Experimental evidence for

Diels-Alder reactions and other transformations. Unfortunately, experimental evidence for these... 

Because of the mentioned leveling effect of the solvent (or excess acid itself acting as such) the acidity cannot exceed that of its conjugate acid. In the case of water the limiting acidity is that of HsO. Proton-ated water, H30 (hydronium ion), was first postulated in 1907, and its preeminent role in acid-catalyzed reactions in aqueous media was first realized in the acid-base theory of Bronsted and Lowry. Direct experimental evidence for the hydronium ion in solution and in the... 

Temperature rather than frequency is the independent variable. We have previously noted the equivalency of these variables. Figure 3.16 may be taken as experimental evidence for that. 

The saturation magnetization, J), is the (maximum) magnetic moment per unit of volume. It is easily derived from the spia configuration of the sublattices eight ionic moments and, hence, 40 ]1 per unit cell, which corresponds to = 668 mT at 0 K. This was the first experimental evidence for the Gorter model (66). The temperature dependence of J) (Fig. 7) is remarkable the — T curve is much less rounded than the usual BdUouia function (4). This results ia a relatively low J) value at RT (Table 2) and a relatively high (—0.2%/° C) temperature coefficient of J). By means of Mitssbauer spectroscopy, the temperature dependence of the separate sublattice contributions has been determined (68). It appears that the 12k sublattice is responsible for the unusual temperature dependence of the overall J). 

Hydroxides. The hydrolysis of uranium has been recendy reviewed (154,165,166), yet as noted in these compilations, studies are ongoing to continue identifying all of the numerous solution species and soHd phases. The very hard uranium(IV) ion hydrolyzes even in fairly strong acid (- 0.1 Af) and the hydrolysis is compHcated by the precipitation of insoluble hydroxides or oxides. There is reasonably good experimental evidence for the formation of the initial hydrolysis product, U(OH) " however, there is no direct evidence for other hydrolysis products such as U(OH) " 2> U(OH)" 2> U(OH)4 (or UO2 2H20). There are substantial amounts of data, particulady from solubiUty experiments, which are consistent with the neutral species U(OH)4 (154,167). It is unknown whether this species is monomeric or polymeric. A new study under reducing conditions in NaCl solution confirms its importance and reports that it is monomeric (168). 8olubihty studies indicate that the anionic species U(OH) , if it exists, is only of minor importance (169). There is limited evidence for polymeric species such as Ug(OH) " 25 (1 4). 

Molecular orbital calculations indicate that cyclo C-18 carbyne should be relatively stable and experimental evidence for cyclocarbynes has been found [25], Fig. 3B. Diederich et al [25] synthesised a precursor of cyclo C-18 and showed by laser flash heating and time-of flight mass spectrometry that a series of retro Diels-Alder reactions occurred leading to cyclo C-18 as the predominant fragmentation pattern. Diederich has also presented a fascinating review of possible cyclic all-carbon molecules and other carbon-rich nanometre-sized carbon networks that may be susceptible to synthesis using organic chemical techniques [26]. 

It was also suggested that the relationship of the two forms (A) and (B) might be that of stereoisomerides of the cis-trans decalin type and a good deal of work was done to provide experimental evidence for this view or the alternative that they are structural isomerides. It was also found that of the methods used to synthesise the pyridocoline system all but one gave norlupinane (A) on reduetion, the exceptional formation of uorlupinane (B) being limited to reduction of the 1-keto compound by the Clemmensen method. 

Other contributions to this discussion have been made by Harvey, Miller and Robson, and by Dewar and King, and important additional experimental evidence for the location of the. CHOH group at has been provided by Witkop, who explains the improvement in the yield of yohimbone, which occurs when the decarboxylation of yohimbic acid takes place under oxidising conditions, as due to the primary formation of a j8-ketocarboxylic acid, which is readily decarboxylated. This may be represented, using part of the yohimbine formula fXIV), as follows MeO. OC. CH. 1 CHOH. ->HO. OC. CH. CHOH. ... 

In a more recent paper Menschikov assigns to trachelantamidine formula (VI, p. 611), which makes it structurally identical with iso-retronecanol (p. 609). He also provides experimental evidence for the view that trachelantic acid is 2-methyl-3 4-dihydroxypentane-3-carboxylic acid, Me CH. C(OH)(COjH). CHOH. CHg, and trachelanta-mine becomes C HjaN. CHaO. OC. C(OH)Pr. CHOH. CHg, C,HjaN being the pyrrolizidine residue (1947). 

The surface X-ray experiments by Toney et al. [151] give experimental evidence for voltage-dependent ordering of water on a silver electrode. They observed a shift of the silver-oxygen distance with applied potential. 

