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Short-lived intermediates

The rates of many reactions are not represented by application of the law of mass action on the basis of their overall stoichiometric relations. They appear, rather, to proceed by a sequence of first- and second-order processes involving short-lived intermediates which may be new species or even unstable combinations of the reaclants for 2A -1- B C, the sequence could be A -1- B AB followed by A -1- AB C. [Pg.690]

Probable mechanisms often have been deduced The reactant forms a short-lived intermediate with the catalyst that subsequently decomposes into the product and regenerated catalyst. In fluid phases such intermediates can be detected spectroscopic ly. This is in contrast to sohd catalysis, where the detection of intermediates is much more difficult and is not often accomphshed. [Pg.2092]

In many gaseous state reactions of technological importance, short-lived intermediate molecules which are formed by die decomposition of reacting species play a significant role in die reaction kinetics. Thus reactions involving die mediane molecule, CH4, show die presence of a well-defined dissociation product, CH3, die mediyl radical, which has a finite lifetime as a separate entity and which plays an important part in a sequence or chain of chemical reactions. [Pg.42]

Under normal conditions of electrophilic substitution, these benzenium ions are short-lived intermediates. The fact that the structures are stable in nonnucleophilic media clearly demonstrates the feasibility of such intermediates. [Pg.556]

Unlike nitric oxide, NO, the monomeric radical sulfur nitride, NS, is only known as a short-lived intermediate in the gas phase. Nevertheless the properties of this important diatomic molecule have been thoroughly investigated by a variety of spectroscopic and other physical techniques (Section 5.2.1). The NS molecule is stabilized by coordination to a transition metal and a large number of complexes, primarily with metals from Groups 6, 7, 8 and 9, are known. Several detailed reviews of the topic have been published. ... [Pg.123]

By using this technique acrylamide, acrylonitrile, and methyl acrylate were grafted onto cellulose [20]. In this case, oxidative depolymerization of cellulose also occurs and could yield short-lived intermediates [21]. They [21] reported an electron spin resonance spectroscopy study of the affects of different parameters on the rates of formation and decay of free radicals in microcrystalline cellulose and in purified fibrous cotton cellulose. From the results they obtained, they suggested that ceric ions form a chelate with the cellulose molecule, possibly, through the C2 and C3 hydroxyls of the anhy-droglucose unit. Transfer of electrons from the cellulose molecule to Ce(IV) would follow, leading to its reduction... [Pg.503]

Enolate ions are more useful than enols for two reasons. First, pure enols can t normally be isolated but are instead generated only as short-lived intermediates in low concentration. By contrast, stable solutions of pure enolate ions are easily prepared from most carbonyl compounds by reaction with a strong base. Second, enolate ions are more reactive than enols and undergo many reactions that enols don t. Whereas enols are neutral, enolate ions are negatively charged, making them much belter nucleophiles. As a result, enolate ions are more common than enols in both laboratory and biological chemistry. [Pg.853]

Some transfer agents react by addition-fragmentation (Section 6.2.3) or abstraction-fragmentation mechanisms. Both of these processes involve the formation of a short-lived intermediate. The reaction scheme for addition-fragmentation can be summarized schematically as follows (Scheme 6.3). [Pg.287]

How can you use the rotating ring disk electrode for detecting short-lived intermediate species ... [Pg.139]

One way to make the short-lived intermediates amenable to study is to increase their lifetime, usually by irradiating in the solid state and at very low temperatures. Then, the intermediates can be measured at the end of the irradiation by optical absorption spectroscopy or ESR. [Pg.890]

Evidence for the occurrence of vinyl cations as short-lived intermediates in solvolysis and other reactions has accumulated in the last few years (reviewed by Hanack, 1970, by Richey and Richey, 1970, and by Modena and Tonellato, 1971), but they have not been observed spectroscopically. It has been shown possible to intercept some vinyl cations—prepared in a system of extremely low nucleophilicity (EHSO3—SbEj 1 1-1 10) by protonation of propyne and 2-butyne— by carbon monoxide (Hogeveen and Roobeek, 1971b). The oxocarbo-nium ions formed in these cases are shown in the following scheme ... [Pg.45]

