Reactive short-lived species

Free carbynes, CR , are exceedingly reactive short-lived species and... 

Peroxynitrous acid, however, is not stable and decomposes to yield HO and NO2 [224]. When NO2 reacts with NO, N2O3 is generated. A comprehensive discussion of these RNS is outside the scope of this paper. However, it should be noted that all these RNS are highly reactive, short-lived species. Therefore, their quantitative assay is a challenging task particularly under in vivo conditions Nitrite (NO2 ) that results e.g. from the decomposition of N2O3, is often used as a marker of the NO production under in vivo conditions and it is known that under inflammatory conditions the concentration of nitrite is strongly elevated. For instance it was shown that nitrite concentrations of up to 4 mmol/1 can be detected in synovial fluids of the patients suffering from RA [226]. 

Because so many interstellar molecules are reactive short-lived species, potential studies in the laboratory can be difficult. These chemical compounds cannot be obtained in bottles or gas cylinders, but have to be created inside the spectrometer reaction cell in situ, as they exist only for fractions of seconds. Their synthesis requires exotic gas-phase techniques employing DC or AC discharges, supersonic jets, as mentioned, and reactor-flow ovens (see Plate 3). There is no guarantee that a species of astrophysical interest can be created in sufficient quantities in the laboratory for spectroscopic studies. 

Hydroxyl radicals are highly reactive, short-lived species. Since they have virtually no time to diffuse in the bulk of the solution, organic molecules have to be in the vicinity of the surface of the catalyst to participate in the oxidation reaction. In other words, in a simple situation the reaction rate should increase with an increase in the surface concentration of organic molecules. Here we refer to adsorbed organic species implying, however, that organic molecules may also be located in a double layer surrounding TiOa-... 

The description of reactive intermediates, which are short-lived species, is the main field of application of quantum chemical model calculations, due to the fact that the intermediates are difficult to observe and characterize. For example, the influence of structure on the stability of various carbenium ions — which have been used as models of the cationic chain end — and the delocalization of the positive charge were treated on this basis. 

C09-0121. Benzyne is an unstable molecule that can be generated as a short-lived species in solution. Suggest a reason why benzyne is very reactive. 

The species present in polymers that can be studied by ESR are often highly reactive, short-lived and are present in low concentrations. However, developments in instrumentation have offered improvements in sensitivity and,combined with more reliable interpretation of data (1), the increasing application of this method of polymer characterization in studying radiation effects on polymers can be expected. 

Divalent silicon compounds (silylenes), one of the most interesting class of low-coordinated silicon compounds, had been known as highly reactive, short-lived transient species that resembled the carbon analogues (carbenes), until the recent... 

The postulation of trimethylene and tetramethylene diradicals as reactive intermediates involved in many thermal isomerization and fragmentation reactions has a long history,but not until 1994 had they ever been detected in real time. The validity of the diradical hypothesis was tested through femtosecond studies, and the tests provided dramatic evidence confirming that these short-lives species are indeed real, directly experimentally accessible chemical entities. 

A radical, often called a. free radical, is a highly reactive and short lived species with an unpaired electron. Free radicals are electron-deficient species, but usually uncharged. So their chemistry is very different from the chemistry of even-electron and electron-deficient species, e.g. carbo-cations and carbenes. A radical behaves like an electrophile, as it requires only a single electron to complete its octet. 

Short-Lived Species in Fluid Solution. - In fluid solution, radical cations derived from saturated hydrocarbons are highly reactive oxidizing species and the rates of their bimolecular reactions are often determined by the frequency of diffusion collisions in solutions. It is known that the reactions of primary radical... 

So far the methods described for measuring excited state lifetime, and hence reactivity, have been indirect methods that rely on a comparison with some standard le.g. actinometer quantum yield or quenching rate constant) that has already been measured. A direct method for measuring the lifetime of short-lived species produced photochemically is flash photolysis. This is a very important technique in photochemistry, though only the basic ideas as they apply to mechanistic studies are outlined here. In flash photolysis a high concentration of a short-lived species (electronically excited state or... 

The reactivity of SiN multiply bonded species has in part been discussed in Section IV. B, dedicated to synthesis, because the existence of reactive and short-lived species is often proven by trapping experiments. This part of the review therefore focusses on more comprehensive reactivity studies of Si=N systems. 

