Living species

Altogether it is not a large reaction database in January 2003 it comprised only about 1533 unique structures and 2175 reactions. Nevertheless, it is a very important reaction database as it contains the essential processes occurring in living species. [Pg.560]

These examples served to show that the Biochemical Pathways database provides a rich source of information on these all important reactions that determine the transformation of nutrients into the broad spectrum of compounds contained in living species and the concomitant production of energy to keep these processes going. [Pg.567]

My work on long-lived (persistent) carbocations dates back to the late 1950s at Dow and resulted in the first direct observation of alkyl cations. Subsequently, a wide spectrum of carbocations as long-lived species was studied using antimony pentafluoride as an extremely strong Lewis acid and later using other highly acidic (superacidic) systems. [Pg.75]

The intermediate diphenylhydroxymethyl radical has been detected after generation by flash photolysis. Photolysis of benzophenone in benzene solution containing potential hydrogen donors results in the formation of two intermediates that are detectable, and their rates of decay have been measured. One intermediate is the PhjCOH radical. It disappears by combination with another radical in a second-order process. A much shorter-lived species disappears with first-order kinetics in the presence of excess amounts of various hydrogen donors. The pseudo-first-order rate constants vary with the structure of the donor with 2,2-diphenylethanol, for example, k = 2 x 10 s . The rate is much less with poorer hydrogen-atom donors. The rapidly reacting intermediate is the triplet excited state of benzophenone. [Pg.755]

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. [Pg.75]

A recent report from the U.S. National Academy of Sciences estimates than less than 1% of all living species have been characterized. Thus, alkaloid chemistry remains today an active area of research, and innumerable substances with potentially useful properties remain to be discovered. [Pg.65]

When you consider how many different ways thirty (or even fewer) different amino acids can be combined in long chains of a hundred or more, you can see why there are so many proteins known, and why different living species of... [Pg.348]

Sequential addition of monomers 6 7-26-27-114) is the most obvious procedure. Once the first monomer has been polymerized, the resulting living species is used as a polymeric initiator for the polymerization of the second one. The monomers are to be added in the order of increasing electron affinity to provide efficient and fast initiation 26 U4). This condition is rather restrictive, and the number of monomer systems that can be used is limited (Table 5). Moreover, when the second monomer contains an electrophilic function (e.g. ester) which could lead to side reactions, it is necessary to first lower the nucleophilicity of the living site. This is best done by intermediate addition of 1.1-diphenylethylene25). The stabilized diphenylmethyl anions do not get involved in side reactions with ester functions, while initiation is still quantitative and fast. [Pg.164]

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. [Pg.190]

Transport and dijfusion. With the exception of N2, O2, Ar, and numerous other long-lived species that are well-mixed in the bulk of the atmosphere, horizontal and vertical transport are closely coupled with chemical reactions in controlling atmospheric trace-substance concentrations. [Pg.132]

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. [Pg.355]

Note that the quasi-steady hypothesis is applied to each free-radical species. This will generate as many algebraic equations as there are types of free radicals. The resulting set of equations is solved to express the free-radical concentrations in terms of the (presumably measurable) concentrations of the long-lived species. For the current example, the solutions for the free radicals are... [Pg.52]

The quasi-steady hypothesis allows the diflficult-to-measure free-radical concentrations to be replaced by the more easily measured concentrations of the long-lived species. The result is... [Pg.52]

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. [Pg.176]

Another means of observation of such radical dissociation of hydrocarbons, to a lesser extent, is achieved in modern esr spectroscopic studies. Dissociation of bitropyl [49] into the tropyl radical [3 ] was observed in m-xylene solution (11). The radical is a long-lived species but only at low concentration, 10 -10 m (Vincow et al., 1969). The radical dissociation (12) of 7-(triphenylmethyl)-l,3,5-cycloheptatriene [50] has been found to be more facile than that of bitropyl (Okamoto et al., 1970). [Pg.184]

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. [Pg.651]

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. "... [Pg.58]

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... [Pg.116]

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