Singlet diradicals are usually extremely short-lived intermediates. For example, trimethylene (TM, 2) was observed to have a fast decay time of 120 fs by femtosecond spectroscopy [84, 85]. Since the localized 1,3-cyclopentanediyl diradical (62) was characterized by Buchwalter and Closs in 1975 [81, 82], experimental efforts have been made to prepare and characterize the persistent, localized singlet 1,3-diradicals. Some experimental achievements of the localized diradicals are collected in Fig. 25 and Table 3. It should be mentioned that the literature of experimental studies selected here is not exhaustive and more related references can be found in [83-115] and others. [Pg.256]

We deal with many reactions that are not elementary. Most industrially important reactions go through a complex kinetic mechanism before the final products are reached. The mechanism may give a rate expression far different than Equation (1.14), even though it involves only short-lived intermediates that never appear in conventional chemical analyses. Elementary reactions are generally limited to the following types. [Pg.6]

Complex reactions can be broken into a number of series and parallel elementary steps, possibly involving short-lived intermediates such as free radicals. These individual reactions collectively constitute the mechanism of the complex reaction. The individual reactions are usually second order, and the number of reactions needed to explain an observed, complex reaction can be surprisingly large. For example, a good model for... [Pg.8]

This reaction is complex even though it has a stoichiometric equation and rate expression that could correspond to an elementary reaction. Recall the convention used in this text when a rate constant is written above the reaction arrow, the reaction is assumed to be elementary with a rate that is consistent with the stoichiometry according to Equation (1.14). The reactions in Equations (2.5) are examples. When the rate constant is missing, the reaction rate must be explicitly specihed. The reaction in Equation (2.6) is an example. This reaction is complex since the mechanism involves a short-lived intermediate, B. [Pg.36]

Many reactions involve short-lived intermediates that are so reactive that they never accumulate in large quantities and are difficult to detect. Their presence is important in the reaction mechanism and may dictate the functional form of the rate equation. Consider the following reaction ... [Pg.49]

The quasi-steady hypothesis is used when short-lived intermediates are formed as part of a relatively slow overall reaction. The short-lived molecules are hypothesized to achieve an approximate steady state in which they are created at nearly the same rate that they are consumed. Their concentration in this quasi-steady state is necessarily small. A typical use of the quasi-steady... [Pg.50]

Figure 6. The Fourier transformed signal AS[r, i] of CH2I2/CH3OH. The pump-probe time delays vary between i = —250ps and 1 ps. The pair distribution function gi i peaks in the 3 A region. If i < 50 ns, the 1—1 bond corresponds to the short-lived intermediate (CH2TI), and if I > 100 ns it belongs to the (I3") ion. Red curves indicate the theory, and black curves describe the experiment. (See color insert.)... Figure 6. The Fourier transformed signal AS[r, i] of CH2I2/CH3OH. The pump-probe time delays vary between i = —250ps and 1 ps. The pair distribution function gi i peaks in the 3 A region. If i < 50 ns, the 1—1 bond corresponds to the short-lived intermediate (CH2TI), and if I > 100 ns it belongs to the (I3") ion. Red curves indicate the theory, and black curves describe the experiment. (See color insert.)...
Based on surface science and methods such as TPD, most of the kinetic parameters of the elementary steps that constitute a catalytic process can be obtained. However, short-lived intermediates cannot be studied spectroscopically, and then one has to rely on either computational chemistry or estimated parameters. Alternatively, one can try to derive kinetic parameters by fitting kinetic models to overall rates, as demonstrated below. [Pg.287]