The position of the equilibrium is determined not only by ring size and polar and steric factors but also by the environment of the molecule. The experimental evidence for the existence of three tautomeric forms has been based on the study of their reactivity and, to a lesser degree, on physicochemical measurements 175-177). Often the existence of the corresponding carbinolamine or its acyclic tautomeric form in addition to the basic dehydrated form is quite important. 

The attack on the aromatic nucleus by hydroxyl radicals is probably analogous to that by phenyl and methyl radicals, Eq. (34a,b). Evidence that the first step is the addition of hydroxyl radical to benzene, rather than abstraction of a hydrogen atom, has recently been adduced from a study of the radiolysis of water-benzene mixtures. The familiar addition complex may undergo two reactions to form the phenolic and dimeric products respectively, Eq. (34a,b). Alternative mechanisms for the formation of the dimer have been formulated, but in view of the lack of experimental evidence for any of the mechanisms further discussion of this problem is not justified. 

Following this, House and Ro presented the first experimental evidence for the existence of metalated epoxides. Treatment of cis- and trons-a-methyl- 3-(phenyl-... 

A number of studies have focused on D-A systems in which D and A are either embedded in a rigid matrix [103-110] or separated by a rigid spacer with covalent bonds [111-118], Miller etal. [114, 115] gave the first experimental evidence for the bell-shape energy gap dependence in charge shift type ET reactions [114,115], Many studies have been reported on the photoinduced ET across the interfaces of some organized assemblies such as surfactant micelles [4] and vesicles [5], wherein some particular D and A species are expected to be separated by a phase boundary. However, owing to the dynamic nature of such interfacial systems, D and A are not always statically fixed at specific locations. 

There is convincing experimental evidence for the following important statement. To a degree of approximation satisfactory for most analytical work, the mass absorption coefficient of an element is independent of chemical or physical state. This means, for example, that an atom of bromine has the same chance of absorbing an x-ray quantum incident upon it in bromine vapor, completely or partially dissociated in potassium bromide or sodium bromate in liquid or solid bromine. X-ray absorption is predominantly an atomic property. This simplicity is without parallel in absorptiometry. 

Substitution of known values in Equation 6-6 gives a lower value of a (3520 as against 4410) than that corresponding to the experimental points in Figure 6-4, which indicates that the counting rates measured for chromium Ka are higher than expected on the basis of the treatment given above. Such an increase in the intensity of this characteristic line could be due to its excitation by the characteristic lines of the substrate, notably molybdenum Ka experimental evidence for this excitation process was obtained.10... 

Dall Asta and Motroni (44, 57) provided direct experimental evidence for the transalkylidenation mechanism in the case of cycloalkenes. With a catalyst system consisting of WOCI4, C2H6A1C12, and benzoyl peroxide they prepared a random copolymer of cyclooctene and cyclopentene, the cyclo-pentene double bond being labeled with 14C. The distribution of the radioactivity in the copolymer formed will depend on the site of ring opening. 

More detailed and theoretical explanations of the role of the catalyst, based on this scheme, have appeared (72, 74, 77-82). In order to obtain experimental evidence for this scheme, some investigators did experiments in which 1,2-dimethylcyclobutane or cyclobutane were brought into contact with an active metathesis catalyst. However, 1,2-dimethylcyclobutane was stable under conditions where propene gave a high conversion to ethene and 2-butene (63). The experiments with cyclobutane led to the same conclusion (83). From this, and from the fact that cyclobutanes are not reaction products, although this can be expected thermodynamically, it follows that cyclobutanes are not free intermediates. This prompted Lewandos and Pettit (83) to propose a tetramethylene complex as the key intermediate ... 

The experimental evidence for the availability of the coordinative insufficiency of the transition metal ion in the propagation centers was obtained (175) in the study of the deactivation of the propagation centers by coordination inhibitors. On the introduction of such inhibitors as phosphine and carbon monoxide into the polymerization medium, the reaction stops, but the metal-polymer bond is retained. It shows that in this case the interaction of the inhibitor with the propagation center follows the scheme ... 

Of the possible sets of scaling laws containing only the dimensionless group of properties pjpv, set 4, which includes A, pL, pv, y, C L, and kL, appears closest to consistency with the available experimental evidence for burn-out in uniformly heated round tubes. 

Unfortunately, even for low molecular weight material it is difficult to obtain clear experimental evidence for a roughening transition [71]. This is mainly due to the fact that during growth the interface generally assumes a metastable shape and relaxation times are long and increase with crystal size. Therefore we certainly cannot expect a definitive answer for macromolecules. We shall therefore just make several comments which hopefully will be of use when reading the literature. 

There is no direct experimental evidence for the intermediate 2.30 in the reaction sequence of Scheme 2-19. In the corresponding diazotization of 2-aminophenazine the proportion of the quinone diazide (isomer of 2.31) amounted to only 16%, but 30% unsubstituted phenazine was also found. The phenazine may have resulted from the overall redox reaction. 

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