the monomolecular reaction of 1 silanol with 1 chlorosilane, according to reaction (A) (cfr. Figure 2). The second surface specie is a bidentate silyl group, created either by a bimolecular reaction (B) or by a consecutive reaction (C). We have reported previously [11] that the surface of MCM-48, prepared by the gemini 16-12-16 surfactant, possesses 0.9 OH/nm2. 

With appropriate adaptations, the other monomolecular reaction schemes can be treated by similar procedures. It may suffice, therefore, to give below the resulting Laplace transforms of the interfacial concentrations in terms of parameters, the meanings of which are given in Table 8. Where possible, references to the literature are given. However, notations and formulations are often quite different due to the personal preferences of the authors. 

Hale calculated the effect of a preceding monomolecular reaction when the disc electrode is galvanostatically controlled [185]. His results can be applied to a wide range of homogeneous rate constants. 

---