Many important processes such as electrochemical reactions, biological processes and corrosion take place at solid/liquid interfaces. To understand precisely the mechanism of these processes at solid/liquid interfaces, information on the structures of molecules at the electrode/electrolyte interface, including short-lived intermediates and solvent, is essential. Determination of the interfacial structures of the intermediate and solvent is, however, difficult by conventional surface vibrational techniques because the number of molecules at the interfaces is far less than the number of bulk molecules. [Pg.71]

Th, Th and Po, all decay by alpha emission and are thus measurable by isotope dilution and alpha spectrometry (Ivanovich and Murray 1992). However, " Th is produced by the alpha decay of and in turn decays by beta emission to via the short-lived intermediate " Pa (half-life 1.18 m) ... [Pg.462]

In this part we dwell on the properties of the simplest radicals and atoms in the adsorbed layer of oxide semiconductors as well as analyse the quantitative relationships between concentrations of these particles both in gaseous and liquid phases and on oxide surfaces (mostly for ZnO), and effect of former parameters on electrophysical parameters. Note that describing these properties we pursue only one principal objective, i. e. to prove the existence of a reliable physical and physical-chemical basis for a further development and application of semiconductor sensors in systems and processes which involve active particles emerging on the surface either as short-lived intermediate formations, or are emitted as free particles from the surface into the environment (heterogeno-homogeneous processes). [Pg.194]

Solutions of tetrazolium salts, e.g., 53, have been reported to both become colored and bleached under the influence of both UV and visible light. Several workers have attributed this phenomenon to photoreduction to the corresponding formazan (51) and the formation of a fluorescent colorless compound (152) through photooxidation.240- 243 The reduction of 152 under UV or blue light to the intense green radical structure (153) has also been reported (Scheme 21).244 A one-electron reduction product (154) is proposed as a short-lived intermediate in the photoreduction.245... [Pg.248]

The element "eka-cesium" had long been suspected. Was detected as a short-lived intermediate product in the decay series of actinium. [Pg.79]

Some economies are possible if equilibrium is assumed between selected compartments, an equal fugacity being assignable. This is possible if the time for equilibration is short compared to the time constant for the dominant processes of reaction or advection. For example, the rate of chemical uptake by fish from water can often be ignored (and thus need not be measured or known within limits) if the chemical has a life time of hundreds of days since the uptake time is usually only a few days. This is equivalent to the frequently used "steady state" assumption in chemical kinetics in which the differential equation for a short lived intermediate species is set to zero, thus reducing the equation to algebraic form. When the compartment contains a small amount of chemical or adjusts quickly to its environment, it can be treated algebraically. [Pg.180]

In contrast, hydrolysis of the methyl derivative, Cp(OC)2Fe-SiMe(H)Cl (Id), yields the corresponding ferrio-silanol, Cp(OC)2Fe-SiMe(H)OH, as a short-lived intermediate identified by means of IR and NMR spectroscopy, which immediately reacts with Id to give the bis(ferrio)disiloxane 6. Conversion of 6 to the fluorine analogue [Cp(0C)2Fe-Si(Me)F]20, characterized by a linear Si-O-Si arrangement [6], is achieved via consecutive H/Cl- and Cl/F-exchange. [Pg.186]

There is evidence that tr[Pg.282]

The rotation speed provides an independent variable increasing to decreases the time taken for a species to travel between the disc and the ring and hence increases the likelihood of detecting a short-lived intermediate. All of the above methods critically depend on the species generated at the electrode being active towards further electron transfer at the ring. [Pg.186]


See other pages where Short-lived intermediates is mentioned: [Pg.47]    [Pg.881]    [Pg.113]    [Pg.321]    [Pg.154]    [Pg.266]    [Pg.330]    [Pg.115]    [Pg.117]    [Pg.124]    [Pg.167]    [Pg.303]    [Pg.369]    [Pg.592]    [Pg.19]    [Pg.56]    [Pg.133]    [Pg.5]    [Pg.423]    [Pg.155]    [Pg.249]   
See also in sourсe #XX -- [ Pg.221 ]

See also in sourсe #XX -- [ Pg.110 ]




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