The phenyl cation (134) firstpostulated by Waters335 is a highly reactive species oflow stability and plays a fundamental role in organic chemistry—for example, in the chemistry of diazonium ions. According to gas-phase studies and calculations, its stability is between that of the ethyl cation and the vinyl cation.336 Since it is an extremely electrophilic and short-lived species, it could not be isolated or observed directly in the condensed phase. For example, solvolytic and dediazoniation studies under superacidic conditions by Faali et al.337,338 failed to find evidence of the intermediacy of the phenyl cation. Hyperconjugative stabilization via orf/zo-Me3Si or... 

The continual evolution of the discipline of chemistry is reflected in our treatment of the elements. The careful reader will note that we have included articles for the first 104 elements the remainder of the elements are recently discovered or exist only as short-lived species and, accordingly, are not readily available for the usual chemical studies that reveal, for example, their bulk properties or reactivity and much of the standard chemistry that is of interest. Much of what little we know about the elements beyond 104 permits us to place these elements in their appropriate places in the periodic table, which nevertheless still turns out to be quite insightful from a chemical point of view. 

To determine the chemical nature, concentration, and kinetics of reactive intermediates, time-resolved techniques are used. To detect short-lived species, an inert matrix at extremely low temperature [7], an extremely high-intensity light source, extremely sensitive detection method, or combination of these methods is used. The method using an intensive light source, called flash photolysis, is a technique of transient spectroscopy and transient kinetic studies in which a light pulse is used to produce transient species. Commonly, an intense pulse of short duration is used to produce sufficient concentration of a transient species for spectroscopic observation. The method can be applied to follow concentrations of substrates, intermediates, and products as a function of time after the flash, which enables in the elucidation of photochemical reaction mechanisms (kinetic spectroscopy) [8,9],... 

The splitting of a Cl2 molecule is an initiation step that produces two highly reactive chlorine atoms. A chlorine atom is an example of a reactive intermediate, a short-lived species that is never present in high concentration because it reacts as quickly as it is formed. Each Cl- atom has an odd number of valence electrons (seven), one of which is unpaired. The unpaired electron is called the odd electron or the radical electron. Species with unpaired electrons are called radicals or free radicals. Radicals are electron-deficient because they lack an octet. The odd electron readily combines with an electron in another atom to complete an octet and form a bond. Figure 4-1 shows the Lewis structures of some free radicals. Radicals are often represented by a structure with a single dot representing the unpaired odd electron. 

The free radicals we have studied are one class of reactive intermediates. Reactive intermediates are short-lived species that are never present in high concentrations because they react as quickly as they are formed. In most cases, reactive intermediates are fragments of molecules (like free radicals), often having atoms with unusual numbers of bonds. Some of the common reactive intermediates contain carbon atoms with only two or three bonds, compared with carbon s four bonds in its stable compounds. Such species react quickly with a variety of compounds to give more stable products with tetravalent carbon atoms. 

These developments have in turn renewed the interest in understanding the structure and reactivity of radical cations. Modern computational chemistry methods, especially density functional methods, as well as the continued exponential increase in hardware performance provided improved tools for a detailed analysis of these interesting species. At the same time, the unique problems of the computational treatment of radical cations as well as the direct and indirect observation of these short-lived species continue to pose new challenges for the development of new theoretical and experimental methods. 

In the Lindemann mechanism, a time lag exists between the energisation of A to A and the decomposition (or isomerisation) of A to products. During this time lag, A can be de-energised back to A. According to the steady-state approximation (s.s.a), whenever a reactive (i.e., short-lived) species is produced as an intermediate in a chemical reaction, its rate of formation is equal to its rate of decomposition. Here, the energised species A is short-lived. Its rate of formation = kJAp and its rate of decomposition k t [A][A ] + k2[A ]. Thus,... 

Compounds containing silicon bonded to only one other atom are unstable and are usually only generated and observed as reactive intermediates of short half-life. Silicon compounds subjected to flash photolysis or electrical discharges in the gas phase produce short-lived species SiX (X = H, F, Cl, Br, I, C, Si, etc.), the band structure of which have been studied in detail. The structures, electronic configurations, and so on of Six (X = H, F, Cl, Br, I, N, O, etc.) have also been the subject of MNDO (modified neglect of diatomic overlap) and other calculations. ... 

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