Short-lived species

All the alkali metals have characteristic flame colorations due to the ready excitation of the outermost electron, and this is the basis of their analytical determination by flame photometry or atomic absorption spectroscopy. The colours and principal emission (or absorption) wavelengths, X, are given below but it should be noted that these lines do not all refer to the same transition for example, the Na D-line doublet at 589.0, 589.6 nm arises from the 3s — 3p transition in Na atoms formed by reduction of Na+ in the flame, whereas the red line for lithium is associated with the short-lived species LiOH. 

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. 

Francium is produced by the a decay of Ac, which decays mostly by /3 emission. However, ca. 1% of the decays are by a emission, giving Fr, the isotope with the longest half life (t,/2 = 1260 s). Rapid separation techniques are necessary to isolate this short-lived species from the complex mixture. One way is to separate the Th and Ra daughters from the Ac mother and then separate the monovalent Fr from the trivalent Ac. Other major isotopes of Fr have even shorter lifetimes. 

The Stability of hydrocarbon ions is discovered intuitively by observing whether the hydrocarbon ion can be isolated as a salt, for example, a sodium salt of the carbanion or a tetrafluoroborate salt of the carbocation. Conversely, a single hydrocarbon ion produced in the gas phase is obviously an unstable and short-lived species. Thus, many of the aliphatic carbocations in the gas phase are merely observable species but are not usable for synthesis. 

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. 

Carotenoid radicals — Many of the important oxidations are free-radical reactions, so a consideration of the generation and properties of carotenoid radicals and of carbon-centered radicals derived from carotenoids by addition of other species is relevant. The carotenoid radicals are very short-lived species. Some information has been obtained about them by the application of radiation techniques, particularly pulse radiolysis. Carotenoid radicals can be generated in different ways. "... 

The catalysis of reactions of short-lived species, such as free radicals generated by radiation, became of interest in more recent times. When the species whose reactions... 

The alternative method is continuous-flow , in which the reactants flow through the detection coil during data acquisition. Continuous-flow NMR techniques have been used for the direct observation of short-lived species in chemical reactions [4—6]. The main difference between stopped- and continuous-flow NMR is that in the latter the sample remains inside the detection coil only for a short time period, termed the residence time, x [7], which is determined by the volume of the detection cell and the flow rate. The residence time alters the effective relaxation times according to the relationship in Eq. (2.5.1) ... 

Two reactions have come to be extensively used with silenes, arising from the need to trap the short-lived species cleanly and in high yield, as evidence either of their formation or of the extent of their formation. These are the addition of alcohols, usually methanol, across the double bond to yield an alkoxysilane, and the Diels-Alder reaction with a diene, often 2,3-dimethylbutadiene. Each is an example of the two different types of addition to the Si=C double bond. 

In order to observe a short-lived species it may be necessary to employ a rapid-scanning spectrometer, such as a diode-array instrument (Sms for a 240nm-800nm spectrum). In addition, the absorbances of electrogenerated species can be very small and signal-averaging or phase-sensitive detection may be necessary to achieve the required signal-to-noise ratio (cf. EMIRS and FTIR). 

We now turn to the addition of hydrogen to 3Fe(CO)3. This is hypothesized to be the process that leads to the short-lived species formed in sc Ar upon photolysis of iron pentacarbonyl in the presence of H2 (24). It is however possible that Fe(CO)3 is present instead in the form of the weakly bound 3Fe(CO)3(Ar) species under these conditions. However, the reaction has also been studied in the gas phase, where a room-temperature rate constant of 2.7 x 10 11cm3molecule 1s 1 was reported (49,55), again corresponding to a very large, near collision-limit value. 

The disappearance of nitroxides may also occur by unimolecular reactions. A good example is found with t-butoxy t-butyl nitroxide, known to be a relatively short-lived species, for which three fragmentation pathways are possible (1 la,b,c). Loss of butoxyl radicals (path a) was shown to be reversible (Perkins and Roberts, 1974), and, in the absence of any substrate which... 

Solvation enthalpy data for neutral short-lived species, like radicals, are even more scant than for long-lived stable molecules. They can only be experimentally determined through indirect methods, namely, by comparing the enthalpies of reactions of those species in solution and in the gas phase. The former are obtained, for instance, by using the photoacoustic calorimetry technique (see chapter 13), and the latter by several gas-phase methods. 

CH4/O2 and CF4 as the reactant gases and observed the formation of [M — 4 H] ions in the Cl plasma (Scheme 13)166. Thus, repeated deprotonation and electron transfer processes appear to offer an efficient access to more highly unsaturated and/or ring condensed trimethylenemethane radical anions. The [M — 4 H]- ion is considered identical to the molecular radical anion (42) of acepentalene (43), which was generated as a short-lived species from the former by neutralization-reionization mass spectrometry167. Efforts to apply Squires methodology to triquinacene 41 and the tribenzotriquinacenes 44 have been made168. 

Flash experiments lo ) show completion of mNP formation within the flash duration time (20 (xs). Therefore the short-lived species (Amax 340—350 nm and 490—510 nm, lifetime 40 ms in presence of OH and 0,3 ms in presence of methylamine) observed cannot be an intermediate and is probably the radical anion of mNA i h... 

Upon flashing (6 ns pulse) an alkaline solution of dNA in acetonitrile/water 1 1 with a frequency-doubled ruby Iciser (347 nm) three short-lived species have been detected i > ... 

A simple method for the production and cryogenic trapping of ion-radicals is mentioned. The technique, cold window radical discharge (CWRD), enables the isolation of short-lived species in rare gas matrices, such as p-dichlorobenzene cation-radical. These species are formed within discharge plasmas, close to the trapping surface (Kolos 1995). 

Flash excitation of solutions of 3,5-dinitroanisole in the presence of nucleophiles produces a short-lived species with an absorption maximum at 550-570 nm (Figure 11, curve a). Decay of this species follows first order kinetics. The lifetime varies rather strongly with the nature of the nucleophile, from 40 ms with hydroxide ion to 0 1 ms with thiocyanate (Table 4). The absorption spectrum is independent of the reagent used. 

The results with the classical discharge flash were positive in that a short-lived species, the radical anion, could be traced and identified. 

The second short-lived species, with X ax = 475 nm, does not depend for its occurrence on the presence of a nucleophile. The... 

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